UNIVERSITY OF GHANA

COLLEGE OF BASIC AND APPLIED SCIENCES

APPLICATION OF THE RAINFALL INFILTRATION BREAKTHROUGH

(RIB) MODEL FOR GROUNDWATER RECHARGE ESTIMATION IN

THE BIRIM NORTH DISTRICT OF EASTERN REGION, GHANA

NYARKO DELAIAH ANTWI

(10703628)

THIS THESIS/DISSERTATION IS SUBMITTED TO THE UNIVERSITY

OF GHANA, LEGON IN PARTIAL FULFILMENT OF THE

REQUIREMENT FOR THE AWARD OF MPHIL IN HYDROGEOLOGY

DEGREE

JULY, 2022

University of Ghana http://ugspace.ug.edu.gh

DECLARATION

I, Nyarko Delaiah Antwi, hereby declare that this thesis is a result of an original research

undertaken under supervision of Prof. Sandow Mark Yidana and Prof. Larry-Pax Chegbeleh

towards the award of Master of Philosophy in Hydrogeology in the Earth Science Department,

University of Ghana. And that, to the best of my knowledge, it has not been presented elsewhere

for another degree except where due acknowledgement has been made in the text.

NYARKO DELAIAH ANTWI Date

(Student)

PROF. SANDOW MARK YIDANA Date

(Principal Supervisor)

PROF. LARRY PAX CHEGBELEH Date

(Co-Supervisor)

University of Ghana http://ugspace.ug.edu.gh

ABSTRACT

Evaluation of Groundwater recharge from rainfall is essential for sustainable water resources

management, particularly in arid and semi-arid environments. The Birim North District, where

majority of the population relies on groundwater due to pollution of surface water sources, has

already experienced a drop in groundwater levels attending to the cumulative impacts of human

activities and climate change. This project applies the Rainfall Infiltration Breakthrough (RIB)

methodology to estimate groundwater recharge in the shallow unconfined, saprolite aquifer system

in the Birimian Province in Southwestern Ghana. The Water level fluctuation (WTF) approach

was used to estimate groundwater recharge in order to check, augment, and confirm the Rainfall

Infiltration Breakthrough (RIB) recharge estimates by comparing such groundwater recharge

estimates. The specific yield values acquired from the previous studies were compared to those

acquired using the linear regression model as a quality assurance measure. The validity of the

analysis, i.e., the association between rainfall and groundwater level, was established as a result of

this. The line drawn in the regression model for determining the specific yield corresponded to

0.06, which was close to the value (0.05) obtained from literature. The RIB model estimated local

recharge at 2.9 % to 21.4% of mean annual precipitation (MAP). The WTF approach estimated

recharge to be between 3.2 % to 22.6 %. The prediction showed that decreased rainfall had no

effect on groundwater levels during the simulation period in the climate scenario analysis.

However, the ratio of recharge rate to precipitation did not alter considerably; it was somewhat

greater than the baseline. Correlation examination of rainfall and observed water level fluctuation

(WTF) data at the monthly scale, along with recharge estimates derived from other approaches,

indicated that the RIB results based on monthly data were plausible and could thus be utilized as

recharge estimates.

University of Ghana http://ugspace.ug.edu.gh

iii

These findings suggested that using these methods to estimate groundwater recharge provides

opportunities for assessing temporal variations in groundwater recharge and thus facilitates

groundwater resources management. The method can estimate groundwater recharge in similar

regions with adequately long time series of rainfall and groundwater levels. The RIB model is

particularly suitable for shallow unconfined aquifers with minimal transmissivity; nonetheless, the

RIB model's utility for application in various climatic locations and hydrogeological circumstances

needs to be further investigated. These strategies could be tested in the future in catchments that

have similar conditions of physiographic and hydrogeologic systems to the current research region.

University of Ghana http://ugspace.ug.edu.gh

DEDICATION

This work is dedicated to my parents, Rev. Daniel Nyarko and Mrs. Beatrice Nyarko, for their

unrelenting support and prayers at all times. To my siblings, Daniel and David Nyarko, for

everything they ever did in my journey to this point. Not forgetting my family, friends, and

everyone else I could not mention here.

University of Ghana http://ugspace.ug.edu.gh

ACKNOWLEDGEMENT

This project was made possible ultimately by God's amazing grace and sufficient loving kindness.

My profound gratitude goes to my supervisors, Prof. Sandow Mark Yidana and Prof. Larry Pax

Chegbeleh, for their guidance, good leadership, patience, and technical advice throughout this

research project. Thank you to the University of Ghana, Department of Earth Science, for giving

me the opportunity to pursue this research project.

I also express my genuine appreciation to Rev. Daniel Nyarko for the financial support and Prof.

Patrick Asamoah Sakyi for his outstanding assistance.

This project would not have been possible without the support from my mates. In particular, I

would like to thank Audrey Claude and George Leslie Gaskin for their time and local insight.

I am extremely grateful to Mulalo Isaih Mutoti for granting me access to the RIB software.

University of Ghana http://ugspace.ug.edu.gh

TABLE OF CONTENT

DECLARATION ............................................................................................................................. i

ABSTRACT ..................................................................................................................................... i

DEDICATION ............................................................................................................................... iv

ACKNOWLEDGEMENT .............................................................................................................. v

CHAPTER ONE ............................................................................................................................. 1

INTRODUCTION .......................................................................................................................... 1

1.1 Background ........................................................................................................................... 1

1.2 Problem Statement ................................................................................................................ 7

1.3 Objectives .............................................................................................................................. 8

1.4 Study Area ............................................................................................................................. 9

1.4.1 Location .......................................................................................................................... 9

1.4.2 Climate.......................................................................................................................... 11

1.4.3 Relief and Drainage ...................................................................................................... 12

1.4.4 Vegetation and Soil ....................................................................................................... 13

1.4.5 Geology and hydrogeology ........................................................................................... 15

CHAPTER TWO .......................................................................................................................... 21

LITERATURE REVIEW ............................................................................................................. 21

2.1 Introduction ......................................................................................................................... 21

2.2 Groundwater Recharge ........................................................................................................ 21

University of Ghana http://ugspace.ug.edu.gh

vii

2.3 Groundwater and climate change ........................................................................................ 22

2.3.1 Impacts of climate change on groundwater in Ghana ................................................. 24

2.4 Groundwater Recharge Estimation Methods ...................................................................... 26

2.4.1 Chloride Mass Balance (CMB) Techniques ................................................................. 26

2.4.2 Water Table Fluctuation (WTF) ................................................................................... 34

2.4.3 Environmental Tracer (Isotope) Technique ................................................................. 42

2.4.3.1 Oxygen-18 and deuterium ...................................................................................... 42

2.4.4 Cumulative rainfall departure method (CRD) .............................................................. 46

CHAPTER THREE ...................................................................................................................... 54

METHODOLOGY ....................................................................................................................... 54

3.1 Introduction ......................................................................................................................... 54

3.2 Desk Study .......................................................................................................................... 54

3.3 Recharge Modelling ............................................................................................................ 54

3.3.1 Data requirement in RIB model.................................................................................... 59

3.4 Conceptual Model recharge processes ................................................................................ 60

3.5 Calibration and Validation Methodology ............................................................................ 65

3.6 Sensitivity analysis .............................................................................................................. 67

3.7 Scenarios Analysis .............................................................................................................. 67

3.8 Study Limitations ................................................................................................................ 69

CHAPTER FOUR ......................................................................................................................... 70

University of Ghana http://ugspace.ug.edu.gh

viii

RESULTS AND DISCUSSIONS ................................................................................................. 70

4.1 Introduction ......................................................................................................................... 70

4.2 Estimation of specific yield ................................................................................................. 70

4.3 Recharge estimation using RIB model ................................................................................ 72

4.3.1 Results from rainfall infiltration breakthrough model ................................................. 76

4.4 Sensitive Analysis ............................................................................................................... 80

4.5 Climate scenarios ................................................................................................................ 82

4.6 Recharge estimation using water table fluctuation (WTF) method .................................... 86

4.6.1 Annual Recharge .......................................................................................................... 91

4.6.2 Water table fluctuation method over the six years period .......................................... 101

CHAPTER FIVE ........................................................................................................................ 106

CONCLUSIONS AND RECOMMENDATIONS ..................................................................... 106

5.1 Introduction ....................................................................................................................... 106

5.2 Conclusions ....................................................................................................................... 106

5.3 Recommendations ............................................................................................................. 108

6.0 REFERENCES ..................................................................................................................... 109

APPENDIX ................................................................................................................................. 133

University of Ghana http://ugspace.ug.edu.gh

LIST OF FIGURES

Figure 1.1: Map of Birim North District Assembly ...................................................................... 10

Figure 1.2: Drainage system of the study area .............................................................................. 13

Figure 1.3: Geological map of the study area ............................................................................... 16

Figure 3.1: Contour map of the study area ................................................................................... 61

Figure 3.2: Location map and polygon of each borehole ............................................................. 63

Figure 3.3: Groundwater Recharge Modelling Process ................................................................ 68

Figure 4.1: Graphical determination of specific yield using the envelope straight line of the

precipitation–groundwater rises points of each recorded rainfall–recharge occurrence. ............. 71

Figure 4.2: Monthly rainfall, observed WLF as well as calculated WLF and groundwater recharge

in borehole MW8 .......................................................................................................................... 73

Figure 4.3: Monthly rainfall, observed WLF as well as calculated WLF and groundwater recharge

in borehole MW8S ........................................................................................................................ 74

Figure 4.4: Monthly rainfall, observed WLF as well as calculated WLF and groundwater recharge

in borehole NMW10S ................................................................................................................... 74

Figure 4.5: Monthly rainfall, observed WLF as well as calculated WLF and groundwater recharge

in borehole NMW12S ................................................................................................................... 75

Figure 4.6: Monthly rainfall, observed WLF as well as calculated WLF and groundwater recharge

in borehole NMW17S ................................................................................................................... 75

Figure 4.7: Baseline conditions of measured rainfall and groundwater level fluctuations for

borehole NMW8S ......................................................................................................................... 82

Figure 4.8: Measured rainfall and groundwater level fluctuations with 10 % less rainfall for

borehole NMW8S ......................................................................................................................... 83

University of Ghana http://ugspace.ug.edu.gh

Figure 4.9: Measured rainfall and groundwater level fluctuations with 20 % less rainfall for

borehole NMW8S ......................................................................................................................... 83

Figure 4.10: Measured rainfall and groundwater level fluctuations with 30 % less rainfall for

borehole NMW8S ......................................................................................................................... 84

Figure 4.11: Water table fluctuation in response to rainfall in MW8 from 2012 to 2017 ............ 86

Figure 4.12: Water table fluctuation in response to rainfall in NMW8S from 2012 to 2017 ....... 87

Figure 4.13: Water table fluctuation in response to rainfall in NMW10S from 2012 to 2017 ..... 88

Figure 4.14: Water table fluctuation in response to rainfall in NMW12S from 2012 to 2017 ..... 89

Figure 4.15: Water table fluctuation in response to rainfall in NMW17S from 2012 to 2017 ..... 90

Figure 4.16: Annual variations in precipitation and groundwater recharge in borehole MW8 (2012–

2017) ............................................................................................................................................. 98

Figure 4.17: Annual variations in precipitation and groundwater recharge in borehole NMW8S

(2012–2017) .................................................................................................................................. 98

Figure 4.18: Annual variations in precipitation and groundwater recharge in borehole NMW10S

(2012–2017) .................................................................................................................................. 99

Figure 4.19: Annual variations in precipitation and groundwater recharge in borehole NMW12S

(2012–2017) .................................................................................................................................. 99

University of Ghana http://ugspace.ug.edu.gh

LIST OF TABLES

Table 3.1: Sampled boreholes and their coordinates .................................................................... 59

Table 4.1: Groundwater recharge estimates from RIB model ...................................................... 78

Table 4.2: The Spearman correlation coefficients between rainfall and observed WLF Borehole

Monthly scale ................................................................................................................................ 79

Table 4.3: Summary of inputs of the sensitivity analysis of groundwater recharge calculated with

the RIB model on a monthly basis ................................................................................................ 81

Table 4.4: Annual precipitation (mm), annual level variation (m), net annual level rise (used to

calculate recharge) (m), net annual level decline (m), and recharges (%P) for Sy = 0.05 for borehole

NW8 .............................................................................................................................................. 92

Table 4.5: Annual precipitation (mm), annual level variation (m), net annual level rise (used to

calculate recharge) (m), net annual level decline (m), and recharges (%P) for Sy = 0.05 for borehole

NMW8S ........................................................................................................................................ 93

Table 4.6: Annual precipitation (mm), annual level variation (m), net annual level rise (used to

calculate recharge) (m), net annual level decline (m), and recharges (%P) for Sy = 0.05 for borehole

NMW10S ...................................................................................................................................... 94

Table 4.7: Annual precipitation (mm), annual level variation (m), net annual level rise (used to

calculate recharge) (m), net annual level decline (m), and recharges (%P) for Sy = 0.05 for borehole

NMW12S ...................................................................................................................................... 95

Table 4.8: Annual precipitation (mm), annual level variation (m), net annual level rise (used to

calculate recharge) (m), net annual level decline (m), and recharges (%P) for Sy = 0.05 fore

borehole NMW17S ....................................................................................................................... 96

Table 4.9: Groundwater recharge from WTF method ................................................................ 102

University of Ghana http://ugspace.ug.edu.gh

CHAPTER ONE

INTRODUCTION

1.1 Background

Groundwater recharge is a crucial step in replenishing groundwater resources. Groundwater has

become increasingly constrained over time as a result of rising population, water shortages, change

in climate, inadequate water service delivery, and degradation of surface water quality, particularly

in semi-arid rural areas (Nemaxwi et al., 2019). Groundwater is a more potentially reliable source

of water than surface water since surface water is more prone to pollution than groundwater

(Banoeng-Yakubo et al., 2005; Duriez, 2005; Ghasemizadeh, 2015; Nemaxwi et al., 2019). The

groundwater recharge rate is one of the most important factors in semi-arid areas for sustaining

long-term groundwater use, making rational groundwater allocation decisions, and establishing

successful water and environmental management strategies.

Estimating recharge is critical in any groundwater system analysis and evaluating the effects of

abstractions for various uses (Sophocleous 2005). Estimating groundwater recharge is a major

challenge for assessing sustainable groundwater development and management, particularly in

arid and semi-arid regions where rainfall and recharge are low while evapotranspiration is

considerable (Sun et al., 2013). Groundwater modelers have faced major challenges due to the

complications of aquifer geology and the unpredictability associated with the meteorological data

of a given area (Ahmadi et al., 2012; Ghafari et al., 2018; Sharda et al., 2006).

Ghana has abundant groundwater resources (Johnston and McCartney, 2010). Many towns and

villages in Ghana have adopted groundwater as their primary source of water since it is both a

practical and economically viable source of water (Nsiah et al., 2018). Aside from the benefits to

University of Ghana http://ugspace.ug.edu.gh

society's economic standing and human health, the advantages of using groundwater are numerous.

To name a few, its high quality in terms of chemical and bacteriological content, its ability to

survive under climatic constraints, its readiness for use referring to the low treatment cost as

compared to surface water, and the ease of installing hand pumps in remote places (Lutz et al.,

2014). According to estimates, potable surface water can cost up to twice as much as water derived

from aquifers in areas with less than 5,000 residents (Krautstrunk, 2012). While many

communities in Ghana and across Africa already have access to potable water, population growth

and the Millennium Development Goals necessitate ever-increasing access (Lutz et al., 2014). As

a result, hand-pump wells are in limited supply.

Despite this, research demonstrates that change in climate will have a global impact on

groundwater resources (Earman and Dettinger 2011; Epting et al., 2020; Wu et al., 2020).

According to Kankam-Yeboah et al., (2009), Ghana will become water-stressed by 2025, even if

climate change and its repercussions are not considered. Change in climate will exacerbate the

problem by creating a 5-22 percent decline in groundwater recharge in 2020 and a 30-40 percent

decrease in 2050 (Kwoyiga and Stefan 2019).

The Akyem area, which comprises unique hydrogeology, abounds in natural resources, mainly

minerals and forest products, is located in the Eastern region of Ghana (Owusu, 2012). Granite

and upper and lower Birimian rock formations comprising phyllite, schist, greywacke,

metavolcanic, and quartzes underlie a large portion of the area. These rocks have a lot of potential

for extracting groundwater (Birim North District, 2006; Nartey et al., 2011). It is part of the Birim

North District and has a history of widespread artisanal gold mining activities in and along river

systems. As a result, water from most rivers has become unsuitable for human consumption,

requiring most communities to rely on wells or boreholes for water supplies (Attiogbe and

University of Ghana http://ugspace.ug.edu.gh

Nkansah, 2015; Attua et al., 2005; Ghana Business News, 13 March 2010). Thus, groundwater is

the primary source of water in these communities (Frimpong et al., 2016). According to Asante

and Asare (2008), the demand for adequate water to satisfy the ever-increasing domestic,

industrial, and agricultural needs is increasing in the Birim North District. In addition to domestic

water abstractions, mine dewatering activities and the impacts of unregulated, illegal surface

mining activities have been threats to the sustainability of groundwater resources, especially in the

shallow unconfined saprolite aquifer system in the area. Therefore, there is the need to assess the

current levels of groundwater recharge to provide data to assist in resource governance and

sustainable management.

Several hydrological and hydrogeological studies have been conducted in the research region. For

instance, Attua et al., (2005) evaluated the water quality of rivers used as drinking sources in

artisanal gold mining settlements in the Akyem-Abuakwa area using a multivariate statistical

technique. Acceptable, reasonably good, bad, and seriously polluted are the four degrees of water

quality discovered using cluster analysis. Only the Subri and Kadee rivers have been recognized

as "safe" to drink. Discriminant analysis indicated that turbidity, arsenic, temperature, phosphate-

phosphorus, and total dissolved solids are the most important variables for identifying the drinking

water quality of river sources. Five components with eigenvalues larger than unity were found in

the principal component analysis, accounting for more than 80% of the variation in water quality.

More than 26% of the variation was due to artisanal gold mining, with arsenic and mercury being

the primary pollutants in the area. Agricultural operations and the disposal of residential garbage

are two further causes of contamination.

University of Ghana http://ugspace.ug.edu.gh

Nartey et al., (2011) measured contamination levels of mercury in rivers and streams near small-

scale gold mining locations in Ghana's Birim North District. The overall concentrations of mercury

recorded upstream were substantially lesser than the measured downstream amounts, according to

the findings. In addition, the contents of mercury in the water samples measured in both seasons

surpassed the WHO drinking water guideline level. During the dry season, however, one

downstream total mercury content was beyond the recommended levels. In both seasons, the

overall contents of mercury in sediments upstream and downstream surpassed the US-EPA

guideline threshold of 0.2 mg/kg. In both seasons, the overall contents of mercury in wells in the

research area exceeded the WHO recommended limit. In the wet season, the number of mercury

contents in boreholes had lesser concentrations lower than in the dry season.

Frimpong et al., (2016) studied the assessment of groundwater quality in the Akyem Mine Area of

Newmont Golden Ridge Ltd (NGRL). The study assessed the groundwater quality in the area and

determined if there were significant changes in parameters, possibly due to activities of the mine

and other anthropogenic activities. Results and analysis indicated that water from both shallow

and deep wells is either neutral or weakly acidic and is as well within the WHO acceptable range

for color and TDS. The study found that though there was external influence responsible for

significant variations in TDS and pH values for shallow and deep boreholes, the influence has not

been significant enough to cause parameters to exceed WHO guidelines. The following mining-

related metal contaminants were below the detection limit and thus posed no health risk to the

consumption of water from both shallow and deep wells. Hence, based on their research, there

have not been significant impacts of the parameters under consideration on groundwater quality in

the Akyem Mine area.

University of Ghana http://ugspace.ug.edu.gh

Although various studies have been conducted in the study area, there has been no previous

research on groundwater recharge estimation, according to the literature in the area. There are no

comprehensive recharge investigations for this specific location, either published or unpublished.

There is, therefore, a need to provide a reliable estimation of the groundwater recharge to manage

the groundwater resources. Furthermore, the geographical distribution of the rate of recharge is

important for groundwater management and analysis. Infiltration of precipitation is the primary

source of recharge to groundwater in the model area (AMEC Geomatrix, 2010). Hence

understanding the groundwater recharge response to rainfall events is critical to understanding the

catchment's aquifer system and its dynamics.

In African countries, several methods for groundwater recharge estimation have been employed

with varying degrees of accuracy in recent decades (Beekman and Xu, 2003; Sun et al., 2013). The

results of using these methods revealed that when alternative techniques and record data sets were

employed, groundwater recharge estimates offered by several professionals differed substantially.

Most groundwater recharge approaches have been designed for large-scale applications, while

little information is available describing processes at the local scale (Sun et al., 2013). Various

studies have been conducted to estimate rainfall-induced groundwater recharge (Baalousha 2005;

Szilagyi et al., 2011; Tshelane et al., 2014). In spite of the numerous studies found in the literature,

uncertainties in parameter estimation continue to be a concern due to the variety of factors that

influence the rate of recharge (Liggett and Allen, 2009). Qablawi (2016) presented an analysis of

four groundwater recharge estimation methods, as well as an assessment of their accuracy and

applicability. The soil water balance, Chaturvedi formula, seasonal recession method (Meyboom

method), and well-level data were the approaches used. Because the climate in the study area was

sub-humid continental, the results of the Chaturvedi formula were less dependable than those of

University of Ghana http://ugspace.ug.edu.gh

the soil water balance and the well-level data method. The Meyboom method's conclusions were

also unreliable for two reasons: the approach predicts average groundwater recharge over a five-

year period, and it cannot calculate a negative figure. There were also uncertainties with the soil

water balance and the well-level data methods. Due to a paucity of evaporation data in some places,

these methods may be difficult to apply. The computation of recharge may be influenced by the

method employed to compute evaporation.

Groundwater is very vital in arid and semi-arid environments than in humid areas because they

receive a small amount of rainfall. After the occurrence of rainfall in a dry season environment,

the element of recharge is the utmost significant aspect. However, it cannot be measured directly,

resulting in a scarcity of data (Majdabady et al., 2020). As a result, most approaches for estimating

groundwater recharge do not appear to be totally appropriate due to the challenges associated with

quantifying groundwater recharge. As a result, the rainfall infiltration breakthrough (RIB)

technique was composed because it is a simple, cost-effective, and easy-to-use tool for estimating

groundwater recharge (Ahmadi et al., 2014; Majdabady et al., 2020; Sun et al., 2013).

This study sought to estimate the rate of groundwater recharge that develops from rainfall by using

rainfall infiltration breakthrough (RIB) to support water resources decision-making, which would

be essential for sustainable water development and groundwater modeling in the study area.

University of Ghana http://ugspace.ug.edu.gh

1.2 Problem Statement

The Birim North district is one of the country's forested areas. Parts of these forests have been set

aside to secure the long-term use of natural resources, culminating in the establishment of nine

forest reserves to preserve a portion of the district's original vegetation (Attiogbe and Nkansah,

2015). However, a variety of variables have played a significant role in the district's forest cover

decline. Poor farming methods, deforestation (particularly the actions of unlicensed chainsaw

operators), bushfires, and reckless mining activities are among the most prominent of these.

According to Attiogbe and Nkansah (2015), these human activities have caused the Pra River's

headwaters to dry down during the dry season, especially along its banks. This usually results in

long-term water shortages in areas that rely on it for their water supply. If mining activities damage

these water resources, the problem may escalate (Birim North District, 2006).

Over the years, the growing use of groundwater and the resultant increased demand on limited

groundwater supplies as a result of the increased usage of these aquifers as a source of water for

domestic, commercial, and irrigation purposes have led to the introduction of mechanized pumps.

Most of the surface watercourses in the district, including the Pra River and Birim River, have

been contaminated by small-scale mining and anthropogenic activities (Frimpong et al., 2016).

Due to the pollution of the surface water, Newmont Golden Ridge Ltd (NGRL) and the

Government have provided the surrounding towns/villages with boreholes fitted with hand pumps.

Thus, borehole and hand-dug wells represent the primary source of freshwater for residents in

these communities (Frimpong et al., 2016). Despite the paucity and insufficiency of drinkable

water in several places due to contamination of surface water bodies, the underground water

reserve is rich (Nartey et al., 2011). Little is known or documented concerning the physical

hydrological parameters of the Birim Basin. Thus, quantifiable parameters that are required to

University of Ghana http://ugspace.ug.edu.gh

conduct a mass water balance analysis that could provide further sustainability investigations in

this area are restricted. The long-term viability of water resources in this region is key to sustaining

the lives of the communities in this area. The goal of this research was to create a modified

groundwater recharge estimates for the Birim Basin using the rainfall infiltration breakthrough

model. Monthly recharge estimates utilizing the modified RIB program and current data in the

research area are provided, accompanied by sensitivity analysis. The effects on groundwater levels

and the expected monthly recharge rate are investigated using scenarios analysis of data from the

research region and varying precipitation inputs.

1.3 Objectives

The main objective of the study was to develop an improved groundwater recharge estimate from

rainfall and groundwater level data in the Birimian Province in Southwestern Ghana.

The specific objectives include to:

➢ Adopt and modify a methodology for estimating groundwater recharge using rainfall and

groundwater level data;

➢ Estimate groundwater recharge using RIB and WTF methods and compare findings;

➢ Determine the relationship between rainfall and groundwater recharge in order to

determine the response of groundwater recharge to rainfall events;

➢ Evaluate various scenarios of climatic variability on the sustainability of the shallow

unconfined saprolite aquifers in the study area.

University of Ghana http://ugspace.ug.edu.gh

1.4 Study Area

1.4.1 Location

The Birim North District was established in 1987 by the former Birim District Council as part of

the government's decentralization effort to promote effective localized administration and speed

up development in the area. It is located in the Eastern Region and is bounded to the north by

Kwahu West, to the west by the Ashanti Region's Asante Akyem South, Amansie East, and Adansi

South Districts to the south by Birim South District, and to the east by Atiwa and Kwaebibirem

Districts. The region, particularly its headquarters, New Abirem, is strategically located near major

commercial towns such as Nkawkaw, Oda, and Kade (Figure 1.1).

University of Ghana http://ugspace.ug.edu.gh

Figure 1.1: Map of Birim North District Assembly

University of Ghana http://ugspace.ug.edu.gh

The district's economy stands a better chance of improving with improved road conditions

connecting it to key commercial areas. New Abirem is a nodal or confluence town since it is

positioned at the intersection of the Nkawkaw-Oda-Kade highways (Birim North District, 2006).

The area is located between latitudes 6.15o N and 6.35o N; and longitudes 0.20oW and 1.05oW

(Owusu, 2012). The district has an estimated total land area of 1,250 km2, accounting for roughly

6.47 percent of the Eastern Region's total land area.

1.4.2 Climate

The research area is located in a wet semi-equatorial climatic zone with a double maxima rainfall

pattern. The climate is tropical, with a daily temperature range from approximately 14.0 ˚C to 36.4

˚C, according to data collected by Clear Creek Consultants in 2009 (Clear Creek, 2010). The first

rainy season begins in late March and lasts until the beginning of July. The second season begins

in mid-August and runs until the end of October. The amount of rainfall received in the district is

between 1500 mm and 2000 mm per annum. Monthly average temperatures in the model area

range from approximately 24.3˚C in August to 26.6˚C in February and March (Clear Creek, 2010).

Monthly average relative humidity at the Akyem Project site range from approximately 66.9

percent in January to 88.8 percent in July (Clear Creek, 2010). The district's agricultural

operations, particularly the growth of food crops and tree plants like oil palm and cocoa, have

benefited from a large amount of rainfall and temperate temperatures.

University of Ghana http://ugspace.ug.edu.gh

1.4.3 Relief and Drainage

The district is primarily undulating and hilly in nature. Nature is undulating and mountainous,

rising to more than 61 meters above sea level in certain places. The highest peak in the district is

Kwasiakwasi Mountain (N 06°28'09.6" & W 00°54'44.3"), which is found in the Kwasiakwasi

Forest Reserve and climbs to a height around 800 m above sea level. Streams such as Nyanoma,

Nkwasua, and Aprokuma originate in the Kwasiakwasi Forest Reserve.

The territory is primarily drained by two rivers, the Pra and its tributary, the Birim. These rivers'

tributaries in the district include the Nwi, Suten, Mamang, Adechensu, Sukrang, Nkwas ua,

Nyanoma, and Afosu. These rivers flow from the northeast to the southwest, eventually joining

the Pra, which flows south and joins the sea near Shama in the Western Region (Figure 1.2). The

Pra River forms the western boundary between the district and the three districts of the Ashanti

Region. The Birim River also acts as the district's southern boundary (Attua et al., 2005; Birim

North District, 2006).

University of Ghana http://ugspace.ug.edu.gh

Figure 1.2: Drainage system of the study area

1.4.4 Vegetation and Soil

The district is located in Ghana's semi-deciduous forest area, which is made up of tall trees with

evergreen undergrowth. It features a considerable number of economically valuable trees for the

lumber sector. There are nine forest reserves in the area. It has much undergrowth, but due to the

rapid expansion of the cocoa and oil palm businesses, illegal chain saw operators' activities, and

frequent bushfires, the original forest is quickly becoming a secondary type (Birim North District,

2006).

University of Ghana http://ugspace.ug.edu.gh

The district's soils can be classified into five major groups. Swedru-Nsaba/Ofin Compound

Association, Atiwa-Atukrom-Asikuma-Ansum Compound Association, Juaso-Manso-Debia

Association, Bekwai-Oda Association, and Birim-Chichiwere Association are among them (Antwi

2010; Birim North District, 2006).

The major soil formation is the Swedru-Nsaba Ofin Compound Association. They are formed over

granite. They can be found in the areas of Prankese, Nkwateng, Otwereso, and Abenase. This

compound association is made up of two simple associations: Swedru-Nsaba and Nta-Ofin. Nta

Offin evolved from the transported products of the former Swedru series. The Swedru Nsaba

series, which is heavy in magnesia and potash, is excellent soil for tree and arable crops, but

especially for cocoa. Because ofin soils are unsuited for tree crops, they are primarily used for

cultivating dry season vegetables, sweet potatoes, sugarcane, and rice (Birim North District, 2006).

The soil in the Atiwa–Atukrom–Asikuma–Ansum compound series is restricted to a small portion

of the district near Amuana Praso. Topsoils are dark brown, somewhat humus, silty, clay, and

loam, with subsoils that are reddish-brown to red soft clay loam. Because of their high acidity and

low base status, these two soil series are infertile. They are suggested for coffee, oil palm, other

tree crops, and forestry (Birim North District, 2006).

The Juaso-Manso-Adubea Association is located near Noyem, Prasokuma, and Atobiaso. It is dark

brown and shallow, and it helps with oil palm output. Bekwai- Oda Association is located in the

vicinity of New Abirem, Ntronang. They are well-drained and suited for growing a wide range of

tree and arable crops, including cocoa, coffee, citrus, oil palm, avocado pear, mangoes, yams,

maize, cassava, and plantain. The Oda series also inhabits flat, rather large regions next to rivers

and streams. They are ideal for mechanized irrigated rice farming (Birim North District, 2006).

University of Ghana http://ugspace.ug.edu.gh

The Birim-Chichiwere Association was discovered at Edubia. It was built on the River Birim

deposits. It is moderately well-drained, deep, and easy to use with machines; it occurs on almost

flat areas with little or no erosion, and it is appropriate for a wide range of tree and arable crops.

Chichiwere is thought to be harmful to tree crops (Birim North District, 2006).

1.4.5 Geology and hydrogeology

Akyem is underlain by the Birimian metasedimentary and metavolcanic rock units (Figure 1.3).

These include northeast-trending belts of folded, metamorphosed volcanic and sedimentary rocks

of the early Proterozoic, which underlie the model area. The southeast side of the model area is

underlain by rocks of the Birimian Supergroup. In this region, the Birimian terrain comprises

northeast-trending belts of volcanic and volcanoclastic material separated by broad turbidite-

dominated sedimentary basins. Tarkwaian sediments unconformably overlay the Birimian

volcanic belts in the northwestern portion of the model area. The Birimian rocks consist of black

phyllites, metasiltstones, metagreywackes, tuffaceous sediments, tuffs, and hornstones (Kesse,

1985). Tarkwaian sediments consist of conglomerates, sandstones, and phyllites. Structural

features of the Birimian and Tarkwaian units display a strong northeast-southwest trend (GRRL

2005 and 2006; Ireland et al., 2001). The sandstone (quartzite) consists of variable amounts of

feldspar, sericite, chlorite, ferriferous carbonate, magnetite, or hematite and epidote. Locally, the

Akyem deposit is a shear zone hosted gold deposit almost at the contact between Birimian

metasedimentary and metavolcanic rock. Mineralization is chiefly in the metasedimentary

separated by graphitic shear. The deposit strike 070° and dips 60° to 65° SSE (Atule, 2007).

University of Ghana http://ugspace.ug.edu.gh

Figure 1.3: Geological map of the study area

University of Ghana http://ugspace.ug.edu.gh

In-situ weathering of bedrock defines the hydrostratigraphy in the model area. Weathering of

bedrock to tens of meters depth is caused by the tropical climate. The pace at which rocks weather

is affected by rainfall and temperature. Chemical weathering is accelerated by higher temperatures

and more rainfall. Highly weathered bedrock (saprolite), moderately weathered bedrock (saprock),

and fresh rock (bedrock) are the three basic hydrostratigraphic units. Localized zones of increased

permeability have emerged from structural characteristics in bedrock and quartz veining. In the

flood plains of streams and rivers, alluvial deposits lie on top of saprolite. The following are brief

descriptions of four hydrostratigraphic units, as well as the hydraulic property values determined

during studies by AMEC Geomatrix (2010), Golder (2004 and 2006), and Lycopodium Knight

Piesold (2005).

(a) Alluvium

In the model area, alluvium is made up of fluvially deposited silt, sand, and gravel along streams

and rivers. Alluvial deposits' lateral dimensions and thicknesses are poorly defined. Limited

investigations conducted in support of the tailing storage facility along the Adenchensu river

indicate that the alluvial deposits are small, measuring less than 200 meters wide and 2 meters

thick (Lycopodium Knight Piesold 2005). Hydraulic conductivity of 1.6x10−3 centimeters per

second (cm/s) was found in an aquifer test conducted in alluvium (AMEC Geomatrix 2010).

(b) Saprolite

The Saprolite is a near-surface unit that, because of its clay composition, may resist infiltration,

but it lacks weathered quartz veins that might act as conduits for infiltration and groundwater flow

(Anon., 2010). Saprolite is made up of decomposing bedrock that has been weathered in place into

University of Ghana http://ugspace.ug.edu.gh

sand-silt-clay-sized particles with remaining quartz veins. The saprolite is usually covered by a

thin layer of lateritic soil that is 1 to 5 meters deep. The thickness of the saprolite-laterite unit is

usually less than 60 meters (Golder 2004). Topography affects the saturated thickness of the

saprolite unit. The saturation thickness of the saprolite near the proposed Akyem mine pit ranges

from 1 to 20 meters (Golder 2004). With topography, the saturated thickness of the saprolite unit

varies. The saturated thickness of the saprolite near the proposed Akyem mine pit ranges from 1

to 20 meters (Golder 2004). Saprolite has a hydraulic conductivity range of 3x10−7 to

3x10−4cm/s with a geometric mean of 8 x10-6 cm/s (Golder 2004 and Lycopodium Knight Piesold

2005). The saprolite's upper, less permeable part can act as a semi-confining layer for this

productive zone. In contrast, the lower, usually saturated part of the saprolite is characterized by

lower secondary clay content, thus creating a zone of enhanced hydraulic conductivity.

The Saprock, on the other hand, is relatively impermeable, allowing groundwater to be stored and

transmitted through fractures within the rock (Frimpong et al., 2016). Saprock is a type of bedrock

that has been moderate to lightly weathered. The transition between saprolite and fresh bedrock is

represented by this 1- to 20-meter-thick unit. The saturated thickness of the saprock around the

proposed Akyem mine pit varies from zero (unsaturated) in upland places to 5 to 10 meters in

lowland locations. Based on falling head tests and pumping tests, hydraulic conductivity

estimations range from 2x10−5 to 2x10−2cm/s with a geometric mean of 4 x 10−4cm/s (AMEC

Geomatrix 2010; GRRL 2005; Golder 2006,).

University of Ghana http://ugspace.ug.edu.gh

(d) Bedrock

Groundwater in the bedrock occurs under confined conditions and is the dominant water-bearing

associated with the fracture system. Greywacke and mafic volcanics are the most common bedrock

types found in the area. Variations in fracture density, as well as the magnitude and degree to

which fractures are interconnected, affect the hydraulic characteristics of bedrock aquifers (Anon.

2010). According to SGS's vertical electronic sounding surveys and dipole-dipole resistive

surveys, most fractures are localized between 30 and 45 meters below ground surface (bgs) (GRRL

2005). The hydraulic conductivity estimates from tests performed on wells completed in bedrock

range from 4.6 x 10−6 to 2x10−3 cm/s with a geometric mean of 1.2 x 10−4 cm/s due to the

variety of fracture conditions (Golder 2006 and 2004, and Lycopodium Knight Piesold 2006,

AMEC Geomatrix 2010).

Although the underlying matrix's general permeability is low, faults, shear zones, and fractures

linked with structural trends are the most common groundwater transmission structures. A

sequence of dykes that have been mapped perpendicular to the dominant structural trend may

produce local barriers to groundwater flow in the area, whereas these primary permeability features

may transmit relatively considerable amounts of water. Golder (2004) used packer testing in four

exploration boreholes, and AMEC Geomatrix(2010) used aquifer testing in four deep wells to

examine the hydraulic parameters of the Akyem fault and its associated structures. Hydraulic

conductivity values for the structural zone ranged from 3x10−6 to 3x10−4 cm/s with a geometric

mean of 1.5x10−5cm/s. Generally, borehole depths drilled through rocks of the Birimian and

Tarkwaian Systems range from 35 to 62 m, with an average of 42 m (Agyekum, 2004). Borehole

depths in granitoid-underlain areas are similar, ranging from 35 to 55 meters, with an average of

50 meters (Carrier et al., 2008). In some areas, the regolith is tapped at relatively shallow depths

University of Ghana http://ugspace.ug.edu.gh

using hand-dug wells. The Birimian and Tarkwaian Systems' productive zones have aquifer

transmissivity ranging from 0.2 m2 /d to 119 m2 /d, with an average of 7.4 m2 /d. Transmissivity

within the regolith is slightly higher than that observed in the integrated aquifer system, ranging

from 4 m2 /d to 40 m2 /d with an average of about 10 m2 /d (Carrier et al., 2008). Borehole yields

for the integrated aquifer systems in the Birimian and Tarkwaian Systems are generally low,

ranging from 0.48 m3 /h to 36.4 m3 /h with a mean yield of 7.6 m3 /h. The lower yields observed

in these rocks are most likely due to differences in the degree of weathering within the granitoid

(Yidana et al., 2008).

University of Ghana http://ugspace.ug.edu.gh

CHAPTER TWO

LITERATURE REVIEW

2.1 Introduction

This chapter gives a review of past research on groundwater recharge estimation, with a focus on

the use of water table fluctuation and rainfall infiltration breakthrough methodologies. It also

thoroughly discusses the recharge estimation models in groundwater studies and how concepts and

methods have been used in other jurisdictions to achieve similar results. The chapter scrutinizes

the methods that the literature suggests for groundwater recharge estimation and how they apply

in the current study aiming to highlight the most beneficial methodology for the study area.

2.2 Groundwater Recharge

One of the essential variables in hydrological studies is groundwater recharge, but it is often one

of the most difficult to estimate accurately. Groundwater can be recharged both by precipitation

and/or surface water sources such as rivers and lakes infiltrating into the soil and rock layers of

the ground (Bhattacharya et al., 2003). When assessing groundwater recharge, there is a need to

distinguish between potential and actual recharge. Soil water percolating beneath the root system

is a possible recharge; thus, soil water entering the aquifer is the actual recharge (Petersen, 2012).

Most of the potential water for recharge would be retained at negative pressure (suction) in the

vadose zone and is ineligible to be exploited. Instead, the actual recharge is the volume of water

that actually makes it to the groundwater table and can be used (Sophocleous, 2004). Groundwater

recharge may be categorized as (1) direct or indirect by reference to the origin of the recharging

water, (2) piston or preferential flow by reference to the flow phase through the unsaturated region,

University of Ghana http://ugspace.ug.edu.gh

(3) point, line, or areal recharge by reference to the area on which it operates, and (4) current, short

or long-term recharge by reference to the time scale over which it occurs (Boerner and Weaver,

2012; De Vries and Simmers 2002; Obuobie et al., 2012).

Groundwater recharge estimation is one of the vital issues in measuring the aquifer's sustainable

yield in dry and semi-arid environments, as recharge rates are typically low relative to mean yearly

rainfall and are therefore hard to estimate effectively (Xu and Beekman, 2003). There are several

factors that govern groundwater recharge. These include the rainfall rate and length, the soil

profile's previous moisture state, geology, soil properties, water table, and aquifer depth

characteristics, vegetation and land cultivation, topography, and landscape (Ng et al., 2009).

Albhaisi et al., (2013) noted that recharge has systematically risen over time as a result of

degradation in South Africa's Berg River watershed. This was after being speculated that due to

this shift in land use, evapotranspiration would decrease, and recharge would increase. In the soil,

recharge is calculated by its composition, thickness, linkage, uniformity, and hydraulic elements,

while the porosity of the aquifer determines the recharge magnitude.

2.3 Groundwater and climate change

Groundwater systems are influenced by climate change in a variety of ways. Climate change has

the potential to influence the amount of soil infiltration, deeper percolation, and groundwater

recharge in the hydrological cycle (Wu et al., 2020). Furthermore, increasing temperatures raise

evaporative demand over land, reducing the amount of water available for groundwater recharge

(Bates et al., 2008; Brielmann et al., 2009; Jesußek et al., 2013; Kipfer and Livingstone, 2008;

Kurylyk et al., 2014; Menberg et al., 2014; Possemiers et al., 2014). Conversely, anthropogenic

impacts on groundwater systems are primarily due to the pumping of groundwater and the indirect

University of Ghana http://ugspace.ug.edu.gh

impacts of land-use changes and irrigation (Wu et al., 2020). Groundwater demand is fast rising

in tandem with increasing population, while the change in climate puts extra strain on water

resources and increases the likelihood of extreme drought (Wu et al., 2020).

Variations in the Earth's climate have the ability to have an impact on groundwater quality and

quantity. There is a general consensus that the Earth's climate is changing and will continue to

change in the future due to the impacts of global warming in the air (Earman and Dettinger, 2011).

Future precipitation projections are not as consistent or reliable, but they often result in extremely

arid areas and extremely moist areas (Intergovernmental Panel on Climate Change, 2007).

According to Earman and Dettinger (2011), overall warming combined with unknown

precipitation changes is predicted to result in less freshwater availability in most situations than if

the precipitation changes were the only factor. That is, even if precipitation stays unchanged (or

slightly rises), warmer conditions will likely result in minimal recharge and runoff. Warming is

expected to amplify runoff and recharge reductions where precipitation declines.

Many studies of the effects of change in climate on surface-water resources make no reference to

groundwater and do not seem to account for groundwater contributions to streamflows in a

meaningful way. This method is based partly on the observation that changes in climate have a

greater impact on surface water than on groundwater, leading to the presumption that climate-

change impacts on groundwater are not really severe. The impacts of variations in climate on

groundwater systems are intrinsically unpredictable due to the uncertainty of future climate

projections. Climate-groundwater interactions are typically incompletely defined and modeled,

which contributes to ambiguity (Bates et al., 2008; Earman and Dettinger, 2011; Epting et al.,

2020; Huggenberger and Epting, 2011). Higher total precipitation increases the quantity of water

available for groundwater recharge at any particular place and may produce higher recharge devoid

University of Ghana http://ugspace.ug.edu.gh

of other effects. On the other hand, less precipitation is likely to result in less recharge (Earman

and Dettinger, 2011; Huggenberger and Epting, 2011).

The majority of research concentrates on entire catchment areas and solely looks at infiltrating

precipitation water changes (i.e., "precipitation-fed aquifers"), with the exception of a few

extensive assessments of extremely small aquifers (e.g., Malcolm and Soulsby (2000)). Similarly,

in a review of Climate Change (CC's) influences on groundwater recharge, for determining the

classification of groundwater resources, Smerdon (2017) emphasizes the need to know

groundwater recharge mechanisms, which includes location and timing. The researcher reviews

six scientific reports (Crosbie et al., 2013; Green et al., 2011; Kurylyk and MacQuarrie, 2013;

Meixner et al., 2016; Moeck et al., 2016; Taylor et al., 2013) that show how the lack of certainty

of future precipitation distribution and pattern from General Circulation Models (GCMs) results

in different recharge estimates, to the point that modeling studies are frequently unable to estimate

the degree and orientation (rise or fall) of prospective recharge situations.

2.3.1 Impacts of climate change on groundwater in Ghana

According to the Ghana National Climate Change Policy, climate change has caused a rise in

temperature and a reduction in average yearly rainfall in the nation's natural borders (Ministry of

Environment, Science and Technology, 2013).

In Ghana, attempts to comprehend climate variability and its influence on groundwater systems

are inadequate, and there are ambiguities, as there are with rainfall, because most existing research

is projection-based. Climate change and its effects on groundwater, on the other hand, have been

a major cause of worry. The Volta Basin Authority conducted a transboundary diagnostic

investigation, which found that climate change and its consequences are one of the Volta Basin's

University of Ghana http://ugspace.ug.edu.gh

primary issues (covering parts of Ghana) (Mul et al., 2015). This supports Oyebande and

Odunuga's (2010) conclusions that in the dry and semi-dry sectors of small areas, the Niger and

Volta Basins will see a fall in streamflow and groundwater recharge as aquifer recharge has already

dropped considerably. There is also a chance that the groundwater level will drop as a result of the

reduced recharge.

According to the Council for Scientific and Industrial Research-Water Research Institute (CSIR-

WRI) research on climate variability and water systems in Ghana published in 2000, climate

change may lead to a broad reduction in groundwater recharge of 5-22% in 2020 and 30-40% in

2050 (Kankam-Yeboah et al., 2009).

In the Volta Basin, McCartney et al., (2012) used the Soil and Water Assessment Tool (SWAT)

and Water Evaluation and Planning (WEAP) techniques to investigate the effects of climate

variability on water systems. According to their research, the yearly mean rainfall, runoff, and

average groundwater recharge would all decline by 2050. Under the A1B scenario, for example,

groundwater recharge and mean yearly discharge will have been reduced by 53 percent and 45

percent by the end of the century, respectively.

There is, however, one aspect where these findings diverge. In a study, Obuobie (2010) estimated

groundwater recharge in the White Volta River Basin, West Africa, in the perspective of future

climate change and discovered that yearly recharge to the basin's groundwater, which is currently

around 7% of annual rainfall, could increase to around 33% in the future for the period 2030-2039.

University of Ghana http://ugspace.ug.edu.gh

2.4 Groundwater Recharge Estimation Methods

Estimating groundwater recharge is critical for the effective and lengthy management of

groundwater systems. Due to the difficulty of measuring groundwater recharge directly, a number

of groundwater recharge estimation systems have been developed, ranging from physical methods

to modeling approaches (Carrier et al., 2008; Scanlon et al., 2002). Since information is always

inadequate and situations change over time and distance, most researchers believe that recharge

estimation is effectively done in stages (Atta-Darkwa et al., 2013; Mare'chal et al., 2006).

Methodologies based on surface water and unsaturated-zone data offer calculations of possible

recharge, while those relying on groundwater data offer estimates of actual recharge, according to

Scanlon et al., (2002). As a result, it is common practice to estimate recharge using numerous

approaches in order to improve the accuracy of recharge estimates (Healy and Cook, 2002; Scanlon

et al., 2002).

Some of the recharge rate estimation methods and techniques used are chloride mass balance, soil

moisture balance technique, water table fluctuation, environmental tracer technique, and

hydrological modeling.

2.4.1 Chloride Mass Balance (CMB) Techniques

The Chloride Mass Balance (CMB) is a very useful tool for calculating recharge in dry and semi-

arid regions due to the difficulty encountered in using conventional water balance methods

(Allison et al., 1994). It is one of the broadly applied approaches in water resources development

and management (Brunner et al., 2004; Martin 2006; Obuobie 2008; Sandwidi 2007). The premise

behind this technique is the fact that the chloride ion has a conservative nature and does not easily

University of Ghana http://ugspace.ug.edu.gh

react with other elements in water when traveling through the unsaturated zone to the saturated

zone (Marei et al., 2010; Zagana et al., 2007). It offers a direct estimate of recharge and is the least

expensive with low-time integrating properties. The CMB technique is easy to use and does not

necessitate the use of any complicated equipment. However, if used alone, the approach may not

provide a reliable representation of recharge rates. When evaluating the results, the method's

uncertainties and assumptions must be taken into account. The Chloride Mass Balance method

for estimating groundwater recharge depends on several assumptions; The following are the

assumptions that must be met in order for the approach to be applied successfully:

➢ Apart from precipitation, there is no alternative origin of chloride in groundwater.

➢ In the system, chloride has a conservative nature, which means that it does not leach from

or cannot be taken by aquifer materials and is not involved in any chemical processes.

➢ With regard to long-term precipitation and chloride content in the precipitation, steady-

state situations are sustained.

➢ Surface run-on and runoff are negligible (Zhu et al., 2003; Marei et al., 2010).

➢ The depth of the groundwater table ought to be sufficient to avoid the vaporization of

groundwater (Zhu et al., 2003; Marei et al., 2010).

➢ Within the basin, there is no chloride recycling.

By using this method, recharge in mm/year (R) can be estimated by equation (1) as described by

Marie et al., (2010);

𝑅 =
𝐶𝑙(𝑝)

𝐶𝑙(𝑔𝑤)
× 𝑃………………………………………………………………………………….2.1

Where R, Cl(p), Cl(gw), and P are respectively recharge rate, chloride content in precipitation,

chloride content in groundwater, and average annual precipitation.

University of Ghana http://ugspace.ug.edu.gh

Consequently, in places where significant amounts of the groundwater chloride originate from

mineral dissolution processes or are affected by reactions that reduce its amounts in groundwater,

the method can be inappropriate and lead to underestimation of recharge. The approach cannot be

used in regions where evaporates underlie the areas or where saline water is evaporating or mixing.

Its application in fractured rock systems is complicated because extra chloride is created by the

breakdown in a groundmass of the rock; time is required to establish a new balance between the

rock matrix and cracks; and if additional chloride is made, CMB recharge rates are thought to be

the bare minimum. The results could be influenced by wind recycling of dried salt, unrecorded

discharge, and assimilation by harvested plants.

Several researchers around the world have adopted the CMB approach to predict groundwater

recharge with varying degrees of success: Obuobie (2008), for example, employed chloride mass

balance to estimate recharge in Ghana's Upper East Region; the latter study's estimates were

significantly higher than the former study's. These results revealed the method's uncertainty and

highlighted that there is no single extensive estimation method that can be applied at regional

scales.

In the same area, Obuobie et al., (2010) again applied the chloride mass balance (CMB) method

in the Upper East region of Ghana to calculate the recharge to groundwater aquifers. Furthermore,

the recharge is linked with water demand in order to determine the groundwater's potential for

enhancing water supply. Their findings indicated that groundwater recharge accounted for 3 to 19

percent of average annual rainfall, indicating a large groundwater potential, as usage in the research

area was estimated to be a little more than 14 percent of recharge. However, their research revealed

that long-term mean chloride contents in rainfall could not be calculated due to a lack of data for

University of Ghana http://ugspace.ug.edu.gh

the study area. Mean monthly chloride readings recorded in 2006 were utilized as the basis for the

study.

Earlier on, the Chloride Mass Balance method, together with the Soil Balance technique, was used

by Carrier et al., (2008) to estimate groundwater recharge in parts of the Voltaian in Northern

Ghana. The results of their estimates indicated that recharge ranges from 1.5% to 11.2% of mean

annual rainfall. These variations in the estimated recharge rate indicated variability of geological

characteristics within the unsaturated zone and the climatic heterogeneity in the study area. The

research indicated that the assumption concerning the origin of Cl might contribute to errors in

recharge estimate as the Cl data for rainfall was only available for a short period. Consequently,

there is a large uncertainty for Cl in rainfall, which obviously translates into large uncertainty in

recharge. As only major events were sampled, the values presented in the study are likely to be

underestimated (by at least 10-15 %, if the Cl detection limit is used for minor non-sampled

events). However, the comparison of Cl concentration values obtained here with those of another

study available for northern Ghana (Martin, 2006) reveals Cl concentrations of similar magnitude,

which would suggest that Cl values estimated here are representative. To reduce the inaccuracy

related to Cl flow from run-on/runoff, field profiles were carefully placed to avoid steep slopes or

depressions.

In portions of the Voltaian Basin in Northern Ghana, estimates of groundwater recharge using the

CMB method by Yidana and Koffie (2014) suggested recharge in the range of 1.8 to 32% of the

yearly precipitation and attributed the maximum estimates of recharge to localities with open wells

which act in encouraging rapid groundwater recharge. According to their findings, the most major

problem in using the CMB technique to estimate groundwater recharge is that other substantial

origins of chloride in the zone saturation and unsaturation might not be effectively measured,

University of Ghana http://ugspace.ug.edu.gh

resulting in inaccurate estimations of precipitation's actual contribution to groundwater chloride.

Another issue with the CMB technique, they stated, was the likelihood of fluctuations in

atmospheric chloride concentration that could cause the chloride amounts in the air to differ

significantly from what existed during the recharge time being calculated. Wells and other

groundwater outlets analyzed in the study area were settled in sandstones and the weathered zone,

where chloride was not seen to be a dominant ion.

In a similar project, Mensah et al., (2014) used the Chloride Mass Balance approach to examine

the representativeness of groundwater recharge in parts of Ghana's White Volta Basin. The

chloride mass balance (CMB) technique, according to their findings, works well in a tropical

climate in terms of obtaining reasonably accurate groundwater recharge estimates for groundwater

resource appraisal. The results of this study are consistent with those acquired through

mathematical model calibration and investigations of infiltrating rainfall evaporation rates.

According to the CMB's predicted groundwater recharge, the shallow aquifer system in the area

receives 0.9 percent to more than 21 percent of annual precipitation. This level of groundwater

recharge indicates that commercially sustainable groundwater development in the region has a

bright future. The analysis also found that recent derived atmospheric water, which might have

been isotopically enhanced in the weightier isotopes of the components of the water molecule

during the recharge process, is the source of groundwater recharge in the terrain. Other tracers

should be utilized to ensure the reliability of the results anytime the CMB approach is used in

groundwater recharge calculations, according to this study.

Adams et al., (2004) published a report on the groundwater recharge evaluation of the Central

Namaqualand basement aquifers in South Africa. The CMB approach underrepresented alluvial

aquifer recharge related to transitory rivers (i.e., Buffels River Town and Rooifontein). Extra

University of Ghana http://ugspace.ug.edu.gh

chloride input from run-on during recharge times could explain this. Their analysis revealed that

the CMB method's calculation did not have chloride data from run-on. Except for the Leliefontein

specimen, which did not match the age expectation and could be due to a sampling or scientific

mistake, the recharge rates correlated with the 14C ages from separate boreholes. They came to the

conclusion that the alluvial aquifer recharge rates were lesser than projected. The unaccounted

chloride in the surface runoff flux was the cause of this.

In a similar climatic zone, Nyagwambo conducted a study in a crystalline aquifer in the tropics of

Zimbabwe in 2006 using a similar methodology. The study concluded that the strategies produced

a varied recharge, around 8% and 15% of yearly rainfall. The study found that all of the recharge

estimating techniques utilized in the research had the flaw of relying too much on one crucial

variable, such as chloride deposited for the CMB technique.

In nations other than Africa, Hay (1997) investigated groundwater recharge on the western edge

of the southern Bridger Range in Montana, and the technique was applied in a more humid location

of the intermountain west where streamflow from the mountain front is considerable. Rather than

springs or mountain front wells, Hay looked at chloride concentrations in groundwater using

groundwater boreholes filtered at varying depths over the valley floor. Rather than recharge

occurring at the mountain front via diffuse surface flow or bedrock flow, the valley aquifer was

projected to be recharged by streamflow from the mountain front and precipitation falling directly

on the broad valley floor below the mountain front. As a result, the CMB equation's input

parameter incorporates both precipitation and streamflow. The flow of several hill-front streams

shown in the study was measured weekly at the mountain front and used to determine stream

inflow into the region. Two streams were tested for chloride concentrations for use in the CMB

calculation on two successive days in July, beginning in the three-year course of the study. When

University of Ghana http://ugspace.ug.edu.gh

these measurements were collected, the streams were barely recovering to base flow, according to

the author. The average mean annual values measured by the National Atmospheric Deposition

Program at Clancy, Montana, over the ten years preceding the study were used to calculate the

concentration of chloride in precipitation. Hay's CMB estimates are sensitive to chloride

concentrations in groundwater and warn about the consequences of inter-annual variability in

precipitation input and spring snowmelt timing, implying that alternating warm and cool

temperatures reduce peak discharges and allow maximum water infiltration into the subsurface.

Contrary to the above-mentioned Nevada and Montana research, Russel and Minor (2002) used

chloride concentrations of groundwater recorded from successive elevation springs inside a

mountain block to calculate CMB for large alluvial or lake sediment-filled catchments. The

altitude-reliant CMB application is based on the concept of changing recharge rates along the

mountain front as a function of elevation. Russel and Minor's analyses revealed a nonlinear

relationship between reducing spring elevation and increased ratios of spring water chloride

concentrations to average precipitation chloride concentration, with lower elevations experiencing

larger improvements in relative spring water chloride concentration. This association, according

to the scientists, shows that recharge occurs within the mountain block at geographically varied,

elevation-dependent rates. Furthermore, recharge occurs within a given watershed as a result of

numerous characteristics such as slope, aspect vegetation, and various elevation-dependent

parameters. From these analyses, the authors obtain CMB estimates of mountain block recharge

to valley aquifers, not as a single calculation for the mountain block, as Dettinger did, but as

elevation-dependent, area-weighted recharge calculations. The chloride concentration of

groundwater obtained from springs along the mountain front, according to Russel and Minor

(2002), is the most vulnerable to error, with estimates of average precipitation coming in second.

University of Ghana http://ugspace.ug.edu.gh

Aishlin (2006) conducted groundwater recharge estimates via CMB application at multiple

watershed scales within the Dry Creek Experimental Catchment in a similar project to determine

the fraction of yearly precipitation that is allocated for recharge, as well as the geographical

diversity within the recharge. Incorporating streamflow discharge into the CMB calculation

validates the recharge estimations as net groundwater recharge rates, indicating the amount of

water accessible for groundwater flow pathways in deeper mountain blocks. Annual precipitation

partitioned to net groundwater recharge was estimated to be zero to eleven percent for the entire

watershed from July 2004 to June 2005. Nevertheless, usage at many drainage scales within the

Dry Creek Experimental Watershed revealed that over the same period, 22 percent of yearly

precipitation was divided into total groundwater recharge into higher altitude sub-catchments. The

data from the second research period, from July 2005 to June 2006, were generally deemed

inaccurate due to the mobilization of inter-yearly deposited unsaturated zone chloride. Using time-

series data from spring and stream chloride concentrations applied to hydrograph separation, the

time it takes for chloride to be mobilized in the unsaturated zone, as well as lateral and vertical

migration toward bedrock infiltration and stream channels, were determined. Furthermore,

gain/loss analysis utilizing time-series data from stream chloride concentrations shows evidence

of streamflow loss to groundwater recharge. The disparities between the water years 2004-2005

and 2005-2006 show the significance of exercising caution when addressing the assumptions

behind the implementation of chloride mass balance to estimate recharge, as well as the

requirement for rigorous demarcation of a suitable multi-year integration interval for calculating

the mean annual groundwater recharge.

In Nebraska, the CMB was employed to predict groundwater recharge in 2012. Sand that has been

well-organized and is rough dominates the terrain. Boerner and Weaver carried out this study,

University of Ghana http://ugspace.ug.edu.gh

which used the chloride mass balance concept, which is similar to the water budget approach, to

predict yearly recharge across Nebraska. According to the chloride mass balance approach, most

part of this area is recharged at a rate of less than 30 mm per year. They later found that CMB

provided a good estimate of recharge across Nebraska and that comparing CMB estimates to

recharge estimates from other methods is valuable.

Ping et al., (2014) conducted a study on a local level that used CMB for groundwater recharge

calculation in a dry mountainous terrain in Southern Interior British Columbia. According to the

research, recharge estimates of 1.1-1.9 percent of precipitation at the valley floor and 1.8-2.7

percent of precipitation on mountain areas appear to be helpful. These results, however, were not

compared or verified utilizing alternative approaches. According to Boerner and Weaver (2012),

comparing CMB estimates with recharge estimates from soil water balance or other approaches

might be beneficial. They suggested that the application of the CMB method at the drainage scale

appeared to be beneficial for predicting groundwater recharge quickly.

The assumptions required by the unsaturated zone CMB approach are generally difficult to verify

and may therefore be causes of error. As a result, CMB-based recharge estimation should be

utilized with caution and in conjunction with other recharge methods.

2.4.2 Water Table Fluctuation (WTF)

The WTF approach has been employed in many studies (Healy and Cook, 2002; Moon et al., 2004)

to estimate recharge in unconfined aquifers. Because of its consistency, simplicity of usage, and

reduced cost of usage in semi-arid regions, this technique is thought to be one of the most

productive and appealing (Xu and Beekman, 2003). WTF is mainly caused by a number of

University of Ghana http://ugspace.ug.edu.gh

different hydrologic phenomena caused by natural and/or anthropogenic means (Varni et al.,

2013). Given the amount of accessible groundwater-level data and the ease of estimating recharge

rates from temporal variations or geographical trends of groundwater levels, the WTF method is

among the most extensively employed methods for predicting recharge rates. To be utilized, this

approach needs information on specific yields and variations in water levels as time passes.

The method makes no assumptions about the transport of water through the zone of unsaturation;

therefore, the availability of preferential flow pathways does not limit its application (Obuobie et

al., 2012). The WTF approach assumes that increases in levels of water in unconfined aquifers are

caused by recharge water entering at the water table and that all other elements of the groundwater

budget, including lateral flow, are null in the time of the recharge period (Crosbie et al., 2005;

Mensah et al., 2014). The recharge rate can be calculated as the product of the rise in water level

and the specific yield of the groundwater aquifer property. The recharge can be expressed

mathematically as

𝑅 =
∆ℎ

∆𝑡
× 𝑆𝑦…………………………………………………………………………………….2.2

Where R, Δh, Δt, and Sy are respectively the amount of recharge from precipitation (LT−1 ),

change in water table head during recharge period (L), period of recharge (T), and specific yield.

The following are some of the main hypotheses that this technique is based on:

➢ The rise and fall of water table levels in shallow unconfined aquifers are primarily

attributable to groundwater recharge and discharge;

➢ The aquifer's specific yield is defined and consistent during the fluctuation of the water

table; and

University of Ghana http://ugspace.ug.edu.gh

➢ Water level rise can be calculated using the pre-recharge water level recession (Healy

and Cook, 2002).

These assertions are not always correct, and in some cases, they could be considered flaws in this

method. Variations in groundwater levels, for example, might not necessarily be due to recharge

or discharge. Other causes such as the combination of evaporation and transpiration, variations in

atmospheric pressure, the existence of entrapped air, and earth tides could cause it, or it could be

a response to fluctuations in stream stage for boreholes located near streams (Delin et al., 2006).

An estimation of the specific yield (Sy) at a depth of water table fluctuation is needed to apply the

WTF method. The Sy is defined as (Healy 2010):

Sy = φ − Sr………………………………………………………………………….………2.3

where ϕ is porosity and 𝑆𝑟 is specific retention

Acquiring a specific yield that is reflective of a vast region has proven to be problematic in

previous research. Furthermore, in contrast to the premise of a stable and defined specific yield,

specific yield values change with time (Delin et al., 2006; Loheide et al., 2005). Laboratory

procedures, pumping tests, water-budget approaches, and water table response to recharge are the

most regularly utilized ways to determine Sy. Typically, laboratory procedures determine Sy by

measuring porosity and specific retention and applying the equation (2.3). Sy readings obtained in

the lab and in the field have a high degree of variability. Depending on the space between measured

and abstraction wells, aquifer tests yield values of Sy integrated over rather extensive areas. Walton

(1970) presented a water-budget method that requires a basin-scale water balance, ideally in winter

(when the soil is almost saturated and evapotranspiration is minimal). The balance can be solved

University of Ghana http://ugspace.ug.edu.gh

for the change in groundwater storage, and Sy may be approximated using the water table

fluctuation. The water budget equation is as follows:

𝑃 = 𝐸𝑇 + ∆𝑆 + 𝑄𝑜𝑓𝑓……………………………………………………………………………2.4

where P denotes precipitation, ET denotes evapotranspiration, ∆𝑆 is the variation in water storage

(in surface storing units, zone of unsaturation and saturation), and 𝑄𝑜𝑓𝑓 denotes surface plus

subsurface water flows out of the basin. The variation in groundwater storage can be represented

as a function of Sy (see equation 2.2, and then Sy can be found by rearranging equation 2.4).

Because the change in groundwater storage must be inferred from the difference of other water

budget elements, including significantly higher water quantities, the method may have huge

inaccuracies. However, this method yields a mean value for a vast region rather than a local value.

The water table's response to the recharge method is determined by determining the ratio of water

table rise to cumulative rainfall (Risser et al., 2005b) for all registered events in the examined area.

The quantity of open pore space accessible in the zone of unsaturation is estimated by the height

of the water table increases following a rainy event (i.e., Sy). For a shallow water table, the

approach is applicable.

Crosbie et al., (2005) investigated various approaches for estimating Sy in two places, calculating

the water retention curve in the laboratory through examination of the water table response and

pumping tests. The rainfall–water table reaction appears to produce the best estimates after testing

multiple methods. As a result, this paper provides a method based on field data (rainfall and water

level fluctuations) that seems to be a version of the water table reaction to recharge events

technique. The approach is a graphical strategy that produces outcomes with higher reliability as

the number of occurrences observed grows (Varni et al., 2013). Because the Akyem catchment has

University of Ghana http://ugspace.ug.edu.gh

comprehensive data records, a large increase in the water table episodes may be studied, making

the Sy determination less questionable. Furthermore, because of the rapid response of the water

table during recharge episodes, the records are particularly well suited for this research. Because

of these considerations, the graphical technique suggested in this study was chosen for Sy estimate.

Shirahatti et al., (2012) cited some challenges with the application of the method. They connected

to defining a typical figure for a specific yield and guaranteeing that water levels variations are

because of recharge and are not because of variations in air pressure, presence of enclosed air, or

other occurrences (such as abstraction). Healy and Cooks (2002) also indicated that this method

for groundwater estimation has its own limitation and stated that:

➢ The water table fluctuation approach is excellent for shallow water table systems with

abrupt rises and falls in water levels. Wetting fronts tend to disperse across vast distances;

therefore, significant rises in deep aquifers are unlikely. The approach could potentially be

used in systems that have substantial unsaturated zones and only have seasonal water level

changes.

➢ Given that recharge results differ significantly within a catchment due to disparities in

altitude, geology, land surface slope, vegetation, and other conditions, the boreholes must

be situated so that the observed water levels are reflective of the area as a whole.

➢ Because the method does not account for a constant rate of recharge, given a constant rate

of recharge equivalent to the rate at which it is drained away from the water table, the water

levels would remain constant, and the water table fluctuation technique could forecast no

replenishment.

➢ Further challenges include recognizing the source of water-level fluctuations and

computing a specific yield value (Beekman and Xu, 2003).

University of Ghana http://ugspace.ug.edu.gh

There are no specific restrictions on the range of recharge that can be approximated in the WTF

approach (Obuobie et al., 2012). The WTF approach can be used to estimate subsurface storage

changes over longer time intervals (seasonal or annual). This technique has been employed by

several researchers for groundwater recharge estimation. Obuobie et al., (2012) used the WTF

technique in Ghana to study groundwater level fluctuations and estimate recharge to groundwater

in the White Volta Basin for the 2006 and 2007 study years. The results of observed groundwater

levels indicate large seasonal and regional water level changes, with a span of 1240–5000 mm in

2006 and 1600–6800 mm in 2007. Because water levels rose only during the wet season, the

rainfall seasonality was revealed to be the primary recharge source in the research area. The

recharge to groundwater in the White Volta basin is expected to range from 2.5 to 16.5 percent of

annual rainfall, with an average of 7–8 percent. The findings are acceptable and within the

recommended limits of estimates found in earlier hydrogeological investigations in other sections

of the Volta Basin and in a number of dry areas in Africa using water table fluctuation and other

approaches. The figures reached in this study, according to their research, were slightly greater

than the findings discovered in several of the earlier investigations listed. Because the specific

yield figures utilized in the research were derived from earlier research rather than examined for

the specific aquifers in the research area, some degree of inconsistency was to be expected. They

observed that adopting specific yield estimates established for the aquifers in the research area

could increase the study's trustworthiness.

Later, Krautstrunk (2012) investigated the groundwater recharge in the Nabogo River Basin,

Ghana, employing the WTF and CMB methods. Calculated estimates for specific yields were

selected on the basis of the hypotheses that specific yield varies in the aquifer, and the aquifer is

not confined. Specific yield estimates from the literature review were used to develop a small

University of Ghana http://ugspace.ug.edu.gh

recharge estimation and a large recharge estimation to allow for a variety of specific yield

projections in the aquifer of fractured rock. The values 0.001, 0.005, 0.002, and 0.008 for specific

yield were chosen from certain areas in Zimbabwe and Australia with comparable climates and

fractured aquifers to depict the small valued estimation representing the “semi-confined” parts of

the catchment affected by Plinthite formation, and the values 0.05 and 0.08 from current

research in the White Volta River Basin to reflect the large estimation representing the “semi-

confined” parts of the catchment. The recharge estimates obtained in this research agreed with the

findings acquired in the area and those expected for fractured rock aquifers. Many sources have

given these approaches excellent marks for accuracy when compared to other ways, and they have

been praised for being a quick and inexpensive way to estimate recharge (Sibanda et al., 2009).

The CMB technique gave a mean rate of 37.06mm per year, or 3 percent of annual precipitation,

in the Nabogo Basin, while the WTF technique produced a spectrum of 10-143 mm per year or 1-

13 percent of annual precipitation. The research also revealed that both CMB and WTF

methodologies have limits when it comes to calculating recharge, particularly when specific yield

values are undetermined and time restrictions are imposed on the sample time. It was further noted

that when the two procedures are used together to explain a specific yield, the margins of error are

lowered, reducing the uncertainty associated with utilizing only one method.

More recently, Lutz et al., (2015) employed water table fluctuations (WTF) techniques in Ghana

to analyze the variation of groundwater levels and trends of recharge in the North. The variety of

recharge trends between the areas are demonstrated using the WTF approach and groundwater and

Sy values acquired from the literature. The variety of trends in recharge is attributable to the varied

spatiotemporal precipitation and the uniqueness of hydrogeological formations at the sites.

Recharge seems to be fast at three of the four sites and is computed to be a higher fraction of

University of Ghana http://ugspace.ug.edu.gh

overall yearly precipitation than certain values in existing works. The recharge at the other area is

comparable to values found in existing works, which are around five percent of precipitation.

According to a seasonal analysis of groundwater level fluctuations, groundwater levels rise in

response to wet-season precipitation in the areas. In certain situations, such as at Kpataribogu, the

increases are significant. All areas demonstrate a total net reduction in groundwater levels during

the time of the research on a yearly basis. The drop could be linked to below-average precipitation

deviations in 5 of the 7 research periods. Future research should look at what kinds of precipitation

conditions contribute to aquifer recharge that is close to considerable, as well as what kind of

temporal buffering (storage) the system can provide to eliminate the impact of drought and over-

pumping.

In other countries, Varni et al., (2013) employed the WTF method to describe the recharge of

groundwater in the Pampa plain, Argentina. An approach for determining the Sy is suggested in

this study; it consists of a visual process that connects increases in groundwater level to sources of

rainfall. As the occurrence being monitored grows, the approach produces increasingly reliable Sy

values. The WTF approach was applied after an analysis of eighteen years of daily measurements

yielded a Sy value of 0.09. The recharge numbers obtained are consistent with those calculated by

other writers for the same area.

The WTF method was used as an additional supplement to the RIB model in this study because

periodic and reliable water level readings were accessible in boreholes that were not impacted by

human activities. It provides more reliable estimations regardless of whether a piston or preferred

flow recharge mechanism is used in a given location. Its temporal scope is broad, ranging from a

single day to a year. Furthermore, the procedure is simple to apply, requires little data, and is

inexpensive (USGS, 2008).

University of Ghana http://ugspace.ug.edu.gh

2.4.3 Environmental Tracer (Isotope) Technique

Recently, the techniques based on the heat or chemical isotopic tracers are gaining much

importance in the estimation of groundwater recharge. The estimation of recharge using tracers is

predicated on the tracer's mass conservation and the premise that the tracer flows easily with water

(Sharma, 1985). Environmental isotopes give insight into the evolution of geochemistry, recharge

mechanisms, rock-water relationships, and the genesis of salinity (Abid et al., 2010; Cartwright et

al., 2009; Chen et al., 2006; Demlie et al., 2007; Demlie et al., 2008). As such, they serve as a tool

in the characterization of groundwater flow systems (Moore, 2002). Plummer (2003) indicates that

the various environmental tracers offer different forms of information about the groundwater

system and the aquifer. Thus, tracers are useful tools in estimating groundwater recharge,

groundwater age; trace groundwater flows directions, identify water sources to the groundwater,

etc. Commonly used environmental isotopes in hydrology include but are not limited to deuterium

(2H), 18O, carbon (13C and 14C) and tritium ( 3H) (Fontes and Edmunds, 1989). Oxygen-18 and

deuterium are described below.

2.4.3.1 Oxygen-18 and deuterium

Precipitation has more weight isotopically than the vapor remaining in the air, including more 2 H

and 18O, but water vaporizing from the ocean has less weight isotopically than the water left

behind. The weightier water particles have a tendency to cluster in the liquid form as phase water

transforms from liquid to gas, fractionating the hydrogen and oxygen isotopes. The standard mean

ocean water (SMOW) isotopic ratio can be weighed to these environmental isotopic ratios

(Petersen, 2012). The isotopic makeup of the groundwater is related to the Global Meteoric Water

University of Ghana http://ugspace.ug.edu.gh

Line (GMWL) of Craig (1961) or a Local Meteoric Water Line (LMWL) built for a given location.

The GMWL is expressed by the equation:

𝛿2𝐻 = 8𝛿18𝑂 + 10………………………………………………………………………...2.5

Allison et al., (1984) created the δD displacement procedure for the zone of unsaturation in

Australia. The displacement from the GMWL or LMWL is applied to estimate recharge using

stable isotope soil moisture data plotted on a δ2H − δ18O diagram and the displacement from the

GMWL or LMWL is applied to compute recharge employing:

∆𝛿 =
𝐶

√𝑅𝑒𝑐ℎ𝑎𝑟𝑔𝑒
…………………………………………………………………………….2.6

∆𝛿 = displacement of either D or 018𝑂 from the MWL

C = gradient through the inverse of the square root of moisture flows derived from displacements

from the MWL for different places. Allison et al., (1984) discovered that in Australia, C equals 20.

The actual 𝛿18𝑂 values of precipitation reaching the ground hinge on numerous elements (Chen

et al., 2006; Mook, 2006), and whether these elements will affect the precipitation in the various

environments are discussed further below.

➢ Latitude: Temperatures that are cold diminish the possibility of weightier water condensing

and precipitating as rain. As a result, the higher the latitude, the more ruined the

precipitation is in 𝛿18𝑂.

➢ Proximity to the ocean: The heaviest precipitation tends to fall first in air masses. As a

result, as air masses migrate inland, precipitation gets depleted in 𝛿18𝑂.

University of Ghana http://ugspace.ug.edu.gh

➢ Altitude: Colder temperatures at greater elevations often lead to precipitation with a lower

𝛿18𝑂 content. This may not affect certain places as some areas drop into a constrained

range of elevations.

➢ Season: Colder winter temperatures result in precipitation that is considerably diminished

in 𝛿18𝑂. This can have an impact in locations where the temperature, especially during the

hot summer months, can cause rainwater to evaporate before it reaches the ground.

➢ Amount: When compared to small rains, heavy rainfall events tend to be depleted in 𝛿18𝑂.

Varying isotopic ratios may emerge from the strength and duration of rainfall events, which

might influence the isotopic signature.

Isotopes are rarely used in hydrological studies in Ghana (Yidana and Koffie 2014). However,

impressive and extremely beneficial results have been documented where the methodology has

been utilized. Acheampong (1996) and Acheampong and Hess (2000) studied groundwater

recharge mechanisms in the southern Voltaian basin, applying stable and radiogenic isotope data

from groundwater, surface water, and precipitation. Their re