Hydrochemical And Isotopic Characterization Of Aquifers In The Lower Tano River Basin, Ghana, West Africa.

Abstract

The Lower Tano River basin is one of the basins endowed with minerals, oil, and gas resources in West –Africa. It covers 104,000 km2, and the major riparian countries are Ghana and Ivory Coast. The basin, underlain by the Birimian supergroup rocks and the rocks of the Apollonian has enough surface water resources to meet current water demands, but there are many challenges including high population growth rate (due to influx of oil and gas industrial workers), land degradation, mining activities, climate change etc. These challenges have put immense pressure on the groundwater resources in the basin. Also, the surface water is unreliable to meet the basin’s water demand for socio-economic development. This therefore makes groundwater the most cost effective and preferred means of supplying water to the dispersed population and rural settlements across the basin. One probable requirement for a sustainable and efficient management of groundwater is the characterization of aquifers in terms of its quantity, quality, and quantification of the recharge sources. Hence in this study, the aquifer hydraulic parameters were estimated, the overall quality of groundwater including the level of trace elements contamination, and its suitability for domestic and agricultural purposes was assessed, the hydrogeochemical processes that produces the chemical characteristics or compositions of the aquifer system was determined, the sources of recharge to the aquifer was investigated and the ages of groundwater was determined. The methodology used involved pumping tests carried out on 55 boreholes drilled across the basin, yearly sampling of surface water and groundwater resources, measurement of cations, anions, trace elements, stable isotopes of deuterium (δ2 H), oxygen-18 (δ18 O), tritium (3H) and measurement of silica. Pumping test together with Theis (1934) graphical method was used to estimate the aquifer hydraulic parameters (transmissivity, hydraulic conductivity, and specific capacity). From the estimation of the aquifer hydraulic parameters, it was observed that the mean transmissivity, hydraulic conductivity, and specific capacity values were 5.96 m2/day, 5.70 m/day and 18.80 L/min/m, respectively. The cations (calcium, sodium, potassium, magnesium) and anions (chloride, bicarbonate, nitrate, and sulphate) show that the aquifer of the Apollonian formation and the Birimian super group produces fresh groundwater with most being soft and few being moderately hard and hard. Na+ and SO42- were the dominant cation and anion in the surface water and ground water sampled in 2016. Likewise, Na+ and HCO3- were the dominant cation and anion in the surface water sampled in 2017, the groundwater sampled from 2013 to 2015, and the hand dug wells sampled in 2017. The boreholes sampled in 2017 had Na+ and SO42- as the dominant cation and anion, respectively. All the measured ions (bicarbonates, sulphate, chloride, nitrate, calcium, magnesium, sodium, potassium) in most of the surface water and groundwater sampled from the rocks of the Apollonian and the Birimian supergroup were within the WHO (2011) allowable limits for drinking water. The results of the measured trace elements that is Fe, Pb, Cd, Ni, As, and Al were high in most of the surface water samples. Also, the results of Cu, Cd, Zn, Cr, Fe, Ni, Mn, Co, Pb, As and Al in most groundwater especially those sampled from the aquifer of the Apollonian formation were higher than the WHO (2011) permissible limits for drinking water. The elevated concentrations of trace elements in the surface water and the groundwater were mostly related to geogenic sources. Based on water quality index (WQI) results, it was observed that majority of the groundwater sampled from the aquifer of the Apollonian formation were unsuitable for drinking whiles most groundwater samples taken from the aquifer of the Birimian supergroup were suitable for drinking. As per sodium adsorption ratio (SAR), 99% of the groundwater sampled from both aquifer of the Apollonian formation and the Birimian supergroup were suitable for irrigation. The sodium percent classification indicates that, minority of the groundwater sampled from the aquifer of the Apollonian formation and the Birimian supergroup falls under excellent category for irrigation. Wilcox diagram and magnesium hazard classification indicate that majority of the groundwater especially those sampled from the aquifer of the Apollonian formation gave out excellent to good water for irrigation. The residual sodium carbonate and Permeability index classification specify that all the sampled groundwater from the aquifer of the Apollonian formation and the Birimian supergroup were suitable for irrigation. The chlorinity index classification indicates that the aquifer of the Apollonian formation and the Birimian super group produce groundwater that were suitable for irrigation. Graphical including hydrogeochemical modelling and statistical approaches applied in the delineation of the major factors influencing the evolution of groundwater and the general hydrochemical characteristics revealed that the chemistry of groundwater in the Apollonian formation aquifer and the aquifer of the Birimian supergroup was mainly rock weathering and rainfall. In addition, ion exchange processes, rock-water interaction and incongruent dissolution were the major natural factors governing the formation of the groundwater chemistry in the Lower Tano River Basin. However, the occurrence of sulphate implies that sulphide mineral (pyrites, arsenopyrites and chalcopyrite) oxidation or mobilization and mining activities including illegal and legal small-scale mining possibly have an impact on the groundwater chemistry in the Lower Tano River Basin. The hydrochemistry of the sampled groundwater transited from Ca-HCO-3 to Na-Cl water type along a flow direction for the groundwater samples taken from 2014 to 2015 and 2017. For the groundwater sampled in 2016, the hydrochemistry evolved from Ca-HCO3- to Ca-SO42- water type and then to NaCl water type along the flow direction. The evolution was influenced by ion exchange processes and rock-water interaction. Based on mineral stability diagram, most groundwater in the aquifer of the Apollonian formation and the aquifer of the Birimian super group appears to be stable within the Kaolinite field suggesting ion exchange processes. Stable isotope composition (δ18O and δ2H) of rainwater indicates that the rainwater was not highly evaporated. Also, the slope and intercept of the rainfall regression lines obtained for this study were slightly similar to the local meteoric water line for Ghana and the global meteoric water line. Stable isotope composition (δ18O and δ2H) of surface water (rivers, streams, lagoon, and seawater) and groundwater (hand dug wells and boreholes) reveal that the mechanism of recharge to the aquifer of the Apollonian formation and the Birimian supergroup was rapid with the source being meteoric. Also, the surface water (rivers and streams) contributed to the groundwater recharge of the Lower Tano River Basin. Tritium in rainfall ranges from 1.27 to 10.11 TU. That of the rivers ranges from 2.04 to 3.08 TU and that of the groundwater ranges from 0.99 to 6.78 TU. The tritium results revealed that groundwater from the aquifer of the Apollonian formation and the Birimian supergroup were young, of modern recharge with short residence time. Also, the groundwater was recharged between 1960- 1965 and 1965 – 1970. The outcome of this study has added to the current hydrogeological knowledge about Lower Tano River Basin. It has also added to the characteristics of semi-confined and unconfined aquifers.