Abstract:
In the Pra Basin of Ghana, groundwater is increasingly becoming the alternative water
supply due to the continual pollution of surface water resources through illegal mining and in discriminate waste discharges into rivers. However, our understanding of hydrogeology and the
dynamics of groundwater quality remains inadequate, posing challenges for sustainable water re source management. This study aims to characterize groundwater recharge by determining its origin
and mechanism of recharge prior to entering the saturated zone and to provide spatial estimates of
groundwater recharge using stable isotopes and water level measurements relevant to groundwater
management in the basin. Ninety (90) water samples (surface water and groundwater) were collected
to determine stable isotope ratios of oxygen (δ
18O) and hydrogen (δ
2H) and chloride concentra tion. In addition, ten boreholes were installed with automatic divers to collect time series data on
groundwater levels for the 2022 water year. The Chloride Mass Balance (CMB) and the Water Table
Fluctuation (WTF) methods were employed to estimate the total amount and spatial distribution
of groundwater recharge for the basin. Analysis of the stable isotope data shows that the surface
water samples in the Pra Basin have oxygen (δ
18O) and hydrogen (δ
2H) isotope ratios ranging from
−2.8 to 2.2h vrs V-SMOW for δ
18O and from −9.4 to 12.8h vrs V-SMOW for δ
2H, with a mean of
−0.9h vrs V-SMOW and 0.5h vrs V-SMOW, respectively. Measures in groundwater ranges from
−3.0 to −1.5h vrs V-SMOW for δ
18O and from −10.4 to −2.4h vrs V-SMOW for δ
2H, with a mean
of −2.3 and −7.0h vrs V-SMOW, respectively. The water in the Pra Basin originates from meteoric
source. Groundwater has a relatively depleted isotopic signature compared to surface water due
to the short residence time of infiltration within the extinction depth of evaporation in the vadose
zone. Estimated evaporative losses in the catchment range from 51 to 77%, with a mean of 62% for
surface water and from 55 to 61% with a mean of 57% for groundwater, respectively. Analysis of
the stable isotope data and water level measurements suggests a potential hydraulic connection
between surface water and groundwater. This hypothesis is supported by the fact that the isotopes
of groundwater have comparatively lower values than surface water. Furthermore, the observation
that the groundwater level remains constant in months with lower rainfall further supports this
conclusion. The estimated annual groundwater recharge in the catchment ranges from 9 to 667 mm
(average 165 mm) and accounts for 0.6% to 33.5% (average 10.7%) of mean annual precipitation. The
total estimated mean recharge for the study catchment is 228 M m3
, higher than the estimated total
surface water use for the entire Pra Basin of 144 M m3
for 2010, indicating vast groundwater potential.
Overall, our study provides a novel insight into the recharge mechanism and spatial quantification
of groundwater recharge, which can be used to constrain groundwater flow and hydrogeochemical
evolution models, which are crucial for effective groundwater management within the framework of
the Pra Basin’s Integrated Water Resources Management Plan
