Abstract
This study employed stable isotopes of δ18O and δ2H in conjunction with other hydrological parameters to understand the origin, inferred residence time, and seasonal effect of groundwater in the shallow aquifers of the eastern Dahomey Basin. A total of 230 groundwater samples (97 in the wet season and 133 in the dry season) were collected from the borehole and shallow aquifer between May 2017 and April 2018. Groundwater analysis included major ions and δ18O and δ2H, isotopes data in precipitation from three selected Global Network of Isotope in Precipitation (GNIP) stations across West Africa, Douala in Cameroon, Cotonou in Republic of Benin, and Kano in Nigeria were used in comparative analysis. Results of the hydrochemical model revealed Ca-HCO3 and Na-Cl as dominant water types with other mixing water types such as Ca–SO4, Ca–Cl, Na–SO4, and K–Mg–HCO3, which characterised early stage of groundwater transformation as it infiltrates through vadose zone into the aquifer. δ18O and δ2H precipitation data from the three stations plotted along with the groundwater samples indicate recent meteoric water origin, with little effect of evaporation during the dry season. The plot of Total Dissolved Solids (TDS) against δ18O showed clustering of the water samples between the recharge and the evaporation zone with dry season samples trending towards increased TDS, which is an indication of the subtle effect of evaporation during this period. Tracing groundwater types along the flow paths within the basin is problematic and attributed to the heterogeneity of the aquifer with anthropogenic influences. Moreover, a comparison of the δ18O and δ2H isotopic compositions of groundwater and precipitation in the three selected stations, with their respective deuterium excess (D-excess) values established low evapotranspiration induced isotope enrichment, which could be due to higher precipitation and humidity in the region resulting in low isotope fractionation; hence, little effect of seasonal variations. The study, therefore, suggested groundwater recharge in the shallow aquifer in the eastern Dahomey Basin is of meteoric origin with a short residence time of water flows from soils through the vadose zone to the aquifers.
Highlights
Rock-water interaction is a process that influences groundwater chemical evolution from recharge along the flow paths through the vadose zone to the phreatic zones
As part of understanding the hydrogeochemical dynamics of shallow coastal aquifers of the eastern Dahomey Basin (EDB), this study aimed to apply stable environmental isotopes of δ18 O and δ2 D in combination with key hydrochemical parameters for a conceptual model of how groundwater chemistry changes across different geological units and formation within the EDB
The physicochemical parameters were measured in the field using a Teckoplus 6-in-1 pen-type water quality tester Model 99720 pH/conductivity meter capable of measuring total dissolved solids (TDS), salinity, temperature, and redox potential (Eh/ORP)
Summary
Rock-water interaction is a process that influences groundwater chemical evolution from recharge along the flow paths through the vadose zone to the phreatic zones. Sci. 2020, 10, 7980 groundwater quality changes [1,2]. The origin of water plays a role before continuous evolution as it flows from the recharge zone downgradient within the geological unit of formations [3]. Sedimentary basins play a vital role as sources of water supply to meet water demand, especially in developing countries in Africa and Asia. The stratigraphical characteristics make aquifers vulnerable to contamination and pollution [8]. Shallow coastal aquifers in some developing countries have been reported to be facing various challenges of groundwater quality deterioration, the causes of which are attributed to both geogenic and anthropogenic influence
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