Abstract
Assessments of hydrochemical quality and recharge source are critical components of sustainable groundwater resources evaluation and management, especially in complex weathered/fractured crystalline bedrock terrains. Assessment of the weathered/fractured basement aquifer of Ekiti area, SW-Nigeria was undertaken with emphasis on hydrochemical quality and stable isotope characterization of shallow groundwater system. The study approach involved field sampling and in situ measurements of physico-chemical parameters followed by hydrochemical and stable isotope analyses of the water samples. In situ measurements revealed EC value of 43–995 μS/cm (av 431.96 μS/cm) and total hardness of 31.3–295.1 mg/L. The pH of the water sample ranged from 7.0 to 8.9 (av 8.00). The concentrations of the major cations are in the order of Ca2+ > K+ > Na+ > Mg2+ with their respective average values of 33.35, 25.61, 24.91 and 8.7 mg/L respectively while that of the anions are in the order of HCO3− > Cl− > SO42− > NO3− with their respective average values of 165.44, 31.85, 21.27 and 5.44 mg/L respectively. However, the results of stable isotope analyses revealed δ18O value of −4.2 to −2.1 ‰ and δD of −22.3 to −10.2 ‰, suggesting meteoric water source (recent precipitation water recharging the associated shallow basement aquifer) while the estimated deuterium excess of 6.80–11.00 ‰ (av 9.6 ‰) suggested little or no imprint of kinetic evaporation. Furthermore, the overall hydrochemical profiles revealed the main hydrochemical facies as Ca(Mg)-HCO3 (dominant) and Na(K)-HCO3 (minor) water types which can be attributed to dissolution of minerals in the various rock units in the study area. Conclusively, the overall hydrochemical and stable isotope evaluations revealed a recharge predominantly derived from meteoric (rainfall) source while the corresponding low TDS is a reflection of low water–rock interaction and limited migratory history of the shallow groundwater system in the study area.
Highlights
Over the years, before the discovery of crude oil in Nigeria, Ekiti area was the least developed and neglected part of the former Western region of Nigeria despite the fact that the area was a major producer of cocoa that formed the backbone of the region’s economy
The concentrations of the major cations are in the order of Ca2? [ K? [ Na? [ Mg2? with their respective average values of 33.35, 25.61, 24.91 and 8.7 mg/l TH TDW(m) SWC Ca (mg/L) respectively while that of the anions are in the order of HCO3- [ Cl- [ SO42- [ NO3- with their respective average values of 165.44, 31.85, 21.27 and 5.44 mg/L respectively
A total number of 73 water samples were collected mainly from domestic hand-dug wells within the study area for hydrochemical analysis, out of which 27 samples were selected for stable isotope analysis
Summary
Before the discovery of crude oil in Nigeria, Ekiti area was the least developed and neglected part of the former Western region of Nigeria despite the fact that the area was a major producer of cocoa that formed the backbone of the region’s economy. The communities are peasant farmers and depend mostly on shallow groundwater (hand dug wells) and surface waters (rivers, streams and springs) for sustainability of their farming activity and other domestic requirements. Surface waters in form of rivers and streams apart from being prone to contamination from anthropogenic activities are subject to seasonal variation as most of them dried off during the dry season. The groundwater in the area were found within two major aquiferous units
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