Hydrogeochemical Suitability of Groundwater for Irrigation and Agriculture: A Sodium Adsorption Ratio, Kelley Index and Irrigation Water Quality Index Analysis of Sedimentary and Basement Aquifers in Southwestern Nigeria

Agricultural water resource management in sub-Saharan Africa increasingly relies on groundwater resources that have seldom been hydrogeochemically characterized for their suitability in irrigation water supply. In Sobe community in Edo State and Elegbeka community in Ondo State, southwestern Nigeria, groundwater from hand-dug well water supplies both domestic and agricultural water uses. However, their suitability for irrigation water supply has never been evaluated with hydrogeochemical indices of irrigation water quality. This study evaluated the Sodium Adsorption Ratio (SAR), Kelley Index (KI), Permeability Index (PI), Magnesium Hazard (MH), and a composite Irrigation Water Quality Index (IWQI) on groundwater from 25 sampling points across two geological terrains: Imo shale geological series and Basement Complex migmatite. The results showed that although concentrations of potassium ion are 10 to 18 times higher than permissible limits for domestic water supply by NAFDAC, they are a major agronomic benefit to low-fertility soils. This poses a domestic/agricultural water trade-off with policy implications. Although SAR values classify all groundwater from these two study areas as low to medium salinity hazard, chloride and sulphate ion concentrations in some of the water samples from Elegbeka community result in a high salinity hazard classification for water from Wells 6 and 10. Magnesium ion concentrations greater than 50% classify water from these two wells in Elegbeka community as unfit for irrigation water supply. From the Kelley Index calculation results, 79% of Sobe community water wells are suitable for irrigation water supply compared to 60% of water wells in Elegbeka community. The hardness-dominated water chemistry of water from Elegbeka community poses a precipitation hazard to soil permeability in fine-textured soil plots. This study provides a first-time characterization of water suitability for these two communities and can be a model for promoting safe water use for agriculture and protecting food security.

irrigation water quality; sodium adsorption ratio; Kelley index; magnesium hazard; Hydrogeochemistry; Nigeria; food security; groundwater agriculture

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