Hydrochemical Characterisation of the Basement Aquifer with a Focus on the Origin of Nitrate in the Highly Urbanised Niamey Region, SW of Niger.

This study investigated the basement aquifer beneath the urbanised city of Niamey and the agricultural fields of Kollo, SW of Niger. During the observation period spanning from 2021 to 2023, groundwater and surface water samples were collected for analysis of major ions and the stable isotopes oxygen-18 and deuterium (δ¹⁸O and δ²H) of water. To trace the origin of high nitrate concentrations (NO₃⁻) found in several observation and drinking water wells in both areas, δ¹⁵N and δ¹⁸O isotope values of NO₃⁻ were analysed in groundwater and eluted soil samples. The observed hydrochemical patterns mainly reflect the heterogeneity of the weathered fringe of the basement aquifer. Decreasing concentrations of NO₃⁻ and δ¹⁸O and δ²H values were observed in relation to the distance of the Niger River and increasing thickness of the clay layer on the surface. The wells close to the river in Niamey show a dilution effect during the flood season, and the NO₃⁻ concentrations displayed a continuous increasing trend. The δ¹⁵N-NO₃ and δ¹⁸O-NO₃ values confirmed that septic tank water is spreading in the region of Niamey and that manure originating from livestock in Kollo is the main source of NO₃⁻. The patterns of δ¹⁵N in the soil samples coincide with those of cattle's manure spread in both areas. The shallow wells show significantly higher values of electric conductivity and NO₃⁻ concentrations compared to the deeper wells, which clearly indicates the influence of shallow septic tanks on water quality. [ABSTRACT FROM AUTHOR]