Understanding the hydrogeochemical evolution of groundwater in Precambrian basement aquifers: A case study of Bugesera region in Burundi

Abstract

This study investigates hydrogeochemical processes controlling groundwater evolution in a Precambrian basement aquifer situated in northeastern Burundi, in Bugesera region. The hydrogeochemistry has been evaluated based on the analytical results of 143 water samples comprising 66 samples from shallow wells, 61 samples from springs, and 16 samples from the shallow lakes. While the stability diagrams, the evolutionary sequence of groundwater inferred from the Piper diagram and bivariate plots highlight the weathering of aluminosilicates as the primary process controlling groundwater chemistry in this aquifer, the increasing trend of chloride levels between the highlands (recharge area) and the depression of Bugesera (discharge area) and the random distribution of high nitrate content even in water samples from springs suggest that additional processes such as evaporative concentration and anthropogenic pollution play a secondary role in the hydrogeochemical evolution of groundwater. The spatial distribution of different hydrogeochemical parameters shows a general increasing trend from the highlands where water samples, mainly from springs, show low mineralisation (TDS=26–152mg/l) and low pH (field pH=4.6–6.6), towards the depression of Bugesera where higher values of mineralisation (TDS=44–3229mg/l) and pH (field pH=5.6–7.7) are observed. The overriding control of aluminosilicates weathering on the hydrogeochemical evolution of groundwater in Bugesera region is clearly demonstrated by the predominance of NaHCO3 and CaHCO3 water types and the cross-plots which show an increasing trend of alkaline and alkaline earth cations (Na++K++Ca2++Mg2+) released from the weathering of aluminosilicates along the flow line. Moreover, the poor correlation between the concentrations of SiO2 and Na+, Ca2+, Mg2+, K+, HCO3– and TDS reflects the incongruent character of the weathering of aluminosilicates which causes part of the silica released from the weathering of silicates to be taken up into the neoformation of clay minerals. The anthropogenic pollution is reflected by the occurrence of NaNO3, CaNO3, KNO3 and NaSO4 types even in spring water.