Groundwater flow and geochemical processes affecting its quality in the basement (Oban Massif) and sedimentary (Mamfe Embayment) environments, southeastern Nigeria

Abstract

Groundwater flow and geochemical processes that affect its quality were investigated in Oban Massif and Mamfe Embayment, southeastern Nigeria. Physical parameters of water samples such as acidity (pH), turbidity, temperature and total dissolved solids (TDS) were measured in the field using potable kits. The waters tested acidic with a pH ranged from 5.59 to 7.23, which is not in agreement with World Health Organization (WHO 2011) permissible limit (6.5–8.5) for drinking water. The sedimentary area shows TDS, turbidity, fluoride (F) and chloride (Cl−) ions exceeding permissible limits of 500 mg/l, 5 mg/l, 1.5 mg/l and 250 mg/l, respectively stipulated by WHO (2011) for drinking water. Ionic ratios of SO4/Cl contoured on topographic map decreased to the center indicating that groundwater flow is from east to west and Cl/HCO3 increased towards the center. Cl/HCO3 also increases towards the west reflecting eastern to western direction of groundwater flow. Existing water table values also showed that groundwater flow direction is from eastern in the basement to western sedimentary part of the study area. Ionic ratio plots and Gibbs plots show that geochemical processes such ion exchange, evaporation, crystallization; precipitation and rock weathering affect water chemistry and quality within the study. Silicate weathering and rock-water interactions are the dominant geochemical processes affecting water chemistry and quality. Multivariate statistical analysis confirmed that the source of elements in water is from geochemical processes such as silicate weathering from basement rocks, saline water intrusion, interaction of rock with aquifer materials and human inputs. Piper diagram revealed that the dominant water types are Ca–Na–HCO3- and Ca2+ + Mg2+-HCO3-. Evaluation of water quality for irrigation using percent sodium (% Na), Magnesium ratio, sodium absorption ratio (SAR), potential and salinity hazards show the water is suitable for irrigation. Water quality in the basement is more suitable for agriculture and domestic uses compared to the sedimentary area. This method used in evaluating groundwater quality is useful for local and regional study here in Nigeria and elsewhere on the globe.