Hydrogeochemical and Isotopic Studies of Ground and Surface Waters in the Lower Anayari Catchment Area, Upper East Region of Ghana

Abstract

The Lower Anayari catchment (LAC) located in the Upper East Region of Ghana, is part of the transboundary (Ghana and Burkina Faso) Anayari catchment. The other transboundary catchments are Atankwidi and Yarigatanga. These catchments are noted for intense farming activities. The population within Lower Anayari catchment area is estimated to be about 19,445. An estimate of about 80% of the population depends on groundwater for domestic and agricultural purposes. Agriculture has direct or indirect effects on the chemical composition of groundwater and aquifer geochemistry. This study was carried out to investigate the dominant geochemical processes and anthropogenic activities that influence groundwater chemistry in LAC. The study employed hydrochemistry and isotopic techniques to assess the chemical quality of groundwater in the Lower Anayari catchment. A total of fifty-one (51) samples comprising of thirty-seven (37) boreholes, four (4) hand-dug well and ten (10) surface waters were sampled from six (6) principal communities (Kulwase, Manyoro, Mirigu, Nakolo, Paga and Pungu) for the study. The samples were analysed in-situ for pH, conductivity and salinity using a HACH potable meter; and bicarbonate using a titrimetry. Analytical methods employed for the determination of major and minor ions were: Na+ and K+ (Flame photometry), Ca2+ (complexometric titration), and F-, Cl-, NO3-, PO43-, and SO42- (ion chromatography). Atomic absorption spectrometry was used for the determination of Mg2+ and heavy metals (Fe, Co, Cd, As, Ni, and Pb). Stable isotope of 2H and 18O composition of the water samples were determined using Liquid-Water stable isotope analyser [Off-Axis Integrated Cavity Output Spectroscopy (off axis ICOS) via Laser Absorption]. The general trend of major cations and anions, and trace element concentration were Na+ > Ca2+ > K+ > Mg2+, HCO3- > Cl- > NO3- > SO42-, and Fe > Pb > As > Co > Cd > Ni respectively. The concentration of Ni [0.001 - 0.019], As [0.001 - 0.008], Co [0.005 - 0.010], and Fe [0.006 - 0.586] mg/L, were averagely below WHO standard values for drinking water. The concentration of Cd [0.002 - 0.006] and Pb [0.001 - 0.020] mg/L, were averagely above WHO standard values (0.003 and 0.01 mg/L respectively) for drinking water. The data was subjected to Principal Component Analysis (PCA) this help delineate principal physico-chemical processes (related to minerals dissolutions, organic and synthetic fertilizers influences) implicated in groundwater quality. Hierarchical Cluster Analysis (HCA) further classified the groundwaters of the study area into two groups. The groups show different degrees of water-rock interaction or mineralisation. Water Quality Index (WQI) estimations indicated that the groundwater in the study area is suitable for drinking with the exception of groundwater in Nakolo and Mirigu which showed deteriorating water quality. Piper Trilinear plot indicated three (3) hydrochemical facies Ca-Mg-HCO3, Na-Ca-HCO3 and Na-HCO3 and are consequence of silicate weathering and silicate mineral dissolution, cation exchange and to a lesser extent fertilizer application which is more evident in Nakolo and Mirigu communities. The Wilcox diagram, and the United States Salinity Laboratory diagram (USSL) method for assessment of the groundwater suitability for irrigation revealed that the groundwater from the LAC are suitable for irrigation. Stable isotopes composition measurements for groundwater River Anayari and irrigation dams were clustered closely along the global meteoric water line (GMWL) suggesting an integrative recharge from meteoric origin, with a few showing evaporation effects before recharge.