Groundwater recharge estimation and quality on the southern slopes of mount Kilimanjaro, Tanzania

Abstract

This study was undertaken to estimate groundwater recharge rates using three methods (water table fluctuation (WTF), chloride mass balance (CMB) and soil moisture balance (SMB), as well as assessing the impact use of the pesticides and fertilizers on groundwater quality in a Kahe catchment located on the Southern slope of Mt. Kilimanjaro, Tanzania. The estimated recharge rates of the WTF method yielded a higher value of about 248.4 million m 3 /year, whereas that of CMB and SMB methods was 156.0 and 132.1 million m 3 /year, respectively. The concentration of pesticide residues was below the detection limit, while, NO 3 - levels of sampled groundwater were averaged about 18.40 ± 4 and 7.6 ± 1.7 mg/l in the wet and dry season, respectively. An elevated level of nitrate (NO 3 - ), sulfate (SO 4 2- ), phosphate (PO 4 3- ), and ammonium (NH 4 + ) was found nearby the large-scale sugarcane farms. Generally, the groundwater chemistry was dominated by magnesium-sodium-bicarbonate (Mg-Na-HCO 3 ) water type. The cations levels were in the trend of Mg 2+ > Na + > Ca 2+ > K + , whereas anions were HCO 3 - > Cl - > SO 4 2- for both seasons. The heavy metals concentration of Pb, Cd, Fe, Mn, Zn and Cu for both seasons were averaged approximately 0.08 ± 0.03, 0.11 ± 0.03, 0.16 ± 0.02, 0.11 ± 0.01, 0.46 ± 0.05 and 0.55 ± 0.02 mg/l, respectively. Whereas the concentration was higher in shallow groundwater than deep boreholes and most of them were located around the western-eastern zone of the Kahe catchment. The findings from this study provided reliable data information for future monitoring programs of groundwater resources management in Kahe catchment, Tanzania.