Abstract Climate change and population growth around Mt. Meru experienced lower availability of water for domestic and agricultural uses. Reduction in quantity of water is compounded by lack of information on water quality which could lead to undesired health risks and agricultural effects when such water is used for irrigation. Thus, major ions from 54 different water types (springs, streams, dug wells, boreholes, and lakes) were used to assess hydrogeochemical characteristics and suitability of water for domestic and agricultural purposes. Results showed dominance of the major cation and anion in the order of Na+ > K+ > Ca2+ > Mg2+ and HCO3 − > CO3 2− > Cl− > SO4 2− > NO3 − > F−, respectively. It is revealed that Mt. Meru is the recharge zone. Geology, water–rock interaction time, and climatic conditions control water chemistry. Major freshwater aquifers were found to be fractured mafic volcanics, breccia, and tuff. Lahars, due to their susceptibility to weathering, were found to host groundwater of low quality. The suitability of water for domestic and irrigation purposes was moderate, in order of lakes < dug wells < boreholes < streams < springs. Fluoride was found to be the major natural contaminant affecting water quality for domestic purposes with mean value of 17.6 mg/L, while elevated Na+ (mean = 118 mg/L), K+ (mean = 59 mg/L) and HCO3 − (mean = 390 mg/L) relative to other ions were found to affect water quality for irrigation purposes. In some few cases, anthropogenic pollutions were recognized through NO3 − and Cl−.