This paper assesses the impacts of farmers’ intensive use of agrochemicals (fertilizers and pesticides) on groundwater quality in the Kahe catchment. Samples were collected during the wet and dry seasons of the year 2018 and analyzed for the presence of agrochemicals in the water. Groundwater chemistry was dominated by magnesium-sodium-bicarbonate (Mg-Na-HCO₃⁻). The cations levels were in the trend of Mg²⁺ >Na⁺ > Ca²⁺ > K⁺, whereas anions were HCO₃⁻ > Cl⁻ > SO₄²⁻ for both seasons. The NO₃⁻ had an average value of about 18.40 ± 4.04 and 7.6 ± 1.7 mg/L in the wet and dry season, respectively. Elevated levels of nitrate, sulfate, phosphate, and ammonium were found in water samples collected near the large-scale sugarcane plantation in the catchment. For both seasons, Pb, Cd, Fe, Mn, Zn and Cu concentrations 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. On the other hand, the concentrations were higher in shallow wells than in the deep boreholes. Pesticides’ residues were below the detection limit in all sampled groundwater. The findings from this study provide important information for intervention in groundwater quality management in Kahe Catchment, Tanzania.

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⁻ > CO₃²⁻ > Cl⁻ > SO₄²⁻ > 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⁻.

Predicting fluoride levels in water within fluoride endemic areas is an issue of high significance. As a result several methods including mathematical models have been reported to suit the task. However, most of these methods have limited practicality to low income communities. This study presents the potentials of employing soil characteristics to predict the level of fluoride in groundwater. The study is based at the areas around Mount Meru in Northern Tanzania. The volcanic sediments around this mountain had been segregated by geological studies into various lithologies. In this study water and soil samples were collected at springs in volcanic sediments categorized as main cone group, mantling ash, Tengeru lahar, Ongadongishu lahar and Ngarenanyuki lahar. Fluoride levels in water were then correlated to elemental composition of the soil. Water samples showed that fluoride was low in the main cone group, mantling ash and Tengeru lahar whereby the median concentration was 1mg/l but it was high in Ngarenanyuki and Ongadongishu lahars whereby the median concentrations were 4mg/l and 9mg/l respectively. Soil analyses indicated that high levels of aluminium do coincide along with low sodium levels, and vice versa. In addition high levels of sodium in soil are accompanied by high levels of calcium. Correlation studies indicated a strong negative relationship between aluminium in soil and fluoride in spring water with r2 = 0.847. On the other hand, a positive correlation was obtained between calcium in soil and fluoride in water with correlation coefficient, r2= 0.765. Likewise, sodium indicated a positive correlation with fluoride in water (r2= 0.458). So long as high levels of Na and Ca in soil or water normally result to formation of salts on the banks of water sources after prolonged evaporation during dry seasons, the correlation established between fluoride and such elements in soil can enable people within volcanic areas to identify water sources with unacceptable levels of fluoride in their areas hence reducing the risks of fluorosis.

Water chemistry, and stable isotopes of oxygen and hydrogen (¹⁸O and ^2H respectively), were used to characterize and quantify Lake Duluti–groundwater interaction. Physico-chemical parameters: temperature, pH, electrical conductivity, dissolved oxygen, total dissolved solids, alkalinity, major cations and anions were used to determine chemical characteristics of the lake and to assess its relationship with groundwater sources. Physico-chemical parameters showed abundance of major cation and anions in the Lake water in the following order Na > Ca > K > Mg and HCO₃ > Cl > F > SO₄ > NO₃. The lake water was predominantly Na-HCO₃ type in both wet and dry season, while spring waters were mostly of Ca-HCO₃ type, and boreholes were of Ca-Na-HCO₃ type during the dry season. In the wet season, springs and boreholes were mostly of Na-HCO₃ and Na-K-HCO₃ types respectively. Isotopic results indicate that evaporation-induced isotopic enrichment prevails in the lake and contributes significantly to water loss from the lake. The δ¹⁸O of the lake water averaged +6.1 ‰ while that of well/boreholes and springs averaged −1.2 ‰ and −2.1 ‰ respectively. Similarly, the δD of lake water averaged +24.2 ‰ while that of well/boreholes and springs averaged −12.9 ‰ and −12.2 ‰ respectively. Stable isotope calculations indicate that the lake loses it water to the groundwater aquifer. Water balance model equations used to quantify the level of lake–groundwater exchange found that the Lake Duluti receives and recharges more groundwater than it receives from precipitation and surface runoff. Groundwater thus plays a major role in the hydrological system of Lake Duluti. The findings in this research are of assistance to policy makers and management personnel to make use of the information provided for better management of the lake water.