Application of isotopes and water balance on Lake Duluti–groundwater interaction, Arusha, Tanzania OF ISOTOPES AND WATER BALANCE ON LAKE DULUTI-GROUNDWATER INTERACTION, ARUSHA, TANZANIA

Abstract. 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.