1 APPLICATION OF ISOTOPES AND WATER BALANCE ON 1 LAKE DULUTI-GROUNDWATER INTERACTION, ARUSHA, 2 TANZANIA 3 Nancy P. Mduma 1 , Hans C. Komakech 1 , Jing Zhang 2 , Alfred N.N Muzuka 1 4 1 Department of Water and Environmental Science and Engineering, Nelson Mandela African Institution of Science and 5 Technology, 23311, Tanzania 6 2 State Key Laboratory of Estuarine and Coastal Research, Shanghai, 200062, China 7 Correspondence to: Alfred N.N Muzuka (alfred.muzuka@nm-aist.ac.tz) 8 Abstract. Water chemistry, and stable isotopes of oxygen and hydrogen ( 18 O and 2 H respectively), were used to characterize 9 and quantify Lake Duluti – groundwater interaction. Physico-chemical parameters: temperature, pH, electrical conductivity, 10 dissolved oxygen, total dissolved solids, alkalinity, major cations and anions were used to determine chemical characteristics 11 of the lake and to assess its relationship with groundwater sources. Physico-chemical parameters showed abundance of major 12 cation and anions in the Lake water in the following order Na>Ca>K>Mg and HCO 3 >Cl>F>SO 4 >NO 3 . The lake water was 13 predominantly Na-HCO 3 type in both wet and dry season, while spring waters were mostly of Ca-HCO 3 type, and boreholes 14 were of Ca-Na-HCO 3 type during the dry season. In the wet season, springs and boreholes were mostly of Na-HCO 3 and Na- 15 K-HCO 3 types respectively. Isotopic results indicate that evaporation-induced isotopic enrichment prevails in the lake and 16 contributes significantly to water loss from the lake. The δ 18 O of the lake water averaged +6.1‰ while that of well/boreholes 17 and springs averaged -1.2‰ and -2.1‰ respectively. Similarly, the δD of lake water averaged +24.2‰ while that of 18 well/boreholes and springs averaged -12.9‰ and -12.2‰ respectively. Stable isotope calculations indicate that the lake loses 19 it water to the groundwater aquifer. Water balance model equations used to quantify the level of lake - groundwater 20 exchange found that the Lake Duluti receives and recharges more groundwater than it receives from precipitation and surface 21 runoff. Groundwater thus plays a major role in the hydrological system of Lake Duluti. The findings in this research are of 22 assistance to policy makers and management personnel to make use of the information provided for better management of 23 the lake water. 24 25 Keywords: Water balance, Isotopes, Crater Lake, groundwater, hydrochemistry 26 Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-176, 2016 Manuscript under review for journal Hydrol. Earth Syst. Sci. Published: 8 June 2016 c  Author(s) 2016. CC-BY 3.0 License.