Sources and hydro-geochemical characteristics of lake Duluti waters, Tanzania

Abstract

Water chemistry, and stable isotopes of oxygen and hydrogen ( 18O and 2H), were used collectively to characterize and quantify Lake Duluti (L. Duluti) – groundwater interaction. Physico-chemical parameters including 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. Furthermore, water balance approaches were used to estimate surrounding ground-water exchange with L. Duluti and addressing the role of groundwater on lake hydrological system. Physico-chemical parameters results showed abundance of major cations and anions in the lake water in the following order Na+ >Ca2+>K + >Mg2+ and HCO3 - >Cl- >F- >SO4 2- >NO3 - respectivelly. Water in L. Duluti was found to be of predominantly Na-HCO3 type while that from springs and boreholes was of the Ca-HCO3 and Ca-Na-HCO3-types, respectively. Isotopic results indicated that evaporation-induced isotopic enrichment prevailed in L. Duluti and contributed significantly to water loss from the lake. The isotopic composition of oxygen (δ 18O) of lake water averaged sigma notation(‰) of 6.1 ‰ while that of well/boreles and spings averaged -1.2 ‰ and -2.1 ‰ respectively. Similarly, the isotopic composition of hydrogen (δD) of lake water averaged 24.2 ‰ while that of well/boreles and spings averaged -12.9 ‰ and -12.2 ‰ respectively. Stable isotope calculations suggested that L. Duluti loses water to the aquifer and it is more recharged by the groundwater relative to precipitation and surface runoff. Groundwater inflow to the lake is approximately 2 430 960 meter cubic per year (m 3 /yr) while lake water discharge to groundwater is 2 902 620 m 3 /yr. The lake is recharged through precipitation by 612 000 m 3 /yr. Hence, groundwater plays a major role in the hydrological system of L. Duluti. Based on these findings from the study, there is more groundwater outflow than inflow, hence citing of boreholes in the area should be properly done so as to maintain the state of the lake or groundwater aquifers. In the long run, pumping water from the lake may also introduce more groundwater inflows and less outflows. 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. The information will also enable the Arusha water supply and sewerage authority to know the hydrological state of L. Duluti.