A hydrogeochemical survey of Kilimanjaro (Tanzania): implications for water sources and ages

Abstract

Kilimanjaro, Tanzania, the highest mountain in Africa, has undergone extensive hydrologic changes over the past century in an area where water resources are critical. A hydrochemical and isotopic synoptic sampling program in January 2006 is used to characterize hydrogeology, hydrology, and water quality of the area. Samples were collected from the summit and southern side of Kilimanjaro and the Moshi region (Tanzania). Sample sources included four glaciers, seven groundwater wells, 12 rivers, 10 springs, precipitation, and a lake. Analyses included major ion chemistry, stable isotopes of water (18O and D); in addition, seven samples were analyzed for tritium. The samples generally have good water quality with the exception of three samples with elevated fluoride concentrations (>3 mg/L) and elevated nitrate concentrations (>2.5 mg/L NO3 as N). There is a strong elevation control on stable isotopes, with an apparent elevation effect of – 0.1 ‰ δ18O per 100 m rise in elevation (R2 = 0.79). The results, including the tritium values, show that the hydrogeologic system is comprised of both local and regional flow systems, and that regional rivers are receiving significant inflow from shallow groundwater, and at very high elevations the hydrologic system is derived from groundwater, precipitation, and glacial melt water.