Assessment of groundwater abstraction and hydrogeochemical investigation in Arusha city, northern Tanzania

Abstract

In this work geological and hydrogeochemical data, radioisotopes and stable isotopes, and groundwater hydrographs were used to assess the groundwater abstraction trends and hydrogeochemical characteristics of Arusha wellfield in Arusha city. Groundwater salinity in terms of conductivity (EC) was also used to delineate salinity occurrence and distribution in different parts of Tanzania including Arusha where this study was carried out. The hydrogeochemical results revealed Na-K-HCO 3 water type. Water-rock interaction seems to be the main process determining the groundwater chemistry in the study area. The analysis of geological sections showed two potential aquifers, volcanic sediment and weathered/fractured both of which yield water with high fluoride. Eighty two (82) percent of the analyzed groundwater samples indicated fluoride concentrations higher than WHO guidelines and Tanzanian drinking water standards (1.5 mg/l). Groundwater hydrographs indicated significant groundwater depletion. Water level decline of about 1.0 m/year and discharges reduction of 10 to 57% were observed from the year 2000 to 2017. The radiocarbon isotope signatures showed that groundwater with mean age of 1400 years BP to modern was being abstracted from the wellfield. Recently recharged water was also evidenced by high 14 C activities (98.1±7.9 pMC) observed in spring water. Both groundwater hydrographs and isotope signatures suggest that the Arusha wellfield is already stressed due to groundwater over-abstraction. Through groundwater salinity mapping, it was revealed that generally Arusha has fresh groundwater but with relatively high electric conductivity (10002000

µS/cm). The high salinity levels are partly due to dissolution of trona (evaporate mineral) commonly found in the East African Rift System. It was further revealed that lack of reliable hydrogeological information including interaction between surface water and groundwater hinders water resources management efforts particularly issuance of water use permit. Based on the findings of this study, it is recommended to carry out groundwater flow patterns modelling to show how unregulated drilling affects the deep wells currently depletion problem.