Evaluation of recharge areas of Arusha aquifer, northern Tanzania: application of water isotope tracers

In Arusha urban, northern Tanzania, groundwater contributes about 80% of the water supply. However, elevated fluoride levels and evidence of anthropogenic pollution have been reported in the groundwater around Mount Meru which is a water source for Arusha urban. This study aims at understanding the recharge areas and flow pathways of groundwater in what has been a poorly monitored area. The study uses the isotopic ratio of oxygen and hydrogen to estimate the groundwater recharge area and flow pathway. The results show the recharge elevation of groundwater is between 1800 and 3500 m above mean sea level on the slopes of Mount Meru. Fluoride showed an increasing trend with elevation while NO3 - decreased with sampling altitude. The high content of NO3 - and Cl in water samples from sources in the lower part of the study area is evidence of anthropogenic contamination. The average fluoride contents in the study area are 5.3 ± 0.4 mg/L greater than the limits of 1.5 mg/L and 4 mg/L set by the World Health Organization (WHO) and Tanzania respectively. The nitrate concentration of 83.9 mg/L at the lower elevation areas (<1400 m above mean sea level) exceeds the 50 mg/L WHO limit. The relationship of F- with δ18O and NO3 - suggests the leaching of fluoride in high altitudes and dilution in lower altitudes. Two flow pathways were identified, that start from the recharge area in the south and south-western slopes of Mount Meru towards the southern part of Arusha urban.