Assessment of interaction between surface and groundwater using isotope hydrology and hydrochemistry: the case of Usangu plain Mbeya, Tanzania

Usangu is sub-catchment within the Great Ruaha catchment in the Rufiji basin. The area hosts major social-economic activities demanding large water uses such as cultivated agriculture, wildlife and environmental water use. The research was designed to investigate the interaction between surface and groundwater for the purpose of understanding the recharge mechanism, hydrochemistry and estimate recharge amount using isotope hydrology technique, hydrochemistry and empirical formulas. Recharge mechanism was determined by the analysis of the stable isotopes (Deuterium and Oxygen-18) and tritium, results for Deuterium and Oxygen eighteen indicated that the groundwater is recharged by the precipitation and rivers. The tritium values showed that groundwater is young of meteoric in origin and has the recharge from the surface and therefore its exploitation doesn’t result in groundwater mining but the water is vulnerable to pollution from the recharge. Interpretation of the Cations and Anions on the Piper diagram showed that the dominant water type in the study area is Sodium-Potassium and Calcium Carbonate. Amount of precipitation was estimated by the use of empirical formulas then validated by the Recharge- Precipitation hydrological distributed, the estimated recharge is 131.47mm/year. The results obtained from stable and unstable isotope analysis correspond to the isotope study conducted in the area by IAEA in collaboration with Rufiji Basin Water Office which indicated that the groundwater is recharged by precipitation and rivers in turn there is seepage of the groundwater to the streams during the dry season, this flow is maintained in the rivers as the Base flow.