Identification of groundwater flow systems using geological mapping and chemical spring analysis in South Pare Mountains, Tanzania

Population growth on the hill slopes of the semi-arid South Pare Mountains in Northeast Tanzania has increased water usage. In this area, water availability is directly dependent on the groundwater/surface water interaction, which in turn is dependent on the hydrogeology of the catchment. In order to better understand the hydrogeology of the area, a groundwater flow system analysis was conducted for two adjacent meso-scale catchments (Makanya and Mbaga, a sub-catchment of the Mkomazi catchment; both around 300 km2). Geological mapping and spring sampling were done. Thirty spring- and groundwater samples were collected from inside and around the catchment and analysed for all major cations and anions, including dissolved silica. Due to dipping and faulting of the geological units, a substantial amount of water drains out of the Makanya catchment into the Mbaga catchment. Springs found in the elevated parts of the catchments, contribute to base flow and are found in old debris flow deposits, consisting of weathered parent rocks. Water quality in these parts is mainly determined by silicate weathering due to oxidation of organic matter and evaporation. Concentrations of ions generally increase with decreasing altitude. Springs found in the lower parts of the catchments contain high amounts of sodium, chloride and sulphate, different from the water quality found uphill. We concluded that two distinct flow systems exist in the catchment; a local system, defined by the limited thickness of the weathered material and debris flow deposits, and a regional flow system that is most likely controlled by regional tectonics. This case-study demonstrates how geological mapping and chemical methods can be used to analyse groundwater flow systems.