Hydrogeochemistry in the Ngorongoro Crater, Tanzania, and implications for land use in a World Heritage Site

Abstract

The chemistry of surface and ground waters in the Ngorongoro Crater, Tanzania, home to thousands of large mammals and a World Heritage Site, is controlled by the volcanic host rock lithology, evaporative concentration, mineral precipitation and redissoluton, and biological factors. Three groups of waters are informally differentiated based on their ranges of concentration: (1) dilute inflow (meteoric runoff and springs from short flowpaths: pH<8, Cl −<10 mg/l); (2) concentrated inflow (concentrated runoff and springs from long flowpaths: pH=8–9, Cl −=10–100 mg/l); and (3) brackish waters (pools and Lake Makat: pH>9, Cl −>100 mg/l). Evaporative concentration and biological activity in swamps commonly produce strong geochemical gradients between dilute sources and peripheral concentrated ephemeral wetlands. Dilute inflow is found in the Lerai, Munge, and Oljoro Nyuki streams, and several large springs near the Crater wall such as Ngoitokitok and Seneto. Concentrated inflow is found in downstream reaches of the Munge stream, discharging from springs away from the Crater wall such as Engitati and Mti Moja, and in dry-season pools. Brackish waters are found discharging from springs on the southern margin of Lake Makat, in mudflats surrounding marshes, in ephemeral pools, and in the lake itself. Although few hydrologic data are available, the persistence of relatively fresh water in vegetated wetlands is consistent with lower sedge-dominated wetland evapotranspiration rates compared with open water. This suggests that wetlands may play an important role in ensuring fresh water availability in the basin, and it demonstrates the need for future hydrologic study. Most of the Ngorongoro waters originate as rainfall outside the Crater, and travel into the basin as surface or ground water flow, emphasizing the need for a watershed-scale approach to land management.