SPRINGS IN THE NGORONGORO VOLCANIC HIGHLAND: GEOLOGY, GEOMORPHOLOGY, AND HYDROLOGY

Rainfall on the Plio-Pleistocene stratovolcanoes of the Ngorongoro Volcanic Highland (NVH) in northern Tanzania provides the source of groundwater for the Ngorongoro Conservation Area, a national park that includes 2 designated Natural Heritage sites: Olduvai Gorge and Laetoli Footprint Site. The ~2300 m high NVH captures moisture from semi-annual monsoonal rains (~1200 mm/yr) from the southeast trade winds casting a dramatic rain shadow on the Serengeti Plain to the west and rift lowlands below. With evapotranspiration (~2500 mm/yr) exceeding annual rainfall, the hydrologic budget is negative and surface runoff is seasonal. Groundwater is shielded from evaporation and discharges into topographic lows or fault conduits to the surface. The water supply is buffered from seasonal variability in rainfall. Regional groundwater systems are critical to the local people, animals, and tourists and the demand for fresh water from springs and wells is growing exponentially.

Nine springs were studied to assess the water quality, longevity, and sustainability of the springs. The data include field observations, water quality measurements (pH, temperature, conductivity, and alkalinity), and geologic, topographic, and GoogleEarth maps. The springs fall into 4 distinct groups: 1. Groundwater table intersects surface; 2. Water intercepts an impermeable layer; 3. Groundwater follows fracture pathways in impermeable rock; and 4. Artesian flow up faults/fractures under a hydraulic head. Geochemical results indicate high water quality. Preliminary groundwater modeling suggests that the spring flows can persist from 100s to 1000 years after groundwater recharge; however, more modelling is crucial to assess spring longevity.

These spring settings provide the framework for understanding the mechanisms of groundwater supply in the region. As the global environment changes, these groundwater systems will be affected and, consequently, so will lives of the people and animals who depend on them.