IDENTIFICATION AND STUDY OF GROUNDWATER DISCHARGE SITES USING REMOTE SENSING, GEOHYDROLOGY, AND WETLAND CORES, LAKE EYASI BASIN, TANZANIA

Groundwater provides a reliable source of potable water in arid regions where surface water is prone to seasonal availability and pollutant contamination. Groundwater seepage is typically associated with impervious beds or occurs at the base of slopes or along faults and fractures. This study focuses on groundwater discharge sites (GDS) and associated wetlands along the northeast margin of Lake Eyasi, northern Tanzania. The Eyasi Basin is a half graben at the southern terminus of the East African Rift System and contains a saline alkaline lake. The lake margin has localized areas of GDS already documented by previous researchers, however the geologic context and sustainability of these freshwater sites are unknown.

Multiple remote sensing techniques were used to identify new GDS. Techniques included: use of Normalized Difference Vegetation Index (NDVI), temporal resolution studies, and visual interpretation. ASTER images spanning 10 years and Landsat images spanning 37 years were available for use. While this provided good temporal resolution, the spatial resolution of this type of imagery is too coarse to identify the smaller GDS. GDS are typically associated with wetlands that are able to support vegetation with high water requirements such as papyrus, palm trees, and sedges. The ability to support vegetation continually makes it possible to identify GDS in arid regions with remote sensing. In addition a number of likely GDS were identified based on visual observations of GoogleEarth imagery. GoogleEarth was also used to confirm GDS sites identified through the remote sensing techniques applied to ASTER and Landsat imagery. The high resolution and true-to-life color of GoogleEarth imagery makes it particularly useful in identifying clusters of vegetation.

The Eyasi Basin was visited in July 2011 to ground truth the sites identified with remote sensing. Numerous additional GDS were identified while in the field. Seven cores (totaling 3.2 m of sediment) were taken at 3 groundwater-fed wetlands. The analysis of the cores will reveal the sediment and organic record preserved in GDS, as well as information regarding their longevity. The determination of the hydrogeologic context of identified GDS should lead to an increased understanding of the general conditions that result in GDS in arid regions worldwide.