GEOCHEMICAL EVIDENCE FOR GROUNDWATER :LAKE INTERACTIONS, LAKE NAIVASHA, KENYA

Lake Naivasha is a freshwater lake in the Central Rift Valley, Kenya that has inflow rivers, but no surface outlet. It is unique among Rift Valley lakes in that it is fresh. Previous speculation suggested that the lake loses water through the subsurface, which keeps it fresh. We have used rare earth elements (REE’s) and strontium isotopes to help establish the hydrologic linkages between the lake and the local groundwater system. Water from shallow wells nearest the southern shores of the lake has ⁸⁷Sr/⁸⁶Sr ratios similar to the lake. The wells to the east of the lake, as well as water from the Olkaria geothermal field to the south, have more radiogenic ratios. We have interpreted this to mean that the more radiogenic waters are older and have longer flow paths. The REE data support the notion that groundwater north of the lake, and the lake water have a similar source. As the lake water flows out of the lake to the south, the REE geochemistry changes dramatically as the light REE’s are lost from solution. The geothermal waters have also been greatly modified geochemically. These data support our earlier work suggesting that the lake-groundwater system is closely connected, and the lake acts as a source of groundwater south of the lake.