LAKE-GROUNDWATER RELATIONSHIPS AND FLUID-ROCK INTERACTION IN THE EAST-AFRICAN RIFT-VALLEY - ISOTOPIC EVIDENCE

The assessment of water resources in the Rift Valley environment is im portant for population, agriculture and energy-related issues and depe nds on a good understanding of the relationship between freshwater lak es and regional groundwater. This can be hampered by the amount of flu id-rock interaction which occurs throughout the rift, obscuring origin al hydrochemical signatures. However, O and H stable isotope ratios ca n be used as tracers of infiltration over sometimes considerable dista nces, while showing that the volcanic edifices of the rift floor have varying effects on groundwater flow patterns. Specific cases from Keny a and Ethiopia are considered, including Lakes Naivasha, Baringo, Awas a and Zwai. In addition to their physical tracing role, stable isotope s can reveal information about processes of fluid-rock interaction. Th e general lack of O isotope shifting in rift hydrothermal systems sugg ests a high water:rock ratio, with the implication that these systems are mature. Carbon isotope studies on the predominantly bicarbonate wa ters of the rift show how they evolve from dilute meteoric recharge to highly alkaline waters, via the widespread silicate hydrolysis promot ed by the flux of mantle carbon dioxide which occurs in most parts of the rift. There appears to be only minor differences in the C cycle be tween Kenya and Ethiopia. Copyright (C) 1996 NERC.