Strontium isotopes and rare earth elements as tracers of groundwater-lake water interactions, Lake Naivasha, Kenya

Strontium isotope compositions and rare earth element (REE) concentrations are presented for groundwater and surface water samples collected from the Lake Naivasha watershed in the East African Rift, Kenya. The chief objective of the study is to test the suitability of REEs, in conjunction with Sr isotopes, as tools for investigating groundwater-lake water interactions. In general, the REE concentrations and ⁸⁷Sr/⁸⁶Sr ratios support the authors' earlier investigations where Cl mass balance, δ¹⁸O, δD, and He isotopes were employed to study groundwater-lake water interactions in the Naivasha watershed. The REE data suggest that a significant amount of the groundwater south of Lake Naivasha (i.e., 50-85%) consists of lake water recharge to the aquifer system. Specifically, mixing calculations conducted using REE data of Lake Naivasha water and groundwaters indicate that between 70 and 85% of groundwater directly south of the lake is likely lake water. These values are somewhat higher than the authors' previous estimates determined with conservative stable H isotopes (δD, 50-70%). For both cases, however, the data demonstrate that water originating in Lake Naivasha contributes significantly to the underlying groundwater flow system, hence supporting earlier evidence that the lake's freshness reflects rapid loss of water and dissolved solutes to the local groundwater system. Overall, lake and groundwater Sr isotope compositions support seepage of lake water into the underlying aquifer along the lake's south shore. The ⁸⁷Sr/ ⁸⁶Sr data also provide additional insight into the geochemical evolution of waters of the Lake Naivasha watershed indicating that the initial source of Sr to these waters is likely chemical weathering reactions involving basaltic rocks within the recharge zones of the watershed along the Rift Valley flanks. Furthermore, with increasing residence time of groundwaters within the aquifer system and flow down and along the rift valley floor, Sr isotope compositions of groundwaters become more radiogenic, reflecting rock-water interactions with chemically differentiated and radiogenic peralkaline rhyolite volcanic rocks. The importance of the longer aquifer residence times and radiogenic source rocks is especially apparent for geothermal waters of the Olkaria Geothermal Field that have ⁸⁷Sr/⁸⁶Sr ratios (i.e., 0.70747) similar to local comendites.