The significance of groundwater flow for the hydrologic budgets of paleo-lakes in the East African rift

Abstract

The hydrogeology of rift lakes is complex due to the potential influence of faults and porous volcanic and volcanoclastic media on groundwater flow. We conducted a comprehensive study that integrated geological and hydrogeological data as well as the application of a linear decay model to estimate the groundwater flow between the two of the best-studied lake systems in East Africa, the adjacent Lakes Naivasha and Nakuru- Elmenteita. Whereas both lake basins host relatively shallow lakes today, paleo-shorelines and sediments suggest >100 m deep lakes. during a wetter climate during the Early Holocene during the so-called African Humid Period. The linear-reservoir depletion model simulates the decline of the Early Holocene lakes in both basins to the modern levels. The altitude difference of ca. 100 m of both paleo-lake levels enables us to estimate the duration of the groundwater decline and the connectivity of the two basins via the Eburru/Gilgil barrier. The results suggest that a decline of the groundwater levels during ca. 5 kyrs if there is no recharge, and between 2-2.7 kyrs with the modern recharge of 0.52 mlyr as the end_members of the delay time introduced by subsurface water flow to the hydrology of the lake system. The latter value suggests that ca. 31.5 cubic kilometres of water flowed from Lake Naivasha to Nakuru-Elmenteita at maximum lake level in the Early Holocene following the hydraulic gradient concurrent to the topographic slope. The unexpectedly large volume, more than half of the volume of the paleo-Lake Naivasha during the Early Holocene, emphasizes the importance of groundwater in hydrological modelling of paleo-lakes in rifts Moreover, the subsurface connectivity of rift lakes also causes a significant lag time to the system introduci.ng a nonlinear component to the system that has to be considered while interpreting paleo- lake records.