Conceptual groundwater model of the north-eastern flanks of Mount Kenya

Abstract

Despite serious concerns over declining river flows and prolonged dry spells in the north-eastern region of Mount Kenya and the Ewaso Ng’iro River watershed many aspects of the groundwater system remain unexplored. In particular, the recharge-discharge dynamics of the Ewaso Ng’iro River have not been studied, and no conceptual groundwater model currently links the recharge areas in the high-elevation humid regions to the drier lowlands. This study aims to address this significant knowledge gap by assessing the recharge-discharge dynamics of the Ewaso Ng’iro River and identifying the relevant groundwater subsystems and the main flow paths within the various lava layers constituting the aquifer system. Hydrochemical and stable isotope analyses revealed three distinct subsystems with slightly different chemistries and different recharge zones, all of recent meteoric origin. Groundwater from Mt. Kenya is of the HCO₃-Na-Mg-Ca type with no dominant cations, whereas groundwater from the Nyambene Range, the other surrounding volcanic hills in the area, is of the HCO₃-Na type. Groundwater in the third subsystem in between is of the HCO₃-Mg type and is confined or semi-confined. In this area, carbon-13 analysis showed a strong influence of mantle-derived CO₂ on the groundwater chemistry (very high electrical conductivity and left-shifted oxygen-18 ratios). Finally, major ions and stable isotopes (ẟ¹⁸O and ẟ²H) confirmed that during the dry season, the river is entirely groundwater-fed, with the Mt. Kenya subsystem contributing half of the maximum river flow rate of 800 l/s.