Geochemical fingerprints of waters in the Lake Baringo-Bogoria region, Kenya: Implications for hydrogeochemical processes and water quality

Lake Baringo and Lake Bogoria lie within an asymmetric half-graben in the East African Rift, Central Kenya. Lake Baringo to the north is fresh because of subsurface outflow (~0.8 g/l TDS; pH=8.9) whereas to the south, Lake Bogoria, which is hydrologically closed, is saline/alkaline (mixolimnion: ~40 g/l TDS; pH=10.2). The climate is semi arid (P<ET). P on the valley floor is ~700 mm/yr and ~1200mm/yr on the adjacent highlands; ET is ~2500 mm/yr. The Loboi Plain (~200 km^{2}) between the two lakes is comprised of Pleistocene and Holocene alluvium and lake sediments >1 km thick. The primary sources of water in the area are rivers draining the highlands, and hot, warm and cool springs and seeps of different salinities associated with the faults. Freshwater wetlands and rivers traversing the Loboi Plain represent a limited resource in this semi-arid environment and are heavily utilized for drinking water by the resident population, livestock, and wildlife, and for crop irrigation. To better understand the hydrogeochemical processes affecting the freshwater sources in the southern Loboi Plain, water samples that were collected from lakes, rivers, springs, and one of the large wetlands (Loboi Swamp) were analyzed for a full suite of major and minor elements. The objective of the study was to determine the extent to which the composition of water from a variety of environments could be used as geochemical fingerprints for hydrogeochemical processes. Conservative mixing between the most dilute river-water endmember (~0.1g/l TDS) and the most concentrated Lake Bogoria surface water, is indicated by the covariation of Na^{+} with (Alk_{T}+Cl^{-}) and with F^{-} content for nearly all surface and spring waters in the region. This trend is consistent with an overall process of evapoconcentration of dilute meteoric water containing dominantly Na^{+}, HCO_{3}$^-, and Cl^- derived from weathering of intermediate (trachyphonolite) and basaltic volcanic rocks in the region. Spring waters feeding the Loboi Swamp are warm (~35°C), with pH~6.4-6.9, and compositions (~0.25 g/l TDS) consistent with interaction of relatively shallow meteoric water with host rocks. Surface waters in the wetlands are more concentrated (~0.3-0.5g/l TDS) and more alkaline (pH=7.1-8.2) than associated springs consistent with degassing of dissolved CO₂, evaporation, and redissolution of previously deposited salts. Porewaters in marsh sediments contain higher concentrations of dissolved inorganic carbon, NH₄^{+}$, and Fe than overlying surface water consistent with biologically-mediated organic-matter oxidation under reducing conditions. Each aqueous setting has a distinctive geochemical fingerprint that reflects the initial composition (i.e. source water) and the effects of subsequent physical, chemical and biochemical processes operating in the environment.