Aquifer stratigraphy and hydrogeochemistry of the lake nakuru basin, central kenya rift

Abstract

The geology of Nakuru area comprises mainly of volcanic soils and rocks (lava and pyroclastics) of Tertiary – Quaternary age, which has been affected by a series of faulting, and are overlain by recent sediments. This study considers groundwater to be the chief source of water for the expanding agricultural, industrial, domestic and wildlife demands, and thus there are possible negative effects on groundwater in the basin due to depletion of this resource and deterioration of its quality. Apart from anthropogenic factors, the geology of the area seems to contain a great potential to pose a risk to the regional groundwater variably depending on their/its elemental composition due to their persistent interaction with the water. The general objective of this study was to relate aquifer stratigraphy and lithology to groundwater geochemistry and quality by assessing and determining the stratigraphy, lithology and spatial extent of aquifers present with respect to the possible influence on hydrogeochemistry. The approaches pursued here include the chemical analyses of major ions and heavy metals for constructing hydrogeochemical maps with focus on aquifer material and distribution. A total of 55 deep boreholes, 3 shallow boreholes, 3 springs and 19 river sites were sampled and water analyses performed for the major ionic composition, together with fluorite and selected heavy/trace metals. Saturation indices were calculated, ionic relationships were studied, hydrochemical facies were determined and water types were identified. Driller’s logs from 33 boreholes were utilized to assess aquifer lithology and hydrogeostratigraphy. The data was analyzed and in effect interpreted by the help of appropriate computer programs such as SPSS, WinLog, AquaChem, Rockworks and PHREEQC. Results indicated that aquifer distribution varies from single to three aquifer systems with three major aquifer lithologies. These include fissured trachyte, volcanosediments and pyroclastics revealing a dominant trend in lowlands (<2000m.a.s.l), midlands (2000 – 2200m.a.s.l) and highlands (>2200m.a.s.l) of the study area, respectively. Lithological logs analyses show aquifer thickness variation that ranges from 2 to 46 meters. The composition of dissolved ions in the groundwater is order of HCO3->Cl->SO42->NO3- (anions) and Na+>K+>Ca2+>Mg2+ (cations), with considerable amounts of some heavy/trace metals (in the order of Zn>Fe>Mn>Pb>Cd>Cr). The principal processes that control the groundwater quality variations are the influence of silicate rocks weathering and anthropogenic contribution which is also significant. Most of these ions surpass local and international recommended limits and areas with lofty values have been defined. The outcome of this study provides information needed for groundwater quality budgeting and management.