Coupling Bathymetric Survey and Sediment Geochronology to Model Impacts of Soil Conservation Practices in Lake Naivasha Basin, Kenya

Abstract

Surface waterbodies are sources of ecosystem functions and support various socio-economic activities. However, these waterbodies are threatened by sedimentation which may result in loss of volume and water quality alteration. Thus, the objective of this study was to assess geochronological sedimentation status of Lake Naivasha in Kenya with a view of modelling impacts of conservation practices. This was achieved by conducting bathymetric survey using multifrequency Acoustic Profiling System (APS), sediment coring, geochronological (using 210Pb and 137Cs radionuclide) and geochemical analysis of sediment. Geochemical analysis of P, Al, As, Cr, Cu, Fe, Mn, Ni, Pb and Zn was conducted using Inductively Coupled Plasma Optical Emission Spectrometry (ICP OES). Sources and contamination levels of these elements were established using multivariate analysis and pollution indices such as enrichment factor. The impacts of adopting conservation practices in Lake Naivasha basin were modelled using Soil Water Assessment Tool (SWAT) and geochronological data. The results showed that between July and October 2016, mean depth, volume and surface area of Lake Naivasha was 4.68 m, 722 x 106 m3 and 154.17 x 106 m2, respectively. Sediment cores, dated up to about 140 years, showed that the mean mass sedimentation rate of the lake is 0.32 g/cm2/yr. The sediment load into lake Naivasha from 1966 to 1996 and 1996 to 2016 was found to be 2.78 x 105 and 4.61 x 105 tons/yr, respectively. The difference in sediment load indicates increased anthropogenic activities upstream. Elements (P, As, Fe, Mn and Zn) in Lake Naivasha sediment were found to be from both natural and anthropogenic sources while the rest (Al, Cr, Cu, Ni, and Pb) were from natural sources only. Adoption of filter strips and terraces, as conservation practices on critical and agricultural land, would reduce cumulative sediment load into Lake Naivasha by up to about 30% and 27%, respectively. The impact of these practices increased by up to 11% when implemented in all sub basins with agriculture. It was found that with use of multifrequency APS in combination with dated sediment cores, it is possible to assess the sedimentation status of a natural lake that has no comparable bathymetric data or known pre-impoundment layer like man made reservoirs. Coupling bathymetric survey, sediment geochronology and SWAT model can inform the choice of plausible intervention measures that would reduce sedimentation of waterbodies.