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Land use controls Kenyan riverine nitrate discharge into Lake Victoria – evidence from Nyando, Nzoia and Sondu Miriu river catchments*

ABSTRACT Nitrate (NO3-) sources and discharge were investigated using isotope and hydrochemical analyses in three river catchments draining Lake Victoria basin, Kenya. Hierarchical cluster analysis grouped Nyando, Nzoia and Sondu Miriu River stations into clusters corresponding to major land use classes of the catchments. Mixed agriculture (MA) in Nyando showed higher NO3− compared to the other land uses. Nitrate levels obtained (0.1–11.6 mg L−1) are higher than those reported in previous studies. Hydrochemistry support isotope data indicating that ammonium-based fertilizers and soil N were the major NO3- sources in tea dominated areas with average δ15N (6.5 ± 1.3 ‰), δ18O (6.7 ± 2.3 ‰) values. Manure/sewage were the main source in MA areas with average δ15N (8.4 ± 2.4 ‰), δ18O (7.8 ± 5.4 ‰) values. Sewage was the likely source in urban, residential & industrial areas recording average δ15N (10.0 ± 2.4 ‰), δ18O (6.9 ± 3.7 ‰) values. δ15N between land uses were significantly different (p < 0.0001) while δ18O were similar (p = 0.4). Seasonally, inorganic/organic fertilizers influenced river NO3− mostly in the wet cropping season. Lower NO3− concentrations observed in Nyando and Sondu Miriu during dry or start-wet season could be a result of in situ denitrification.