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

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.