Tracking Sources of excess nitrate discharge in Lake Victoria, Kenya for improved Nitrogen use efficiency in the catchment

Benjamin K. Nyilitya1, Stephen Mureithi2, Pascal Boeckx3

1 Department of Water Resources Management, Ministry of Water and Irrigation, Kenya ([email protected])

2 Department of Land and Water Management, University of Nairobi, Kenya

3 Department of Applied Analytical and Physical Chemistry, Ghent University, Belgium ([email protected])

Abstract

A study was conducted in three major rivers (Nzoia, Nyando, Sondu) draining the Kenyan side of L. Victoria catchment to establish sources of excess nitrate deposition into the Lake using isotopic techniques and hydrochemistry. Results show spatial variation in isotope signatures with enrichment in δ15N and δ18O values near towns and industries. For instance R. Nzoia after Eldoret town had δ15N and δ18O values of 13‰, 6‰ respectively while the river after Mumias sugar factory had 9‰, 10‰ isotope values respectively. A plot of δ15N versus δ18O indicates that most of nitrate from the three catchments originates from Soil Nitrogen and manure or sewage. This may be due to deforestation, charcoal burning, untreated effluent discharges amongst other environmental degradation and poor sanitary practices rampant in the area. However, sewage and industrial effluents has high contribution to river and ground water nitrate near towns and densely populated areas as observed by the enriched δ15N values for Kisumu City Rivers ranging 10‰ to 18‰. Low nitrate content with corresponding high δ15N and δ18O signatures was observed in ground water near Kisumu city indicating denitrification takes place in the area. In addition, surface water in Kisumu had similar and enriched nitrate isotope signatures to ground water indicating high ground water susceptibility to pollution by surface water. This observation is supported by stable water isotope data (δ 18O, δ 2H) which show that the source of groundwater in Kisumu area is evaporated surface water.