Nitrate source apportionment in the complex Nyando tropical river basin in Kenya

Excess nitrate (NO3-) discharge into fresh water resources poses detrimental effects on ecosystems and human health, yet the understanding of its potential sources is lacking in many parts of Sub-Saharan Africa. This study integrated hydrochemistry, multi-isotope tracers (delta N-15-NO3-, delta O-18-NO3-, delta B-11) and a Bayesian mixing model (mixSIAR) to improve on the apportionment of multiple NO3- sources in the Nyando River basin of Lake Victoria, Kenya. River water was monitored spatially in the basin for hydro-chemical and isotopic parameters from July 2016 to May 2018. The data shows that NO3- concentrations in the basin are governed by the predominant land use. Mixed agriculture (MA) land use recorded significantly higher NO3- concentrations (8.8 +/- 10.6 mg L-1), compared to other land use zones: residential & industrial (RI) 3.4 +/- 2.2 mg L-1, sugarcane (S) 3.2 +/- 1.5 mg L-1 and tea & forest areas (TF) 3.0 +/- 1.1 mg L-1. Stable isotope data and hydrochemistry complemented each other in identifying the potential NO3- sources and their spatial-temporal variation in the basin. Boron isotope (delta B-11) data was categorically helpful in overcoming the limitations of delta N-15-, delta O-18-NO3- to discriminate between manure and sewage sources. delta B-11 specifically identified manure as the dominant source of river NO3- input in the MA, RI and S land use zones and also indicated soil N as the main river NO3- source in the TF land use. These findings were substantiated via nitrate source apportionment using a Bayesian isotopic mixing model which showed manure and/or sewage were the leading sources of river NO3- input in the basin, contributing 70% (MA), 65% (RI), 60% (S) and 46% (TF). Soil N ranked second with its highest proportional river NO3- contribution of 41%, recorded for the TF land use, followed by 29% (S), 24% (RI) and 18% (MA). Ammonium in fertilizers and/or rain was third contributing about 10% NO3- for all land uses, while NO3- fertilizers showed 1-2% contribution across the basin. Nitrate in precipitation was the least NO3- source, contributing <1% across the basin. In order to control excess NO3- discharge in the basin, better guidelines for animal manure use, land conservation measures and improvement of sanitation systems should be prioritized.

Automated summary

This study identified the main sources of nitrate in the Nyando River basin of Kenya and emphasized the importance of optimizing animal manure use, land conservation measures, and improving sanitation systems to control excess nitrate discharge. Multiple methods, including hydrochemistry, stable isotopes, and a Bayesian mixing model, were used to determine nitrate sources and their spatial-temporal variation in the basin, revealing the significance of manure and sewage as leading sources of nitrate input.