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

Abstract

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 (δ15N-NO3−, δ18O-NO3−, δ11B) 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 (δ11B) data was categorically helpful in overcoming the limitations of δ15N-, δ18O-NO3− to discriminate between manure and sewage sources. δ11B 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.