Identification of the sources and fate of NO3−-N in shallow groundwater around a plateau lake in southwest China using NO3− isotopes (δ15N and δ18O) and a Bayesian model

Abstract

Pollution by NO3−-N seriously threatens the quality of shallow groundwater (SG) around Erhai Lake, which is the 2nd largest source of freshwater in the plateau area in southwest China; further, NO3−-N affects the lake water quality and human health. We collected SG samples during the dry and wet seasons in 2018 and 2019, and the potential NO3−-N sources and their fates were identified in SG by NO3− isotopes and hydrochemical methods. Our results showed that the NO3−-N concentrations in the SG in the wet season in farmland were far higher than those in the dry season in residential areas. The high variation in δ15N–NO3- and δ18O–NO3- (from −12.78‰ to +18.10‰ and −27.62‰ to +23.07‰, respectively, in the farmland and from −5.34‰ to +34.54‰ and −20.04‰ to +17.47‰, respectively, in the residential area) indicated multiple NO3−-N sources in the SG. The NO3−-N in the farmland mainly originated from chemical nitrogen fertilizer (NF, 36%), soil nitrogen (SN, 33%) and manure and sewage (M&S, 24%) in the dry season and from SN (61%) and NF (33%) in the wet season. The NO3−-N in the residential area mainly originated from M&S (57%), SN (23%) and NF (14%) in the dry season and from SN (50%), NF (25%) and M&S (24%) in the wet season. Nitrogen transformation was dominated by denitrification in the SG. The most polluted SG area was observed on the east bank of Erhai Lake, NO3−-N mainly originated from NF. But the NO3−-N pollution slowed down from high altitude to lakeside and had multiple NO3−-N sources on the west bank of Erhai Lake. The SG was contaminated by nitrogen from NF, SN and M&S along the flow path and flowed into Erhai Lake. Therefore, reducing soil nitrogen concentrations and chemical nitrogen fertilizer applications and improving sewage facilities are significant ways to mitigate nitrate pollution in the SG.