Relationship between land-use and sources and fate of nitrate in groundwater in a typical recharge area of the North China Plain

Abstract

Identification of different nitrate sources in groundwater is challenging in areas with diverse land use and multiple potential inputs. An area with mixed land-uses, typical of the piedmont-plain recharge area of the North China Plain, was selected to investigate different nitrate sources and the impact of land use on nitrate distribution in groundwater. Multiple environmental tracers were examined, including major ions, stable isotopes of water (δ2H-H2O, δ18O-H2O) and nitrate (δ15N-NO3− and δ18O-NO3−). Groundwater was sampled from four land-use types; natural vegetation (NV), farmland (FL), economic forestland (EF) and residential areas (RA). A mixing model using δ18O and Cl− concentrations showed that groundwater recharge predominantly comprises precipitation and lateral groundwater flow from areas of natural vegetation in the upper catchment, while irrigation return water and wastewater from septic tanks were major inputs in farmland and residential areas, respectively. Land use variation is the major contributing factor to different nitrate concentrations. In total, 80%, 49% and 86% of samples from RA, FL and EF, respectively exceeded the WHO standard (50mg/L NO3−), compared to 6.9% of samples from NV. Isotopes of δ15N-NO3− and δ18O-NO3− verified that nitrate in groundwater of the NV (with δ15N ranging from 1.7‰ to 4.7‰) was sourced from soil and precipitation. Examination of δ15N-NO3− vs δ18O-NO3− values along with multivariate statistical analysis (principle component and cluster analysis) helped identify sources with overlapping isotopic values in other land-use areas (where δ15N values range from 2.5‰ to 10.2‰). Manure and septic waste were dominant sources for most groundwater with high NO3− and Cl− concentrations in both farmland and residential areas. The lack of de-nitrification and fact that the area is a recharge zone for the North China Plain highlight the importance of controlling nitrate sources through careful application of manure and fertilizers, and control of septic leakage.