Extreme precipitation accelerates the contribution of nitrate sources from anthropogenetic activities to groundwater in a typical headwater area of the North China Plain

Abstract

Identification of diffused nitrate (NO3–) sources in groundwater is challenging in headwater areas with diverse land-uses, multiple potential inputs and intensification of precipitation. Although NO3– sources in headwater areas with different land-uses have been observed, the mechanisms of the impact of extreme precipitation on NO3– transport to groundwater and NO3– sources in these areas are not well understood. In this study, dual isotopic compositions of NO3– (δ15N-NO3– and δ18O-NO3–) and hydrochemical data were measured in groundwater samples during August 2014-January 2017 under four land-uses: natural vegetation (NV), farmland (FL), economic forestland (EF), and residential areas (RA). An isotope mixing model (SIAR) was applied to determine the main NO3– sources and the governing factors. Affected by extreme precipitation, the rates of exceeding the WHO standard of NO3– (45 mg/L) increased to 93.3%, 65.6%, 100%, and 46.7% for groundwater samples from RA, FL, EF, and NV, respectively, when comparing with 88.7%, 56.6%, 92.9%, and 6.1% of samples collected before extreme precipitation from RA, FL, EF and NV, respectively. SIAR results showed that the contribution of anthropogenetic activities to NO3– in groundwater was accelerated after extreme precipitation. Before extreme precipitation, soil N (43.5%) and organic matter of soil (32.3%) were the dominant NO3– source in NV, and manure/sewage made substantial contributions to NO3– concentrations in groundwater, accounting for 55.8%, 56.2%, and 52.7% in RA, FL, and EF, respectively. In NV, the contributions of organic matter of soil increased to 46.8% and the contribution of soil N decreased to 28.9%. The contributions of chemical fertilizers in soil increased from 15.2% to 31.3% and there was a corresponding decrease in the contribution from manure/sewage (41.1%) in FL, although the contribution of NO3– from manure/sewage increased in EF (66.3%) and RA (57.6%). Assessing temporal variation of NO3– sources in the headwater areas of the North China Plain can help to protect water resource and avoid further deterioration of water quality during land-use management and extreme precipitation in the future.