Tracing nitrate sources in the groundwater of an intensive agricultural region

Agricultural non-point contaminants are the main source of groundwater nitrate in China. The source and transformation of groundwater nitrate in the intensive agricultural region must be understood. To elucidate the overall groundwater nitrate situation in Weifang (a typical representative intensive planting area), groundwater was sampled in 2009 and 2019, respectively. The hydrochemical compositions, nitrogen, and oxygen isotopes were measured to trace the source and transformation of the nitrate and quantify the proportional contribution of nitrate sources of groundwater (including sewage and manure (M&S), soil nitrogen (SN), chemical fertilizer (CF), and atmospheric deposition (AD)) in Weifang, China. The concentration of nitrates in groundwater generally decreased, and the groundwater quality improved significantly. The reasons for the variation of nitrate sources between 2009 and 2019 were likely due to the reduction in the amount of fertilizer, the number of livestock and the area of agricultural land. Land-use affected the concentration of nitrates in groundwater, which ranked as follows: vegetable greenhouse (VG) > winter wheat (WW) > drinking and domestic water (DW). Nitrification was the main process in the study area, and the effect of denitrification was weak. The study area was affected by mixed CF, SN, and M&S contaminants. According to the Bayesian isotope mixing model (SIAR), the proportional contribution of each source was ranked as follows: M&S (65.09%) > SN (17.97%) > AD (9.32%) > CF (7.63%). M&S is the main source of NO3−. In this research, identifying the source of nitrate pollution can cut off the source of pollution in a targeted way, ensure the safety of groundwater in Weifang and protect the agricultural environment of intensive agricultural area, which has significance for nitrate prevention and control in other places.