Sources of Shallow Groundwater Nitrate in the Vegetable Planting Area of Northern Henan and Its Response to Heavy Rainfall Events

In the vegetable cultivation regions of northern China, intensive use of chemical fertilizers and various types of manure has led to a significant rise in nitrate concentration in shallow groundwater. Despite this, there remains a deficiency in comprehensive isotope-based evidence regarding the origins and transformation pathways of elevated nitrate levels in these environments, coupled with an incomplete understanding of the impacts of heavy rainfall events on this matter. The oxidation process of ammonium salts within chemical fertilizers to nitrate is a key factor contributing to elevated nitrate levels in the shallow groundwater. During heavy rainfall, elevated water tables prompt the oxidation of residual ammonium in the soil and unsaturated zones into nitrate. This process will increase the concentration of nitrate in groundwater. To verify this hypothesis, we investigated the shallow groundwater in a vegetable plantation in northern Henan Province. Through the analysis concentrations and isotopic compositions (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) during the dry season (April 2021) and the wet season (October 2021), as well as hydrochemistry and hydrogen/oxygen isotopes of water (δD-H₂O and δ¹⁸O-H₂O), the sources of high concentration nitrate in shallow groundwater and their responses to heavy rain events were studied. Our findings shows that: (1) Nitrate levels during the wet season exhibited a broader range and a higher median (177.47 mg·L^-1) compared to the dry season (114.68 mg·L^-1); (2) Isotopic values (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) showed wider ranges during the wet season, with a median δ¹⁵N-NO₃^- increase from 7.3‰ to 7.8‰ and a δ¹⁸O-NO₃^- decrease from 6.4‰ to 5.1‰; (3) δ¹⁸O-H₂O range in shallow groundwater was narrower during the wet season, with a higher median (-8.8‰ vs. -9.2‰). Deuterium excess (d_e) values also exhibited a narrower range but a lower median of 9.5‰ vs. 6.9‰ during the dry season and wet season, respectively; (4) Heavy rainfall events resulted in the washout and conversion of residual ¹⁵N-enriched ammonium in the unsaturated zone into nitrate, leading to increased nitrate concentration, higher δ¹⁵N-NO₃^- values, and lower δ¹⁸O-NO₃^- values; (5) Bayesian isotope mixing models revealed that in the wet season nitrate primarily originates from increased manure contribution and decreased soil organic nitrogen. While ammonium nitrogen from fertilizers maintained a consistent contribution, nitrate nitrogen's contribution increases, and atmospheric nitrate contribution decreases. These findings confirm the influence of heavy rainfall events on the nitrification of residual ammonium and the subsequent rise in nitrate levels in shallow groundwater. Furthermore, rainfall acts as a carrier, transferring nitrate derived from ammonium conversion in surface manure to the shallow groundwater. Therefore, adopting effective fertilizer management practices in vegetable cultivation areas is imperative to prevent excessive nitrate accumulation and preserve shallow groundwater quality.