Spatiotemporal changes of nitrate retention at the interface between surface water and groundwater: Insight from watershed scale in an elevated nitrate region

Abstract

AbstractUnderstanding the spatiotemporal nitrate retention in streambed is essential for developing management practices in reducing nitrate enrichment. However，the process of nitrate change in the profile of streambed at an elevated nitrate across a watershed scale is still not sufficiently investigated. In this study, we used a combination of hydraulic and hydro‐geochemistry methods to quantify total nitrate retention in streambeds of an agriculture‐intensive watershed in Central China. To conduct surface and groundwater measurements, we collected 1440 water samples for nitrate analysis from 40 shallow drilled wells during the dry and wet seasons from 2018 to 2020. The results showed a clear spatiotemporal variation of nitrate retention in streambed in the watershed. Spatially, nitrate retention in the midstream and downstream reaches was higher than that of the upstream reach. The lowest point of nitrate retention in downstream both in dry and wet seasons was at the depth of 0.75 m. While the lowest nitrate retention was found in midstream and upstream reaches, both in the dry and wet seasons at the depth between 1.5 and 2.5 m. Temporally, nitrate retention was higher in the wet season (1.56 μg N m−2d−1) than in dry season (1.41 μg N m−2d−1). DO min at 3 mg/L was found to the nitrate retention zero threshold in up and midstream. Water change fluxes and nitrate retention both have positive and negative relationship at watershed scale. Nitrate retention at the watershed scale was strongly affected by streambed lithology, precipitation, surface water ‐ groundwater exchange, and human activities. Those findings can provide reference for nitrate removal in international important agricultural areas.