Biogeochemical Mechanisms of HCO3–Ca Water and NO3− Pollution in a Typical Piedmont Agricultural Area: Insights from Nitrification and Carbonate Weathering

Water hardening and NO₃^- pollution have affected water quality globally. These environmental problems threaten social sustainability and human health, especially in piedmont agricultural areas. The aim of this study is to determine the biogeochemical mechanisms of HCO³-Ca water and NO₃^- pollution in a typical piedmont agricultural area (Qingshui River, Zhangjiakou, China). Here, an extensive biogeochemical investigation was conducted in a typical piedmont agricultural area (Qingshui River, China) using multiple hydrochemical, isotopic (δ²H-H₂O, δ¹⁸O-H₂O and δ¹³C-DIC) and molecular-biological proxies in combination with a forward model. In the region upstream of the Qingshui River, riverine hydrochemistry was dominated by HCO₃-Ca water, with only NO₃^- concentrations (3.08-52.8 mg/L) exceeding the acceptable limit (10 mg/L as N) for drinking water quality. The riverine hydrochemistry responsible for the formation of HCO₃-Ca water was mainly driven by carbonate dissolution, with a contribution rate of 49.8 ± 3.96%. Riverine NO₃^- was mainly derived from agricultural NH₄⁺ emissions rather than NO₃^- emissions, originating from sources such as manure, domestic sewage, soil nitrogen and NH₄⁺-synthetic fertilizer. Under the rapid hydrodynamic conditions and aerobic water environment of the piedmont area, NH₄⁺-containing pollutants were converted to HNO₃ by nitrifying bacteria (e.g., <i>Flavobacterium</i> and <i>Fluviimonas</i>). Carbonate (especially calcite) was preferentially and rapidly dissolved by the produced HNO₃, which was attributed to the strong acidity of HNO₃. Therefore, higher levels of Ca²⁺, Mg²⁺, HCO₃^- and NO₃^- were simultaneously released into river water, causing riverine HCO₃-Ca water and NO₃^- pollution in the A-RW. In contrast, these biogeochemical mechanisms did not occur significantly in the downstream region of the river due to the cement-hardened river channels and strict discharge management. These findings highlight the influence of agricultural HNO₃ on HCO₃-Ca water and NO₃^- pollution in the Qingshui River and further improve the understanding of riverine hydrochemical evolution and water pollution in piedmont agricultural areas.