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

Abstract

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., Flavobacterium and Fluviimonas). 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.

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