Coupled processes involving ammonium inputs, microbial nitrification, and calcite dissolution control riverine nitrate pollution in the piedmont zone (Qingshui River, China).

Rivers in agricultural countries widely suffer from diffuse nitrate (NO ₃ ^- ) pollution. Although pollution sources and fates of riverine NO ₃ ^- have been reported worldwide, the driving mechanisms of riverine NO ₃ ^- pollution associated with mineral dissolution in piedmont zones remain unclear. This study combined hydrogeochemical compositions, stable isotopes (δ ¹⁸ O-NO ₃ ^- , δ ¹⁵ N-NO ₃ ^- , δ ¹⁸ O-H ₂ O, and δ ² H-H ₂ O), and molecular bioinformation to determine the pollution sources, biogeochemical evolution, and natural attenuation of riverine NO ₃ ^- in a typical piedmont zone (Qingshui River). High NO ₃ ^- concentration (37.5 ± 9.44 mg/L) was mainly observed in the agricultural reaches of the river, with ~15.38 % of the samples exceeding the acceptable limit for drinking purpose (44 mg/L as NO ₃ ^- ) set by the World Health Organization. Ammonium inputs, microbial nitrification, and HNO ₃ -induced calcite dissolution were the dominant driving factors that control riverine NO ₃ ^- contamination in the piedmont zone. Approximately 99.4 % of riverine NO ₃ ^- contents were derived from NH ₄ ⁺ -containing pollutants, consisted of manure & domestic sewage (74.0 % ± 13.0 %), NH ₄ ⁺ -synthetic fertilizer (16.1 % ± 8.99 %), and soil organic nitrogen (9.35 % ± 4.49 %). These NH ₄ ⁺ -containing pollutants were converted to HNO ₃ (37.2 ± 9.38 mg/L) by nitrifying bacteria, and then the produced HNO ₃ preferentially participated in the carbonate (mainly calcite) dissolution, which accounted for 40.0 % ± 12.1 % of the total riverine Ca ²⁺ + Mg ²⁺ , also resulting in the rapid release of NO ₃ ^- into the river water. Thus, microbial nitrification could be a new and non-negligible contributor of riverine NO ₃ ^- pollution, whereas the involvement of HNO ₃ in calcite dissolution acted as an accelerator of riverine NO ₃ ^- pollution. However, denitrification had lesser contribution to natural attenuation for high NO ₃ ^- pollution. The obtained results indicated that the mitigation of riverine NO ₃ ^- pollution should focus on the management of ammonium discharges, and the HNO ₃ -induced carbonate dissolution needs to be considered in comprehensively understanding riverine NO ₃ ^- pollution in piedmont zones.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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