Impact of occurrence conditions on NO 3 − -N source apportionment in groundwater: insights from PCA-APCS-MLR and MixSIAR methods

Nitrate-N (NO3--N) contamination in groundwater poses a significant threat to drinking water safety and ecosystem health, with accurate source identification being crucial for effective pollution control. Previous studies on NO3--N source apportionment in groundwater have largely neglected groundwater occurrence conditions. In this study, groundwater samples from aquifers with different occurrence conditions were collected and analyzed using an integrated approach combining hydrochemical analysis (PCA-APCS-MLR) and the stable isotope mixing model (MixSIAR) to identify and quantify NO3--N pollution sources. The results demonstrate that NO3--N concentrations in 75 % of the groundwater samples exceeded the standard for drinking water quality of China (≤ 10 mg N L −1 ). NO3--N in unconfined groundwater predominantly originates from soil nitrogen (58 %), with a non-negligible contribution from chemical fertilizers. NO3--N enrichment in confined groundwater is primarily attributed to manure and sewage (37.9 %). In addition, ignoring the groundwater occurrence conditions leads to marked deviations in the source apportionment results derived from both the PCA-APCS-MLR and MixSIAR approaches. This study highlights that considering the occurrence conditions is crucial for distinguishing the primary sources of NO3--N in groundwater, which can enhance the accuracy of source apportionment and the effectiveness of management measures.