Distribution and source apportionment of groundwater nitrate contamination in shallow and deep aquifers of the Hebei Plain: A combined interpretation of PCA-APCS-MLR, hydrochemical methods, and land use characteristics

Study region Hebei Plain, China Study focus Groundwater nitrate contamination is a global concern, and methods for its source apportionment are generally complicated. This study aims to investigate the distribution of nitrate in shallow and deep groundwater across different land use types in the Hebei Plain, and to estimate the relative contributions of potential nitrate sources via the integration of principal component analysis-absolute principal component score-multiple linear regression (PCA-APCS-MLR), conventional hydrochemical methods, and land use information. New hydrological insights for the region Proportions of high-nitrate (> 50 mg/L) groundwater (PHNG) in shallow aquifers in both of piedmont and littoral plains were 5 times or more that in shallow aquifer of the central plain and deep aquifer of various sub-plains. In the piedmont plain, PHNGs in shallow aquifers in both of bare and urban lands were more than 50%, followed by woods-grasslands (43.8%), agricultural lands (27.6%), and rural areas (18.8%); by contrast, the PHNG in deep aquifers in urban lands was 40% and > 10 times that in other land use types. In the central plain, the PHNG in shallow aquifers in rural areas was > 9 times that in agricultural lands. This is opposite to the littoral plain where the PHNGs in shallow aquifers in agricultural lands was 1.6 times that in rural areas. The relative contribution results calculated by PCA-APCS-MLR suggest that manure and septic sewage are preliminarily inferred as the dominant potential sources (58.8%) of nitrate in shallow groundwater of the piedmont plain, followed by mixed sources (25.8%), fertilizers (11.7%), natural sources (2.3%), as well as domestic sewage and nitrification in vadose zones (1.4%). A considerable proportion of mixed sources for groundwater nitrate in the piedmont plain indicates that overlapping nitrogen sources cannot be fully separated by the PCA-APCS-MLR model. In contrast, fertilizers (40.6%) and manure (37.7%) are inferred to be the two major contributors to nitrate in shallow groundwater of the littoral plain. This study reveals that the combination of PCA-APCS-MLR, conventional hydrochemical methods and land use information serves as a cost-effective approach to assess source contributions of groundwater nitrate in large alluvial plains such as the Hebei Plain. It should be noted that this method has limitations in distinguishing overlapping nitrogen sources, and the results require comprehensive interpretation.