Groundwater Nitrate in Multilayer Aquifer Systems of Alluvial Fans on the North China Plain: Spatial Distribution,...

Groundwater nitrate contamination is a global environmental concern, especially in regions with high demand of groundwater. This study presents a comprehensive investigation into the spatial distribution, source apportionment, natural background levels (NBL), and contamination status of groundwater nitrate in shallow and deep aquifers across two typical alluvial fans (AFs) on the North China Plain. Multiple methods were adopted in this study, including principal component analysis-absolute principal component score-multivariate linear regression, a combined method of oxidation capacity and iterative box plot, NBL-based contamination assessment method, and Empirical Bayesian Kriging. A total of 1428 groundwater samples and 11 surface water samples were collected, with 15 chemical parameters analyzed. Results show that median nitrate concentrations in shallow aquifers were more than five times as much as those in deep aquifers in both AFs. The proportion of high-nitrate (> 50 mg/L) groundwater (PHNG) in shallow aquifers of the Dasha River AF (AFD) was approximately twice that of the Hutuo River AF (AFH), whereas the opposite trend was observed in deep aquifers. Shallow groundwater nitrate contamination in both AFs appeared to be independent of land-use types. By contrast, the PHNG in deep aquifers beneath agricultural lands was more than four times that in rural areas in both AFs. In AFH, nitrate in shallow groundwater mainly originated from chemical fertilizers (40.5%) and manure-septic effluent-thickening vadose zone (32.5%). In comparison, shallow groundwater nitrate in AFD was predominantly derived from mixed sources (34.0%) and sewage-thickening vadose zone (29.0%). The NBLs of nitrate in shallow groundwater were 11.0 mg/L in AFH and 21.9 mg/L in AFD, which were roughly twice and three times those in the corresponding deep aquifers, respectively. Nitrate-contaminated shallow groundwater accounted for 41.2% of total samples in AFH and 59.3% in AFD; both proportions were more than four times those of deep aquifers. Nitrate-contaminated shallow groundwater was mainly distributed at the top of the AFs and decreased downgradient along groundwater flow paths. Its spatial pattern was primarily controlled by hydrogeological conditions (e.g., aquifer media size and groundwater flow velocity) rather than anthropogenic factors. Nitrate-contaminated deep groundwater was concentrated in eastern Shijiazhuang City and was likely caused by a thickening vadose zone and vertical leakage of nitrate-contaminated shallow groundwater due to long-term groundwater overexploitation.