Assessment of the natural background levels of ammonia nitrogen and chloride in alluvial fan groundwater using an improved pre-selection method: Implications for groundwater management

Natural Background Levels (NBLs) play a pivotal role in groundwater management. Ammonia nitrogen exceeding national standards is a significant concern in the alluvial fan area of Tuzuoqi, Hohhot, Inner Mongolia. The commonly used pre-selection method relies on empirical judgment with predefined pollution thresholds, making it highly subjective and less adaptable. In this study, an improved pre-selection approach was used to assess the NBLs of ammonia nitrogen and chlorides in the study area, complemented by pollution indices to evaluate the extent and scope of contamination. The improved method first employed Hierarchical Clustering Analysis (HCA) to identify characteristic pollution indicators, including NH₄⁺, Cl⁻, TDS, Ca²⁺, Na⁺, NO₃⁻, and Chlorinated HydroCarbons (CHCs). Subsequently, the contamination levels of these indicators were analyzed to establish pollution thresholds and remove contaminated samples, thereby completing the pre-selection of original samples. The pre-selection criteria for the study area were: For ammonia nitrogen, NH₄⁺-N > 0.5 mg/L combined with Cl⁻ > 250 mg/L; for chloride, Cl⁻ > 250 mg/L and Cl⁻ > 100 mg/L with simultaneous detection of CHCs. Pre-selected samples were further analyzed using Grubbs' test to identify NBLs samples. The validity of the NBLs samples was confirmed through significance tests based on historical data. The 95th percentile of the NBLs samples was used to calculate a unified NBLs for pollution indices. The results indicate that ammonia nitrogen contamination is concentrated in the central-southern part of the industrial park, with a limited spatial extent and severe pollution near pollution sources. The exceedances of ammonia nitrogen in water source wells are due to high NBLs, which are attributed to organic-rich alluvial-lacustrine interbedded deposits. Chloride pollution has spread from the central-southern to the northern areas, causing slight contamination in some wells over a broader region. Compared to other alluvial fans, elevated chloride NBLs are related to the high content of water-soluble salts in the lacustrine sediments within the strata. This study validates the effectiveness of the improved preselection method in determining groundwater NBLs and emphasizes the role of NBLs in identifying sources and contamination levels of exceeding components.