Chu Yu; Ze-peng Zhang; Yi-long Zhang; Heng-xin Zhang; Xiao-han Li

doi:10.26599/JGSE.2026.9280076

doi: 10.26599/JGSE.2026.9280076

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出版历程
- 收稿日期: 2025-03-25
- 录用日期: 2025-10-27
- 网络出版日期: 2026-04-30
- 刊出日期: 2026-06-30

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

Chu Yu^{1, 2},
Ze-peng Zhang^{1, 2
, ,},
Yi-long Zhang^{1, 2},
Heng-xin Zhang^{1, 2},
Xiao-han Li^{1, 2}

1.
Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang 050061, China
2.
Key Laboratory of Groundwater Science and Engineering, Ministry of Natural Resources, Shijiazhuang 050061, China

More Information

Corresponding author: 358288386@qq.com

摘要

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Abstract: 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.
- NBLs /
- Preselection method /
- Ammonia nitrogen /
- Chlorine /
- Hohhot Basin /
- Groundwater contamination

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Figure 1. The geographic position and fault structure diagram (lower left) of the study area

Notes: The distributions of surface water and groundwater sampling sites, potential contamination sources, typical hydrogeological boreholes, and preindustrial groundwater samples (upper), and lithological structures of strata in typical hydrogeological boreholes (lower).

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Figure 2. Box plots showing the distribution of general chemical parameters in groundwater, surface water, and potential pollution source samples, compared with the national Class III groundwater quality standards.

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Figure 3. Relationships of Na⁺ vs. Cl⁻ (a), Cl⁻ vs. CHCs (b), and Cl⁻ vs. NH₄⁺-N (c) for the analysis of factors controlling groundwater chemistry.

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Figure 4. The HCA-derived dendrogram for NH₄⁺ and other characteristic pollution indicators.

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Figure 5. Diagrams showing the relationships between NH₄⁺, Cl⁻, and other characteristic pollution indicators for uncontaminated surface water, potential contamination sources, and groundwater in the study area.

Notes：The red lines in the diagrams denote the determination threshold for contamination. For samples below the detection limit, the detection limit value was taken as their ammonia nitrogen concentrations for plotting.

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Figure 6. HCA of groundwater samples

Note: (a) shows the selection of the optimal number of clusters, and (b) presents the clustering results, validating the identified contaminated samples.

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Figure 7. NH₄⁺-N and Cl⁻ outliers in the PS dataset, identified using Grubbs' test.

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Figure 8. Distributions, pollution indices, and preliminarily delineated contaminated zones of NH₄⁺-N- and Cl⁻-contaminated samples.

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Table 1. PCA-derived factor profile as the extraction criterion.

Species	PC 1	PC 2	PC 3
TDS	0.43	0.05	0.04
SO₄²⁻	0.33	−0.09	−0.08
Cl⁻	0.42	0.03	0.08
Na⁺	0.39	0.17	−0.08
Ca²⁺	0.39	−0.06	0.04
Fe	0.03	0.44	−0.48
Mn	0.09	0.38	−0.57
COD	0.00	0.61	0.36
NH₄⁺-N	0.30	0.03	0.21
NO₂⁻-N	−0.08	0.47	0.49
NO₃⁻-N	0.35	−0.19	0.10
Eigenvalue	5.37	1.81	1.51
Percentage of Variance	48.8%	16.5%	13.8%

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Table 2. Statistical characteristics of NH₄⁺-N and Cl⁻ in groundwater of the study area based on the preindustrial, original, and NBLs datasets.

Statistics	NH₄⁺-N			Cl⁻
Statistics	Original dataset	NBLs dataset	Preindustrial dataset	Original dataset	NBLs dataset	Preindustrial dataset
Sample size	76	67	11	74	62	11
Max. (mg/L)	529.32	6.6	5.4	2641	118.0	75.9
Med. (mg/L)	0.2	0.06	0.05	43.8	37.3	20.6
Mean (mg/L)	8.2	0.8	0.96	176.4	41.9	28.1
St. Dev. (mg/L)	60.6	1.4	1.89	462.3	27.4	23.8
Skewness	8.7	2.4	2	4.0	1	1.7
Kurtosis	75.8	5.4	2.9	16.2	0.8	1.5
One-sample t-test (P value)	<0.001	0.79	/	<0.001	0.08	/

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