the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Nov 2025
Impact of burial conditions on NO3--N source apportionment in groundwater: Insights from PCA-APCS-MLR and MixSIAR methods
Abstract. 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 aquifer burial conditions. In this study, groundwater samples from aquifers with varying burial conditions were collected and analyzed using an integrated approach combining hydrochemical analysis (PCA-APCS-MLR) and stable isotope mixing modeling (MixSIAR) to identify and quantify NO3--N pollution sources. The results demonstrate that NO3--N concentrations in 75 % of the groundwater samples exceeded the WHO drinking water standard. PCA-APCS-MLR analysis revealed that the dominant NO3--N sources in unconfined groundwater and confined groundwater were chemical fertilizers (52.5 %) and manure & sewage (53.9 %), respectively. The MixSIAR model further identified soil nitrogen (58 %) and manure & sewage (37.9 %) as the primary contributors to NO3--N in unconfined and confined groundwater, respectively. These findings suggest that unconfined groundwater in regions with high soil nitrogen reserves is at persistent risk of NO3--N contamination. In addition, neglecting aquifer burial conditions would introduce absolute errors of 22 %–24 % in source apportionment results obtained from both PCA-APCS-MLR and MixSIAR approaches. This study highlights that aquifer confinement must be rigorously considered as a critical factor in NO3--N source identification and pollution control strategies to enhance the accuracy of source apportionment and the effectiveness of management measures.
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Status: open (until 02 Jan 2026)
- RC1: 'Comment on egusphere-2025-5482', Anonymous Referee #1, 07 Dec 2025 reply
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RC2: 'Comment on egusphere-2025-5482', Anonymous Referee #2, 07 Dec 2025
reply
This manuscript investigates the sources and quantification of nitrate in groundwater, with emphasis on the role of burial conditions. PCA-APCS-MLR and MixSIAR are combined to apportion nitrate origins in groundwater samples collected from different layers of aquifers. The results are interesting, highlighting innovative concepts and methodologies in source apportionment and groundwater pollution control strategies, which finally could merit publication.
I have a number of comments:
- The term "NO3--N" is used extensively in the introduction. While it is likely that readers familiar with groundwater contamination will understand this abbreviation, it would be helpful to define it explicitly at its first appearance in the text. For example, you could write "NO3--N (nitrate-N)" the first time it appears.
- The objectives of the study are listed clearly at the end of the introduction, but transition from the discussion of existing methods and challenges to the specific aims of your study is somewhat abrupt. I recommend adding a brief paragraph to explicitly link the identified gaps in current research to the specific goals of your study.
- The introduction provides a comprehensive background on the problem of NO3--N contamination and the methods used for source apportionment. However, it lacks a clear statement of the hypothesis. Pleaseclearly state the hypothesis in the introduction.
- In the materials and methods section, the descriptions of sample testing methods are brief and there are no references provided to support these methods. Please provide detailed descriptions of the sample testing methods and cite relevant literature.
- The materials and methods section on data analysis briefly mentions the use of Pearson correlation tests but lacks details on the specific procedures used for these analyses.
- In the results section, the terms "generalized single-aquifer layer" and "actual double-aquifer layer" are used to describe different scenarios, but they are not clearly clarified. Define these terms explicitly at the beginning of the results section.
- In the discussion section, please include a more detailed comparison with previous studies on NO3--N pollution and source apportionment. Discuss how your results align with or differ from those of other studies and provide possible explanations for these differences.
Citation: https://doi.org/10.5194/egusphere-2025-5482-RC2 -
RC3: 'Comment on egusphere-2025-5482', Anonymous Referee #3, 11 Dec 2025
reply
This study investigated nitrate sources in groundwater by collecting and analyzing samples from aquifers with different burial conditions. The authors employed an integrated approach combining PCA-APCS-MLR and MixSIAR for source identification and quantification. The results were compared between confined and unconfined aquifers, highlighting distinct dominant pollution sources for each setting, which enhanced the accuracy of pollution source identification and the effectiveness of protection strategies. It is an interesting work. The approach and results in this study seem acceptable, but some minor revisions are needed. My specific comments are as follows.
(1) Title: The term “burial conditions” is non-standard. Replace it with a precise hydrogeological expression such as “aquifer confinement conditions” or “aquifer occurrence conditions”.
(2) L. 56: Add the WHO safe limit of 11.3 mg L⁻¹ NO₃⁻-N in drinking water for readers to see the exact threshold behind the health risk.
(3) L. 58: Clarify how groundwater NO₃⁻ reaches surface water; without this pathway the link to aquatic eutrophication appears as a logical leap.
(4) L. 61: NO₃⁻-N instead of this contaminant.
(5) L. 77: Add citations.
(6) L.93-94: Delete.
(7) L.132: Explain why both hydrochemical tracers and MixSIAR are needed together instead of using just one of them.
(8) L. 144: Add research hypotheses.
(9) L.228, L.257: Add citations.
(10) L.275: Add detailed information of the Pearson test.
(11) L. 313: “Generalized single-aquifer layer” and “actual double-aquifer layer” are used without definition. Clarify at the beginning of the section.
(12) L.405: How did the authors derive their fractionation data? What consequences would disregarding fractionation have for the MixSIAR results?
(13) L. 470: Add citations.
(14) L. 498-499: Without a statistical analysis, reporting absolute errors is inappropriate. Replace these percentages with a qualitative statement.
(15) To translate research findings into actionable groundwater protection strategies, the authors should explicitly outline how their results can guide targeted management practices.
Citation: https://doi.org/10.5194/egusphere-2025-5482-RC3
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General comments:
Contamination of groundwater systems by high nitrogen emissions from agricultural land use are one of the most common and serious problems of water resources management and research at the global scale. Thus the paper addresses a highly relevant problem. The manuscript combines hydrochemical analysis (PCA-APCS-MLR) and stable isotope mixing modeling (MixSIAR) to identify and quantify NO3--N pollution sources in groundwater for unconfined and confined aquifers. It compares the result of computing the apportionment, assuming the two aquifers as a single water body, or as two water bodies. The argument for such comparison is that previous studies on NO3- -N source apportionment have largely neglected aquifer type. I tend to accept it after appropriate revisions.
Detailed comments: