the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Nov 2025
Evaluating Long-Term Effectiveness of Managed Aquifer Recharge for Groundwater Recovery and Nitrate Mitigation in an Overexploited Aquifer System
Abstract. Managed aquifer recharge (MAR) has been widely recognized as an effective strategy for groundwater restoration and has been implemented globally. In the North China Plain, over-extraction of groundwater has led to a continuous decline in water levels, forming one of the world's most significant groundwater depressions. Recent riverine MAR operations have shown significant local groundwater recovery, yet the regional-scale hydrological and geochemical impacts of sustained MAR remain insufficiently understood. Most existing studies rely on short-term field monitoring and emphasize localized responses. This study, focusing on Xiong’an depression area, develops a coupled flow and multi-component reactive transport model to evaluate the long-term impacts of MAR on groundwater recovery and the spatiotemporal evolution of water quality. The results indicate that MAR substantially accelerates groundwater recovery and mitigates the regional depression, though the central funnel exhibits a delayed response due to its distance from recharge sources. Nitrate reduction is dominated by dilution effects from recharge water rather than denitrification, with heterogeneity exerting strong control on the spatial pattern but limited influence on overall concentration levels. These findings highlight the dual hydrological and geochemical benefits of sustained MAR and provide quantitative insights for optimizing large-scale recharge strategies in overexploited aquifer systems.
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Status: open (until 31 Dec 2025)
- RC1: 'Comment on egusphere-2025-5534', Anonymous Referee #1, 22 Nov 2025 reply
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CC1: 'Comment on egusphere-2025-5534', Nima Zafarmomen, 03 Dec 2025
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The manuscript presents a 3D variably saturated flow and multi-component reactive transport model (PFLOTRAN) to evaluate the long-term impacts of managed aquifer recharge (MAR) on groundwater recovery and nitrate mitigation in the Xiong’an depression, North China Plain. You explicitly separate the contributions of dilution and denitrification and explore the role of geological heterogeneity using T-PROGS-based realizations.
The topic is timely and important, the study area is of high practical relevance, and using a regional 3D reactive transport model for long-term MAR evaluation is scientifically interesting. The overall narrative is clear and the paper is generally well organized. I recommend publication after considering these comments:
- The TPROGS-based heterogeneous fields are central to the conclusions, but the conditioning data (only 14 boreholes), transition probabilities, variograms, and convergence of 20 realizations are not sufficiently documented. Please add a dedicated subsection describing the limitations of representing heterogeneity with such sparse conditioning.
- The description of pumping, irrigation returns, and lateral boundary conditions is quite brief relative to their importance, and there is no explicit groundwater or nitrate mass balance. Please provide a brief summary in the paper.
- The introduction outlines gaps but does not clearly state what this work adds beyond existing MAR–nitrate modeling in the NCP and globally. Please sharpen the problem statement, explicitly contrast your framework and findings with key prior regional-scale MAR studies, and clearly articulate the unique methodological and management contributions in the last paragraph of the Introduction. I strongly recommend to consider "Assimilation of sentinel‐based leaf area index for modeling surface‐ground water interactions in irrigation districts"
- The conclusion that ~91% of nitrate reduction is due to dilution is based primarily on domain-average concentration differences between scenarios. Please support this attribution with explicit nitrate mass-balance terms (advective–dispersive fluxes vs. reaction sinks).
Citation: https://doi.org/10.5194/egusphere-2025-5534-CC1 -
RC2: 'Comment on egusphere-2025-5534', Anonymous Referee #2, 04 Dec 2025
reply
This manuscript presents a regional-scale assessment of MAR impacts on groundwater level recovery and nitrate dynamics in the Xiong'an New Area using a three-dimensional coupled flow and reactive transport model. The topic is relevant for managing groundwater depletion and nitrate pollution in intensively cultivated regions. The integration of MAR hydraulics with nitrate biogeochemistry is an important contribution, and the authors provide clear insights into how dilution, denitrification, and geological heterogeneity jointly shape water-quality outcomes. The discussion is generally well-structured and highlights both the strengths and limitations of MAR. However, several aspects require clarification and improvement before the manuscript can be considered for publication. The model structure, boundary conditions, and parameterization of reactive processes need more transparency, particularly regarding organic carbon availability, redox controls, and the justification for simplifying nitrogen pathways. Some interpretations appear to overstate the certainty of denitrification estimates given the strong assumptions applied. While the authors have mentioned the influence of spatial resolution, recharge configuration, and heterogeneity, this part would benefit from more rigorous sensitivity analyses. The discussion could also more explicitly connect findings to broader MAR design principles and regional management implications. Furthermore, I suggest the authors discuss how their exclusion of other nitrogen transformation reactions might affect the conclusions regarding denitrification's minor role.
Overall, the study has clear potential, but refinements in methodological justification, uncertainty communication, and the contextualization of results will substantially strengthen the manuscript. Therefore, I recommend minor revisions.Specific comments:
1. Line 16: The abstract states that “regional-scale hydrological and geochemical impacts remain insufficiently understood,” but several recent MAR modeling papers at regional scales exist. It is unclear what is truly new: the model structure? the scale of the simulation? integrating heterogeneity? long-term simulation horizon? nitrate processes?
2. Line 13-24: Currently the abstract provides interpretation but no numbers, making the conclusions feel generic. Consider adding a sentence on the role of heterogeneity in shaping nitrate reduction patterns.
3. Line 33: “become as”
4. Line 113: The rate expression multiplies terms for electron donor (ED), terminal electron acceptor (TEA) and inhibition. However, there is no mention of an explicit microbial biomass pool (or active biomass concentration). Denitrification in aquifers is often biomass-mediated and can be substrate-limited and biomass-limited; excluding biomass (or at least an active biomass term) risks mischaracterizing dynamics, especially under long-term MAR where biomass may grow or be flushed.
5. Line 117: The Introduction criticizes prior studies for omitting microbial OM degradation and C–N coupling, but the method section does not convincingly demonstrate that these processes are represented.
6. Line 123: The manuscript specifies that the model domain is discretized using 100 m horizontal grid spacing but does not provide justification for this choice. Given the scale of the study area (tens of kilometers) and the strong emphasis on preferential flow paths, MAR-induced infiltration fronts, and heterogeneity, did the authors perform mesh and time-step sensitivity analyses (e.g., Courant number assessment or grid refinement tests) to ensure numerical stability and accuracy of both flow and reactive transport simulations?
7. Line 129: Only four lithofacies are represented based on 14 boreholes. This is a very sparse conditioning dataset for a 3D domain of this size. The uncertainty introduced by such limited conditioning should be acknowledged.
8. Line 203: How sensitive are these values to recharge rates or hydraulic conductivity? The results repeatedly attribute attenuation to “distance and connectivity,” but this remains qualitative. A clearer linkage between MAR response zones and the permeability map is needed.
9. Line 215: The concept of comparing points with different distances and permeabilities is good, but the interpretation mixes permeability effects and distance effects. For example, line 220 states that “their amplitudes diverge with permeability: Point1 rises by ~4–7 m, while Point2 increases by ~3–5 m across the projection horizon”. But figure 5b clearly shows a larger amplitude than 5a, which means MAR posed a higher effect on Point2. Also, no lag time between Point1&2 is surprising.
10. Line 255-256: Be cautious with statements such as “strongest decreases” when the absolute reductions remain quite small (10-5 mol/L is extremely minor relative to typical groundwater nitrate levels). You may want to discuss ecological significance, not just the mathematical magnitude.
11. Line 278: Obviously, denitrification is effectively negligible in this system. How reliable this result is? Plus, it is surprising that the spatial pattern is nearly uniform, given strong variability in residence time, electron donor availability, and organic carbon distribution in real systems.
12. Line 353: You highlight the differing effects of heterogeneity on dilution vs. denitrification but do not clearly link these insights to the broader literature on preferential flow and residence-time control on reactive transport. Adding comparison to previous heterogeneity–redox studies would position your results more convincingly.
13. Line 362: “he”
14. Line 373: While you note that the recharge scheme assumed constant conditions, you do not discuss how sensitive the nitrate response is to recharge timing, distribution, or water chemistry. Without acknowledging potential model sensitivity, the management implications may appear overstated.
Citation: https://doi.org/10.5194/egusphere-2025-5534-RC2
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Yuguang Zhu
Zhilin Guo
Sichen Wan
Kewei Chen
Yushan Wang
Zhenzhong Zeng
Huizhong Shen
Jianhuai Ye
Chunmiao Zheng
Groundwater is rapidly being depleted in many regions, threatening water security and food production. We studied a major groundwater depression in northern China to test whether recharging aquifers with diverted river water can help recovery. Using long-term computer simulations, we found that recharge raises water levels and dilutes nitrate pollution, offering an effective way to restore overused aquifers.
Groundwater is rapidly being depleted in many regions, threatening water security and food...
My comments are as follows:
1. The paper is interesting and well written, and it is relevant to the water management of a vital area of China: Xiongan New District.
2. There are two major issues that require a much more careful analysis, and I hope the authors can revise their paper accordingly. The first is the description of denitrification. In this manuscript, the authors used a two-step reduction to simplify the process. This is OK, but requires a much more careful and detailed analysis to justify its correctness. It is well known that denitrification is a complex process controlled by many factors. So, the authors should carefully justify why such a two-step reduction treatment is acceptable for this site!
3. The second issue is related to Eq. 5. The author stated that “ For example, according to the redox gradient theory, sulfate reduction may be inhibited by oxygen, nitrate, and trivalent iron dissolved in groundwater. These effects are modeled using an inhibition term in equation (5)”. This is not sufficient. I would like the authors to explain what “inhibition term in Eq. (5)” is used and why. In one sentence: more elaboration is needed here!
4. L60, there should be a question mark “?” after the two questions mentioned there.
5. L65, when NCP is mentioned for the first time, the full name of NCP must be provided.
I will recommend moderate revision.