| 07 Jul 2026
A Novel Modeling Framework to Track Contaminant Sources and Transport in a Groundwater-Dominated Watershed
Abstract. Backwards particle-tracking is widely used in groundwater modeling to gain insights into transport processes in terms of source areas, travel paths, and travel times. In this study we develop a novel modeling framework to apply this approach to SWAT-MODFLOW, a coupled surface water-groundwater model, using the particle-tracking model MODPATH. We tested the framework by applying MODPATH to a SWAT-MODFLOW model of a groundwater-dominated watershed and tracking particles backwards from a river reach with significant spring discharges to the locations where they entered the MODFLOW domain. By combining the source area information of the particles with the nitrate loading from the SWAT model, we were able to simulate nitrate concentrations in the discharged groundwater as well as to simulate changes in nitrate concentrations resulting from different land use and management scenarios. Critically, our approach developed here permits simulation of travel time distributions and source areas of water volumes and nitrate masses. We show how these source and age components can be used to guide the prioritization of water quality mitigation strategies that consider both intrinsic (i.e., hydrogeological) vulnerability as well as specific management actions that could be taken to improve surface water and groundwater quality.
This study aims to develop a new modeling framework by integrating SWAT-MODFLOW with MODPATH to identify nitrate source areas, quantify groundwater travel-time distributions, and evaluate the responses of spring-fed waters under different land-use and management scenarios. Overall, the topic is interesting and relevant, and the proposed framework has methodological novelty and potential value for groundwater quality management. However, several major issues related to model assumptions, validation, parameterization, and interpretation of the results need to be further clarified before the manuscript can be considered for publication. Below, I have listed comments, hoping they may help improve the manuscript’s quality.
Major comments
The Introduction needs to include a broader and more critical review of previous studies. The current discussion does not clearly show the research gap or the novelty of the proposed framework. Please better explain the relationship between this study and previous particle-tracking, nitrate-source identification, and coupled surface water-groundwater modeling studies.
The claimed computational efficiency should be quantitatively demonstrated. Please compare the runtime, memory requirement, and numerical performance of the proposed framework with SWAT-MODFLOW-RT3D under similar model settings. The authors should also discuss whether the computational savings are achieved at the cost of accuracy because of particle tracking and temporally averaged flow fields.
The Upper Floridan Aquifer is strongly karstified, but it is represented as an equivalent porous medium in the model. Preferential pathways and karst conduits may significantly affect capture zones, groundwater travel times, and mitigation responses. Please test alternative conceptual models, such as high-conductivity preferential pathways, or at least evaluate the sensitivity of the main conclusions to the possible influence of conduit flow.
The adjustment of horizontal hydraulic conductivity needs further clarification. Although the authors maintained the same effective horizontal conductivity across the model layers, changing its distribution among different hydrostratigraphic units may affect interlayer flow, particle pathways, and travel-time distributions. Please provide more hydrogeological justification for these modifications and evaluate their influence on the main results.
Minor comments
Lines 1-2: The title uses the broad term “contaminant,” whereas the framework is mainly applied to nitrate and includes nitrate-specific loading and attenuation assumptions. Please consider replacing “Contaminant” with “Nitrate” or more clearly explain whether the proposed framework can also be applied to other contaminants.
Lines 13-15: Please replace “Backwards particle-tracking” with “backward particle tracking.” The terminology and hyphenation should also be consistent throughout the manuscript.
Line 98: Typo. “SWAT-MODLFOW” should be corrected to “SWAT-MODFLOW.”
Line 181: “Travel time probably density function” should be corrected to “travel time probability density function.”
Line 268: The labels in Figure 4b are difficult to read. Please increase the font size or improve the figure resolution.
Lines 272-279: Please include a figure showing the vertical model discretization, hydrostratigraphic units, and spatial distribution of the main MODFLOW parameters.
Line 280: Please replace “variably” with “variable.”
Lines 378-384 and 417-420: The fonts and legends in Figures 6, 7, and 10 are relatively small. In addition, the large number of source components makes these figures difficult to understand. Please enlarge the text and legends to improve their readability.
Lines 394-395 and 414-415: The legends or color bars in Figures 8 and 9 are missing clear names and units. Please clearly indicate the variables shown and provide units where applicable.
Lines 414-415: Figure 9b should include its own land-use legend instead of asking readers to refer back to Figure 5.
Lines 409-411: In this case, the NNC is achieved…
Line 556: Should “Figure A1b” be corrected to “Figure B1b”?