Exploiting Chloride Conservative Tendencies as Contaminant Surrogates in Groundwater Transport Modeling in a Typical Hydrogeological Environment of Northern New Jersey

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

I have the following coments that the authors should address.

1. Please elaborate on advatages and limitations of the model you used. There are several models that account for unsaturated processes.
2. Please provide the calibration results of temporal evolution.
3. Please organize and discuss your results more systemmatically. 

Author Response

Thank you very much for taking the time to review this manuscript. Please find the detailed responses attached.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The study provides a numerical based approach to understand the transport of chloride in a fractured aquifer in New Jersey and provide insights about the role of local and regional groundwater flow on chloride transport and salinization in urbanized fractured systems. I recommend minor revisions.

The introduction provide sufficient background information of urbanization and de-icing salts in particular as a driver for groundwater contamination, numerical models previously used for New Jersey aquifers, outlines clear knowledge gap and research objective.

Methodology: It would be helpful to include details about the field data collection even though they are explained in another study, since the data is used in this study. It is also recommended to provide with figure of model domain, boundary conditions and model inputs in a table format.

The results provide clear story of Cl transport and the estimation of time duration to reach to important local water sources. The results section is clearly written and supported by the model data. The comparison between particle tracking and MT3DM analysis is important to provide more realistic scenarios taking into account different hydrogeochemical processes that Cl may be subject to.

Figure 9: Why Cl plumes extends towards Passaic river over the 450 year period? What processes contribute to such behavior?

The authors provided discussion on model limitations and the implications of the study findings for groundwater management both in local settings and in similar urbanized fractured aquifer settings.

Figure captions should be more informative. What does the legend indicate on Figure 3 and 4, is it Cl concentrations in mg/L? What are the red triangles? I would recommend to create one figure and subplots indicating in what duration Cl plume reached certain important local benchmarks for easier comparison. And perhaps even provide the side by side comparison of particle tracking plume and plume using MT3DMS for different time periods.

I would also recommend to revising the whole manuscript to make it more concise as some ideas are being repeated throughout the manuscript. Some parts (e.g. introduction, methods and results) almost provide too much background information (e.g. the importance of hydraulic conductivity, MODFLOW and MT3DMS use, the difference between particle tracking and MT3DM), which makes the article unnecessary long.  Consider to make it more concise.

Finally, why the authors tried to refer to other similar settings where the findings will be helpful, it would be great to see the discussion of similar studies in similar settings and comparison of their findings.

Author Response

Thank you very much for taking the time to review this manuscript. Please find the detailed responses attached.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The article presents the results obtained during the continuation of research on the groundwater system and chloride contamination migration in the fractured Brunswick aquifer of northern New Jersey. In the first stage of the work, a numerical model of groundwater flow was developed, and contaminant migration was simulated using an advection model. According to the Authors – and rightly so – this model did not accurately represent chloride migration. Therefore, in the reviewed article, the model simulations also take into account the specific characteristics of fractured rocks, namely their dual porosity, as well as dispersion and diffusion.

An important element of the article is the Authors’ quantitative demonstration of how crucial it is to include, in contaminant transport modeling, not only advection but also dispersion, diffusion, and the dual porosity of rocks. The article fits well within the scope of the journal Hydrology.

In the article, the numerical groundwater flow model is described in detail, whereas the description of the mass transport model is very general. In my opinion, this should be the other way around. If possible, I suggest conducting a sensitivity analysis of the mass transport model to assess how changes in input data, such as porosity or the dispersion coefficient, affect the results. This can be done by performing simulations with different values of porosity and the dispersion coefficient.

Detailed comments:

1. Please add a geological cross-section. This will help readers better understand the proposed conceptual model.
2. I suggest providing all values in SI units.
3. Figure 2 – please add units for the ground surface elevations (e.g., meters above sea level) and groundwater head. I also suggest changing the way groundwater flow directions and velocities are presented (Fig. 2b). Instead of arrows, it would be better to show  particle pathlines, which would also prevent issues with arrows crossing model boundaries.
4. Page 9, line 332 – please add information about the dimensions of the computational blocks.
5. In the model description, please specify whether the wells were only observation points or if they were also pumping wells simulated using a Type II boundary condition (Well Package).
6. Figures 3–9 – please add units for chloride concentrations. In Figure 3, instead of using the term “few days”, please provide the exact simulation time.
7. I suggest adding a cross-section showing changes in chloride concentration with depth after  few years and after 450 years.
8. In the description of the MT3DMS simulations, please specify which boundary condition was used to simulate chloride contamination.
9. Page 17 – please provide a more detailed description of the mass transport model calibration and include a calibration plot in the article.

Author Response

Thank you very much for taking the time to review this manuscript. Please find the detailed responses attached.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for your detailed responses to my comments and for making appropriate changes to the manuscript. I suggest publishing the manuscript in its present form.