New Plume Center of Mass Analysis Tool

Introduction

Groundwater contaminant plume remedy plans that are considering Monitored Natural Attenuation {MNA} are often required to demonstrate a stable plume. i.e. they must provide data or an analysis to verify that the plume is no longer advancing downgradient from the source area toward potential receptor locations.

One method to graphically depict plume stability involves determining the center of mass of the plume over some period of time.

 

Previous Guidance and Methods

In 2000, the Air Force Center for Environmental Excellence (AFCEE) published MNA guidelines (Wiedemeier et al. 2000) that described methods for evaluating the feasibility of an MNA remedy. Tracking plume center of mass to determine plume stability was cited as one of the primary methods.

In 2008, Ricker published methods for evaluating stability of a groundwater plume. For one of the methods, Ricker included a formula for calculating plume center of mass in two dimensions (Ricker 2008).

In 2022, New Jersey Department of Environmental Protection (NJDEP) issued MNA guidance that discussed methods for performing spatial analysis of trends in contaminant plume mass. In particular, the guidance cites to Ricker’s methods and to the AFCEE 2000 guidance center of mass analysis.

In 2024, Golden Software published information on its website describing the use of its Surfer spatial analysis software to apply the Ricker method to contaminant plume center-of-mass analysis.

It is clear from this history that regulatory guidance and industry practice has highlighted the need for a method, or methods, to demonstrate that a groundwater contaminant plume is stable when considering MNA as a part of the overall remedy.

 

TS-CHEM Center of Mass Analysis Tool

Building on the previously identified usefulness of software tools for demonstrating plume center-of-mass stability, the latest version of TS-CHEM - - v2024-3 released in June 2024 - - includes a new Analysis Tool that is capable of calculating the center of mass for any plume, or set of commingled plumes, generated by the software.

 

As an example, a simple benzene plume transport model was developed and run for 10 years, with output every 2 years. 

The TS-CHEM Plume Center of Mass analysis tool operated on the model output shown above to generate data depicted in the figure below.

The output calculated by the TS-CHEM Center of Mass Analysis Tool demonstrates that the benzene plume center of mass location relative to the source (X distance from the source) stabilizes at a time about 7 years after the initial benzene release.

 

Conclusion

One of the key analyses that can prove beneficial in supporting a Monitored Natural Attenuation remedy is a demonstration that the plume is stable in time, which can be verified by demonstrating that the location of the plume center of mass has stabilized. This concept, as well as several methods of making this demonstration, are described in regulatory guidance and in the scientific literature.

 

TS-CHEM has recently incorporated a method similar to the Ricker 2008 center of mass method as one of its built-in analysis tools. In the simple example shown above, the analysis demonstrates that the plume is in a stable configuration after about 7 years, which could support consideration of Monitored Natural Attenuation as a feasible remedy.

 

References

Golden Software 2024. The Ricker Method for Plume Stability Analysis. Golden Software website:

https://www.goldensoftware.com/ricker-method-for-plume-stability-analysis/

 

NJDEP 2022. Monitored Natural Attenuation Technical Guidance. Contaminated Site Remediation & Redevelopment Program, 178 pp; Appendix F – Selected Reference Summaries.

 

Ricker, JA 2008. A practical method to evaluate ground water contaminant plume stability. Ground Water Monitoring & Remediation, 28(4), p. 85 – 94.

 

Wiedemeier, TH, MA Lucas, and PE Haas 2000. Designing Monitoring Programs to Effectively Evaluate the Performance of Natural Attenuation. Air Force Center for Environmental Excellence, 55 pp.

Simplify Source Model Set-Up with the New CSV Import Feature

One exciting addition in the latest release of TS-CHEM (version 2024-3) is the new “CSV Import” feature for time-varying source concentrations. Several of the TS-CHEM solutions allow the user to specify source concentrations that change with time (ATRANS3 and ATRANS4; AT123D-AT FT and AT123D-FT). The gradually varying concentration being released from the actual source (smooth red curve in the chart below), is represented in the TS-CHEM analytical model as a series of concentration steps (blue step changes in the chart below).

The CSV Import feature can be used to efficiently set up and solve a time-varying source model by allowing users to import time-and-concentration source histories that have been created manually by the user, or exported from other user software, in CSV format. For example, the SILT soil source leaching model is capable of calculating and exporting a CSV file of groundwater source concentrations from a soil zone contaminant source (see the SILT Blog Post for more information).  

User-created source history files can be used to analyze various scenarios for releases from an industrial facility over time. Concentration versus time output from software that calculates source release concentrations - - for example, SILT leaching a contaminant from soil to groundwater or SourceDK representing dissolution and dissipation of a NAPL source - -  can be saved as a CSV file and imported directly into TS-CHEM.

Using the new CSV Import feature is simple. In the Model Data tab, for transient (stepped through time) concentration data sets, begin by clicking in the data entry field, and a data entry pane is opened on the right side of the window to facilitate data entry (see Figure 1).

Figure 1. Source Concentration Data Entry Pane

To import a time-and-concentration source history file (in CSV format), click on the Import button to open the CSV data import control panel (see Figure 2).

Figure 2. Import CSV Data List Window

Click on the Choose button and navigate to the CSV data file. The file contents will be displayed in the Preview window. Use the “Significant Digits” setting to format the concentration values to improve viewability (excessively long concentration values with many digits to the right or left of the decimal point can be difficult to read). If the file was created with a header row, the check box can be used to eliminate that row before import.

Checking the “Replace” check box will completely replace (overwrite) all data pairs currently in the source history list. Unchecking the “Replace” check box will merge the CSV file data pairs into the current source history list, and will sort the list to properly organize the specified history of concentrations. Note: If there is a time-and-concentration data pair in the CSV file with a time identical (within 1E-04) to a time already in the source history list, the CSV file data pair will overwrite the current list data pair; thereby effectively specifying an updated concentration for that time.

Click the Import button and the CSV file data will be added to the source history list (see Figure 3).

Figure 3. Source Concentration Data Entry Pane (with imported data)

In addition to the new CSV Import feature, several other updates and improvements have been incorporated into TS-CHEM version 2024-3, including:

• Plume Center of Mass analysis tool for MNA and other analyses
• Data entry enhancements
• Other minor improvements and bug fixes

To take advantage of these updates and improvements and test out the new CSV Import feature, head over to the TS-CHEM Website to download version 2024-3 today!