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!

Supplemental TS-CHEM Installation Instructions for Mac Users

Currently, TS-CHEM is an Intel-based application, and as a result, certain Apple products (MacBook Pro, MacBook Air, iMac, Mac Mini, etc.) that contain Apple silicon chips (e.g., M1, M2, or M3) may not be able to solve TS-CHEM models, producing a message in the run window noting that there is a “Bad CPU type in executable” (see example below).

To solve TS-CHEM models on Apple Macintosh computers that contain Apple chips, users can install the Rosetta 2 software*, which allows those machines to run Intel-based applications.  To install the Rosetta 2 software, follow the steps below:

    1.    Open up the Terminal application

    2.    Copy and paste the following text in the command prompt:

                softwareupdate --install-rosetta

    3.    Press Enter, and follow the subsequent prompts to install the software.

Once the Rosetta 2 software is successfully installed, machines with Apple chips should be able to run TS-CHEM models without any issues.  

Please contact the TS-CHEM Support Team (Support@TS-CHEM.com) if you have any questions, or if you require any assistance with installing the Rosetta 2 software.

*Rosetta is an Apple utility that enables a Mac with an Apple silicon CPU to use apps built for a Mac with an Intel processor. (see https://support.apple.com/en-us/102527). Computers in this category are listed on the Mac computers with Apple silicon web page (https://support.apple.com/en-us/116943).

 

New Modeling Tools – SCL Plume Model and PVM Calculator

The TS-CHEM team is pleased to announce the release of two new Microsoft Excel-based spreadsheet utilities – the SCL Plume Model, and the Plume Volume and Mass (PVM) Calculator.  With straightforward inputs and intuitive numeric and graphic outputs, these tools allow environmental practitioners to quickly estimate the extent of plumes at specified points in time, and get a sense of key plume characteristics, including the volume of impacted groundwater within the contaminant plume, the mass contained within the plume, and/or the mass flux across a user-specified plume transect.

SCL Plume Model Spreadsheet

Since the mid-1980s, one of the most widely used analytical plume transport models has been what is generally referred to as the Domenico model. The Domenico model has been applied by regulatory agencies as the basis for a number of software tools (e.g., USEPA’s BIOSCREEN and BIOCHLOR; CA RWQCB spreadsheet; PADEP Quick Domenico spreadsheet), and has been widely applied and reported on in the professional and scientific literature.  Beginning in approximately 2005, investigators began commenting on perceived inaccuracies in the approximate Domenico solution, and in 2007, an improved solution that maintains the efficiency of the Domenico solution, and reportedly improves on its performance was developed and published by Srinivasan et al. (2007).  That efficient solution has been programmed into the SCL Plume Model spreadsheet, which can quickly generate calculated data outputs and model results charts that can assist practitioners in examining and better understanding plume behavior at their site of interest.

Input data for the SCL Plume Model include the common parameters required for an analytical plume transport model, including:

• source information (source dimensions and source concentration);
• aquifer parameters (hydraulic conductivity, horizontal hydraulic gradient, and effective porosity);
• contaminant transport parameters (dispersion, retardation rate, and degradation rate);
• time information (time at which source concentrations are to be calculated); and
• plot parameters (several inputs that control numeric and graphic model outputs).

Users can also specify a groundwater standard (which allows the user to examine where the plume concentration along the centerline drops below the specified concentration) as well as the distance to the nearest receptor (which allows the user to examine the model estimated constituent concentration at the receptor location).

The SCL Plume Model spreadsheet generates calculated data outputs (including concentrations along the plume centerline at a specified point in time) and model results charts (including a concentration vs. distance chart and plume contour chart).  

Figure 1. SCL Plume Model spreadsheet interface, including model input tables, plume centerline chart, and plume contour chart

Plume Volume and Mass (PVM) Calculator Spreadsheet

The slow dissolution of a chemical of concern (COC) from a subsurface waste zone or spill through soil into slowly seeping groundwater can form a sizeable area of downgradient contamination above some water quality standard (e.g. maximum contaminant level MCL), frequently referred to as a plume. It is often helpful in an environmental site investigation to develop certain metrics that describe properties of the groundwater plume to aid in forming a more quantitative conceptual site model.  The Plume Volume and Mas (PVM) Calculator is designed as a Microsoft Excel spreadsheet-based tool that can assist site investigators with estimating key plume metrics, allowing them to easily input parameters describing the geometry of the plume as it has been mapped in the field, and then to calculate a variety of outputs, including:

• Areal extent and overall volume within the aquifer that is occupied by the plume;
• Volume of contaminated groundwater within the aquifer pore space;
• Mass of dissolved and sorbed chemical of concern in certain concentration contour zones of the aquifer;
• Overall mass of chemical in the plume; and,
• Mass flux across a user-specified transect.

The basic concept underlying the method applied in this tool is that, because of the nature of most groundwater plumes, their overall shape, as well as the shape of the various nested concentration contour zones, can be represented fairly well by a set of nested ellipsoid-shaped volumes (see Figures 2 and 3 below).

Figure 2. Diagram of a typical groundwater contaminant plume (plan view) mapped from a horizontally spaced array of monitoring wells.

Figure 3. Diagram of a typical groundwater contaminant plume (cross-section view) mapped from vertically spaced monitoring wells (e.g. shallow, intermediate, and deep intervals).

The “field mapped” plan view and cross-sectional plume diagrams generated by the environmental investigator are used as the basis for measuring the ellipse dimensions (i.e., the length, width, and thickness for each isoconcentration contour) that form the required set of nested ellipsoid data to be input into the PVM Calculator.

Figure 4. Nested ellipsoids calculated by the PVM Calculator based on user-specified inputs.

In addition to entering estimated dimensions for each plume isoconcentration contour interval, the user can also specify parameters for the constituent of concern (including the name of the chemical and its associated density) and aquifer properties (including hydraulic conductivity, horizontal hydraulic gradient, and effective porosity).  The user can also specify a location along the plume at which mass flux is calculated through a vertical plane (i.e., YZ plane) that is oriented perpendicular to the X direction of groundwater flow.

Based on the user-specified inputs provided, the PVM Calculator estimates the following for each isoconcentration contour interval:

• area
• volume
• dissolved and sorbed chemical mass
• chemical volume

Additionally, the PVM Calculator also estimates the contaminant mass flux across a vertical plane transect (if specified by the user).

These plume metrics are useful for a variety of conceptual site model development and remedy planning project steps.

The SCL Plume Model and PVM Calculator spreadsheet each include a detailed User Guide which provides a comprehensive overview of the tools, step-by-step instructions for all inputs, descriptions of the calculations that are performed, and references to literature on which the tools are based. And, the tools are available FREE of charge on the TS-CHEM website.

To learn more about the SCL Plume Model spreadsheet, or to download a copy, click HERE.  To learn more about the PVM Calculator spreadsheet, or to download a copy, click HERE.