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.
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