Plume Stability & the Ricker Method

After soil and groundwater impact have been fully delineated and all risks to human health and the environment have been evaluated, the next step to getting a Certificate of Completion or No Further Action (NFA) status for your site is to confirm that any impact to the groundwater is decreasing or stable in concentration, and is not migrating away from the point of release. This is important because an increasing plume could migrate off-site and pose additional risks. Plume stability can also be used to evaluate whether remediation at a site has been successful at reducing or eliminating risks.

Plume Stability & the Ricker Method

Plume stability is performed using statistical methods to evaluate the groundwater data that has been collected at each of the monitoring wells at the site. In order for the results to be statistically significant and conclusive, regulators generally require a minimum of four to six sampling events, depending on the statistical method to be used.

The Mann-Kendall Analysis (or MKA) method can determine a trend of increasing or decreasing with as few as four sampling events – however, if the data is still indeterminate with four events (i.e., it does not show a clear increasing or decreasing trend), a minimum of six sampling events are required for a trend result of “stable” to be considered statistically valid using the coefficient of variation calculation (CV). However, one of the downsides of the MKA/CV method is that it tends to focus on results in individual wells, rather than on the plume as a whole, resulting at times in situations where consultants and regulators “can’t see the forest for the trees,” as the expression goes.

Example of a Mann-Kendall Analysis with Coefficient of Variation.

Example of a Mann-Kendall analysis with coefficient of variation

Another more comprehensive method for evaluating plume stability as a whole was developed by Joe Ricker in 2008 and is generally referred to as the “Ricker Method®.” The Ricker Method® utilizes multiple evaluation tools (including 3D modeling) to determine whether a plume is expanding, shrinking, or stable in relation to the point of release. Rather than just relying on one statistical calculation like the MKA/CV, the Ricker Method® includes qualitative evaluations such as “concentration vs. time” and concentration isopleth maps that can show increases or decreases in concentrations at the plume boundaries.

Isopleth Diagrams showing concentration change over time.

Isopleth diagrams showing concentration change over time

The method also calculates the total mass and area covered by the plume, and calculates the “center of mass” for the plume during each sampling event. These calculations of total area and mass can then be compared for each event to show whether they are increasing or decreasing. The center of mass from each event can be compared to the others to show whether the center remains in the same spot (stable), moves closer to the original source area (shrinking), or away from the source area (expanding).

Figure showing Center of Mass (COM) receding back toward tank pit.

Figure showing Center of Mass (COM) receding back toward tank pit

If you have a site with a complex groundwater plume and need assistance with evaluating plume stability, Environmental Works is here to help. We can provide Ricker Method® evaluations for property owners and as a stand-alone service to other consulting firms. We are also experienced in working with state insurance and trust funds to ensure that costs for these evaluations are recognized and reimbursed. To learn more, please click here to  contact us online.