Case study

Use of Dissolved-Phase Plume Stability to Evaluate LNAPL Body Stability with GWSDAT

Lisa Reyenga, P.E.,

GEI Consultants, Inc.

For more information on LNAPLs please see the original publication here.

Case Study

The Site was the location of a pipeline release of LNAPL. The data set includes 10 years of dissolved-phase concentrations of benzene from 2007 through 2016. During this time hydraulic recovery was conducted to remove LNAPL mass, but no active compositional change remedies were utilized to preferentially remove benzene from the LNAPL.

Figure 1 presents a spatial animation showing the interpreted concentration in the aquifer for each year of monitoring. The model interpolates through both space and time, so concentrations can be interpreted even where a location was not included in a particular monitoring event. Individual concentration data points are shown next to the well location. The red line indicates the extent of the plume corresponding to a benzene concentration of 5 µg/L (e.g., the Tier 1 screening threshold). The red cross indicates the location of the center of mass of the plume. The animation provides an excellent visual tool to illustrate the shrinking plume.
Figure 1 – Spatial animation showing the interpreted concentration of benzene in the aquifer for each year of monitoring.
Figure 2 presents a series of graphs showing the calculated plume mass, area, and concentration over time using GWSDAT. These graphs illustrate a steady decrease from 2007 through approximately 2013. At that point concentrations greater than 10 µg/L are largely absent and the plume mass, area, and concentration stabilize. There is an apparent expanding trend in plume area from approximately 2014 through 2016. However, this is an artifact of changes in the monitoring program rather than actual plume expansion. During this time, only the perimeter wells were monitored, and no data points were collected in the interior of the plume.
Figure 2 – Benzene plume mass, area, and concentration over time estimated with GWSDAT
Figure 3 demonstrates the benzene concentrations over time for an example individual well. The concentration is on a log-scale, indicating that the benzene concentration is decreasing exponentially, consistent with natural attenuation. The Mann-Kendall p value (0.0864) is also shown on the graph. This result indicates a statistically significant decreasing trend.
Figure 3 – Benzene concentration over time at an individual well
The shrinking benzene plume is a critical risk evaluation component for the Site. It confirms there are no potential new risks as the result of LNAPL or dissolved-phase migration into new areas, barring new releases or significant hydrogeologic changes at the Site.
A Word of Caution
Like any analytical model, the GWSDAT results are only as reliable as the data provided. Professional judgement must be utilized to ensure that the monitoring network is sufficient in both space and time to characterize the plume extent and behavior. Dissolved-phase plume stability may also vary, depending on the constituent. Evaluation of more than one constituent may be required to fully characterize the Site.