Theme: 04. Addressing Challenging Site Conditions

Topic Area: 4d. Large, Dilute and Commingled Plume Case Studies

Formatted Title:

Re-Evaluating Existing Point of Compliance Well Locations at a Site with Commingled Plumes

Background/Objectives:

Point of compliance (POC) wells are often assigned early in the planning process, prior to implementation of remedial actions. In many cases, the POC well locations are chosen based on the potential site-derived plume migration direction under ambient groundwater flow conditions. However, during groundwater extraction remedies the groundwater flow direction in the vicinity of the site may shift and POC well locations are often not re-evaluated following remedial action implementation. In this case study the project’s primary objectives were to identify the underlying cause of persistent concentrations of chlorinated solvent compounds of concern, specifically tetrachloroethene (PCE) and trichloroethene (TCE), at POC wells and to develop a revised conceptual site model and modified corrective action Site plan based on the findings. Several factors were considered during this evaluation including: TCE and PCE concentration trends/particle path analyses that indicated the orientation plume axes were not consistent with former source areas at the subject Site, the presence of known groundwater source sites adjacent to the Site, and the natural degradation of PCE to TCE.

Approach/Activities:

Three main lines of evidence were used to identify whether the persistent PCE/TCE concentrations at POC wells were derived from former sources at the Site or off-Site sources: Mann-Kendall trend analysis of PCE/TCE concentrations within the core of the capture zone of the groundwater extraction and treatment (GET) system coupled with successful remediation of the former source area in the unsaturated zone at the Site; reverse particle tracking at individual POC wells that exhibited persistent PCE/TCE concentrations; and the reorientation of PCE/TCE plume axes during long-term operation of the GET system. These three lines of evidence were consistent and indicated persistent PCE/TCE concentrations at POC wells were not associated with the Site. Despite the presence of known nearby remediation sites, more definitive hydrogen isotope analysis could not be conducted due to low concentrations of PCE/TCE at key monitoring wells.

Results/Lessons Learned:

Based on the evaluations performed, it was concluded that the decreasing Mann-Kendall trends within the core of the capture zone and increasing Mann-Kendall trends at POC wells were attributable to off-Site PCE/TCE source(s). Additionally, it was determined that migration of the off-Site plumes towards the Site was caused by operation of the Site GET system, as demonstrated by reverse particle tracking analysis. These findings lead to two key recommendations: 1) temporary suspension of the GET to evaluate the off-Site and Site-derived plumes under non-pumping conditions and 2) re-evaluation of POCs based on the data collected during the temporary shutdown. Both recommendations were approved by the overseeing agency and the GET is currently in its temporary shutdown phase. Over the course of evaluation there were multiple lessons learned, but two stood out from the rest. First, the presumption that the concentrations at POC wells are representative of Site sources needs to be carefully evaluated to avoid prolonged operation and/or adjustments to the remedy. Second, that the origin of contaminants in areas with commingled plumes can be identified using multiple lines of evidence.

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Nadja Scholl