Theme: 04. Addressing Challenging Site Conditions

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

Formatted Title:

A Case Study of Approaches to Expedite Closure and Reduce Client Liability for Diffuse Multiple-Source Commingled Plumes

Background/Objectives:

More than 25 years after divesting a large industrial complex to multiple industrial-use owners and tenants, our client was notified that historical activities undertaken during occupancy caused or contributed to contamination. Although the client began divesting in 1977, the notification was based on a 2013 study identifying several tetrachloroethene (PCE) and trichloroethene (TCE) groundwater plumes contributing to soil vapor (SV) beneath an adjacent residential area. Up to 16,000 micrograms per cubic meters of TCE were detected in SV despite associated low-concentration groundwater plumes. In addition to the complexity added by multiple sources and associated potential chlorinated releases over time, the site is underlain by highly complex piedmont slope deposits resulting in abrupt lithologic changes and hydraulic conductivities across discrete lithologies that differ by several orders of magnitude. Industrial use across the 74-acre area resulted in various releases of PCE and/or TCE by different owners/tenants over decades. To address the agency’s concerns, the client entered into a Voluntary Site Contamination Assessment Proposal (VSCAP), established under Section 103I of the South Australia Environment Protection Act 1993, a formal agreement that is publicly available and was one of the first in the state.

Three years into the investigation of chlorinated compounds, the environmental agency indicated that our client needed to also investigate per- and polyfluoroalkyl substances (PFAS). The ubiquity of these chemicals and likelihood of widespread use post-1977 increased our client’s risk.

Four years into the project, changes were made to the audit team (EPA-accredited, third-party reviewer) and the VSCAP to better align with the project’s roadmap to closure. Currently, the project is on track to close within 3 years, likely with limited long-term monitoring or corrective action.

Approach/Activities:

Unlikely to be the only responsible party to identified contamination, we were able to reduce scope to seven (audit lots) from the original 59 divested lots. To expedite closure and identify other potentially responsible parties, the project team used the following approaches: 1) staged assessments using qualitative and quantitative screening tools; 2) maintaining stakeholder engagement; 3) sample SV and indoor air (IA) at residences (beyond VSCAP requirements) to show good stewardship to the public and ensure no risk to residents; 4) focus investigations on risk drivers (IA); 5) hold regular in-person meetings with the environmental agency; 6) conduct concurrent source differentiation work (identify chlorinated plumes migrating onto audit lots) and time series monitoring (assess trends); 7) identify PFAS use by all known owners/occupants including up- and downgradient of the audit lots to isolate other sources/known PFAS releases; and 8) use a dynamic management plan for stakeholder engagement and regulatory team collaboration.

Results/Lessons Learned:

This case study identifies lessons learned and approaches used to expedite closure and identify client and others’ timeframes of contaminating activities. Navigating the VSCAP processes required clear understanding of expectations from stakeholders. Vague language in the VSCAP objectives significantly increased the amount of investigation required – we will present lessons learned to clarify scope. Revisiting the preliminary site investigation with subsequent investigation results improved understanding of contaminating activities and conclusions related to responsibility. Managing to and engaging with a risk-based focus furthered investigations only in areas where the auditor/agency was still uncertain – we will compare Auditor relationships and present what is needed for successful engagement. Conducting voluntary IA sampling at residents provided evidence of low and acceptable risk. Shifting to a source differentiation focus to help identify others’ plumes and contamination timeframes. Conducting source differentiation concurrent with ongoing semiannual monitoring to expedite data collection, assess trends, and increase stakeholder confidence that risk was low and acceptable.

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Stanley Howell