Theme: 04. Addressing Challenging Site Conditions

Topic Area: 4a. Adaptive Site Management: Lessons Learned for Site Characterization and Remedy Implementation

Formatted Title:

Case Study: Adaptive Management to Achieve Closure of a Dry Cleaner Site in California

Background/Objectives:

This case study outlines steps taken to accelerate and secure closure of a former dry-cleaning site in the City of Santa Barbara, California (Site). The project began over 20 years ago and responded to a regulatory shift in the risk focus to vapor intrusion (VI). This necessitated adaptive management to pivot and provide value-added solutions to remediate groundwater (GW) and VI risk. Initial soil sampling (2001) identified tetrachloroethene (PCE) in soil and GW as the principal chemical of concern. Following investigations, sampling, and soil remediation, three closure requests (2004, 2006 and 2009) of the Central Coast Regional Water Quality Control Board (Board) were made and denied, and in each case the Board requested additional investigation, GW monitoring, investigation of upgradient PCE sources, and remediation.

Approach/Activities:

OTIE assumed management of the project in 2013 and installed multi-level soil vapor (SV) probes responding to current regulatory concerns. Elevated PCE SV and GW results prompted installation of an air sparging/soil vapor extraction (AS/SVE) system which operated from 2015 to 2018. Operation of the AS/SVE system proved successful, reducing PCE concentrations by an average of 98% and initially stabilizing concentrations though later showing rebound in the source area. Collection of additional GW data in 2013 confirmed an upgradient source. Collaborative sampling of surrounding properties established a regional GW gradient to support that PCE was migrating onto the Site from upgradient sources.

SV rebound results in 2019 triggered sub-slab (SS) and indoor air (IA) sampling of the occupied building. Some IA samples showed PCE concentrations above San Francisco Board Environmental Screening Levels (ESLs) for IA which had been established as cleanup objectives. Up to eight vapor pins were installed inside the building to monitor SS vapor concentrations. The SVE system was restarted for VI mitigation and remediation with an additional temporary shallow indoor SS SVE well in the source area. Declining SVE performance prompted adaptive site management requiring a focus shifted to a permanent VI mitigation system (VIMS). An innovative passive sub-slab venting system (SSV) system and a membrane barrier were installed, along with contingency SS depressurization and SS pressurization systems to ensure VI performance objectives were met. The VIMS was installed and operational in 2021.

Results/Lessons Learned:

VIMS compliance sampling of two of four SS/IA monitoring events was completed in 2021 over two seasons. IA results were below ESLs for PCE for both events, and results from sampling of the vent risers proved the passive SSV system was effective, therefore additional SS/IA sampling was not required. The Board initially declined site closure and required additional GW monitoring based on an elevated PCE concentration in an onsite well. Following two GW events, a statistical trend analysis was completed that showed stable to declining trends in all monitoring wells. Armed with confirmation of an upgradient source, an effective VIMS, and declining groundwater trends, case closure was granted along with a land use covenant. The project succeeded in proactively addressing SV risks by quickly pivoting when active remediation was no longer effective, addressing VI risk with an innovative passive VIMS, and proving impacts from upgradient sources and statistically showing stable groundwater conditions to secure site closure. This case study provides additional elements of adaptive site management that will be explored in the presentation.

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Enrique Molina