Case Study – Challenges Encountered during Concurrent Implementation of Remedial Strategies at a Solvent Site

Groundwater, soil, and soil vapor below a former dry-cleaning facility are contaminated by tetrachloroethene (PCE) and various petroleum hydrocarbon compounds. Plumes of the various chemicals of concern are commingled onsite and have been delineated to extend below several city blocks. Fugro was retained by California Department of General Services to address risks posed, and to remediate the site for redevelopment. A Soil Vapor Extraction (SVE) system was the first remedy installed to mitigate vapor intrusion risk to nearby receptors. Thermal Desorption using Electrical Resistance Heating (ERH) was selected to remove contaminant mass in the Source Area. During ERH activities, Enhanced Reductive Dechlorination (ERD) in groundwater was implemented for the distal plume to promote the breakdown of PCE to its less harmful by-products. During operation of concurrent remedial technologies, several challenges required swift and dynamic changes in remedial system management. Notable challenges that arose include:

• Chemical desorption of petroleum by-products required a change in the SVE monitoring program to address the increase in contaminant vapor stream waste loading.
• Recovered steam contained more sediment than anticipated which complicated condensate treatment and disposal.
• Heavy rainfall during the winter of 2016-2017 caused groundwater elevations to rise above a semi-confining layer, complicating vapor control and contributing to desorbed contaminant migration beyond the initial capture zone.

Positive effects have also been observed with the addition of ERH into the remedial strategy. These include:

• Warmer groundwater temperatures induced by ERH have allowed dehalococcoides bacteria colonies to thrive.
• Lateral spread of the inoculants injected during ERD near the Source Area appears to have extended beyond the application area.

Our collaborative working relationship with the ERH contractor allowed real-time solutions to challenges in treatment, waste capture, and monitoring methodologies, leading to a successful project.