Background
A natural gas transmission company intended to sell an abandoned property and needed to assess whether the property had any impacts from historical uses prior to selling. After sampling, benzene was detected at elevated concentrations in groundwater in fractured bedrock at this facility. Light non-aqueous phase liquid (LNAPL) was also present in several measuring points. Through several characterization events, it was determined the source of the Benzene and LNAPL was historic releases from an on-site, used oil underground storage tank (UST). To ensure no further releases would occur, the tank was removed. All affected soils surrounding the tanks were also removed and transported to an approved landfill.
UST Remediation Project Work
Remedial Design
To address this site, AGES designed a Multi-Phase Extraction (MPE) system, through which both groundwater and soil vapor were extracted simultaneously. To apply the Soil Vapor Extraction (SVE) process to newly exposed soil, the groundwater table was lowered to dewater the saturated zone. This allowed the Volatile Organic Compounds (VOCs) in the previously saturated soil to be stripped by the induced vapor flow and extracted. Soluble VOCs present in the extracted groundwater were also removed.
Implementing the MPE System
To refine the MPE system requirements, AGES conducted a pilot test at the site for a total of 60 days. Approximately 28,000 gallons of groundwater were extracted from the subsurface through three (3) four-inch diameter bedrock wells. A recovery rate of approximately four gallons-per-minute (gpm) reflects the average rate during system operation. Air and water samples were collected periodically throughout the test and analyzed for Benzene, Toluene, Ethylbenzene and Xylenes (BTEX) as well as Total VOCs. Based on lab analytical data, over 10 pounds of VOCs were removed during the pilot test. The MPE pilot test was also successful at removing a significant volume of LNAPL from the subsurface.
Monitoring of Natural Attenuation
Based on the success of the pilot test, additional active remediation was not required. Monitoring of natural attenuation for the remaining dissolved-phase Benzene has, therefore, been performed. Benzene levels in groundwater have been significantly reduced over time. LNAPL has not been detected in measurable quantities in any wells or piezometers at the site since the conclusion of the pilot test. In addition to removing LNAPL, application of the MPE technology expedited the clean-up process at this site and will allow the client to sell the property without deed restriction.
Special Features
1.) AGES used MPE technology because it is more effective at removing VOCs than pump-and-treat or SVE alone. This is because of the increase in groundwater and contaminated soil vapor removal rates, and the volatilization of contaminants in the dewatered soil.
2.) The increased mass removal rates resulted in a decrease in total removal costs and shorter remediation time-frame.
3.) MPE technology allowed for re-sale of the properties without deed restriction, significantly increasing the sale price of the facility.
