UST Release Remediation & Design Plan

Soil and groundwater samples collected during underground storage tank (UST) system removal activities revealed that the UST system had leaked gasoline into the subsurface and impacted soil and groundwater. A “Suspicion of Release” report was made to the Oklahoma Corporation Commission – Petroleum Storage Tank Division (OCC-PSTD), the regulatory agency with jurisdiction over the operation of petroleum storage tank systems and any associated contamination. Envirotech Engineering & Consulting, Inc., assisted the tank owner with gaining access to the Indemnity Fund, which provides reimbursement for corrective action performed on OCC-PSTD-regulated pollution sites that have resulted from releases of petroleum from storage tank systems. Envirotech began investigating the release in accordance with OCC-PSTD guidelines and protocols. Groundwater plume delineation activities were conducted through several rounds of monitoring well installation projects both on-site and off-site. Once the plume was adequately delineated, a risk assessment was conducted to establish a Site Specific Target Level (SSTL) for soil and groundwater. In order to achieve the SSTL for groundwater, active remediation was required. Air Sparge and Soil Vapor Extraction (AS/SVE) were the remediation technologies selected for the site due to favorable subsurface conditions and proven success.

Envirotech designed and installed the remediation system and associated wells at the facility.  The remediation system consisted of wells, trenches, piping, and road bore activities. Thirty-two (32) air sparge (AS) wells were installed to 20-ft deep and thirty-seven (37) soil vapor extraction (SVE) wells were installed to 5- to 8-ft deep, and three (3) groundwater recovery wells were installed in the most impacted portion of the plume to aid in recovery. This process was completed to provide gradient control to protect the adjacent creek. The AS and SVE wells were used for remediating the hydrocarbon-impacted soil and groundwater. The recovered groundwater was routed to an air stripper, where it was treated to remove hydrocarbons before being discharged to the municipal sewer. The remediation system was housed in two (2) separate buildings at the facility. All soil generated during drilling and piping trench construction was containerized in roll-off containers, characterized, and disposed of at a landfill permitted to accept non-hazardous industrial waste.

Unique situations encountered during the project included a modification to the design and construction of the remediation system buildings to be compliant with flood zone requirements. The base flood elevation was determined, and the buildings were elevated 4-ft off the ground to satisfy the flood zone requirements. Also, the grid of remediation wells and associated piping extended off-site to another property which required crossing a city road. A road bore was constructed to run the remediation system piping through to the property across the street in order to preserve the integrity of the road.

Upon activation of the remediation systems, field technicians checked the systems several times a week to ensure systems were running as designed. The checks included inspections, adjustments, and recording of the operational parameters of the system including temperature, pressure/vacuum, and operating hours of the AS and SVE units. Any accumulated water in the knockout tank of the SVE system was drained into a drum for disposal through the air stripper. The exhaust of the SVE system was monitored with a PID (photoionization detector) to provide information on the vapors recovered from the vadose zone.  In addition, necessary maintenance activities were conducted on a scheduled basis.  Groundwater samples from key monitoring wells were collected on a quarterly basis to monitor the efficacy of the remediation systems.

After approximately three years of monitoring, it was determined that although most of the wells had experienced reductions in contaminants to levels below the established SSTLs, one (1) monitoring well remained above. It was decided to supplement the existing remedial efforts with In-Situ Chemical Oxidation (ISCO). Sodium persulfate was injected into the subsurface and subsequent sampling activities 60 days later revealed successful application. Subsequent sampling activities 90 days later revealed a rebound in benzene to a concentration just above the SSTL. A second ISCO injection event is planned to use more volume and more injection points to achieve the SSTLS and ultimately close the site. If the additional ISCO injection event is successful, closure activities are expected to commence in the winter of 2023.

The key monitoring wells were sampled quarterly to determine the effectiveness of the remediation system for three years.  The groundwater was sampled on the twenty-six (26) monitoring wells to determine if the clean-up goals had been met.  The surface water was sampled as well from a creek nearby. Purged water was contained in drums and processed through the on-site airstrip for remediation prior to discharge into the sewer system.