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Project Case Studies

MTBE Source Project

Consulting for British Petroleum (BP), FCE geologist Roger Edens conducted a special project evaluating MTBE impacts in groundwater across a large portfolio of sites that were under active remediation. The EPA had mandated that the region have Stage 2 Vapor Recovery (Stage 2) and many sites were retrofitted for this.

While managing various remediation sites, Edens noted that the success of MTBE remediation was highly variable and was not readily accounted for by known site conditions. However, when repairs to a Stage 2 system at one managed site correlated with a rapid drop in MTBE concentration in groundwater, Edens conducted a regional study of sites with unexplained MTBE impacts to groundwater where Stage 2 systems were present.

Pressure decay tests were conducted at a broad suite of selected sites. The study determined that leaks in Stage 2 systems correlated with elevated MTBE in groundwater. Repairs were made to these systems and MTBE concentrations at many of the subject sites quickly dropped.

Mr. Edens’ findings indicated that vapor phase gasoline could partition MTBE from gasoline through the vadose zone to groundwater without the release of free phase gasoline to the environment. Notably, sites with soil vapor extraction systems appeared more susceptible to this process.

Minimum Hydraulic Divide

In another consultation with British Petroleum, FCE geologist Roger Edens, facilitated an Interim Remedial Action compromise between BP and a regulating agency that had been working to improve the quality of stormwater runoff into a local wetland. The regulator determined that the retail service station was leaking petroleum related compounds to the stormwater system and demanded emergency remedial action.

Mr. Edens evaluated the site’s hydrogeologic and hydrologic conditions and proposed a unique solution to reduce groundwater flow into the stormwater system. He devised a groundwater extraction trench between the site and the stormwater line that created a hydraulic divide preventing contaminated groundwater from entering the stormwater system.

His solution met the regulators demands, reduced the on-site footprint and lowered the site owner’s costs, all without interrupting day to day operations. Additionally, a monitoring well network was installed that compared groundwater elevations across the hydrogeologic system and controlled pumping rates. This allowed dewatering efforts to be dynamically adjusted to achieve the technical objective at the lowest flow rate. The result minimized energy consumption, materials consumption, system size, and overall costs.

Expedient Remedial Excavation

When consulting for a medium-sized retail petroleum operator, FCE geologist Roger Edens, conducted a sitewide soil excavation in coordination with a scheduled underground storage tank (UST) system replacement. Tank replacement was mandated as a result of updated regulations requiring secondary containment for the UST system. The owner had been gifted the existing USTs by the former branded operator at the cessation of operations and rebranding.

The initial site investigation was conducted by a regulatory agency to identify all the parties responsible for a regional conjoined plume. Undifferentiated impacts to soil and groundwater were present from this and adjacent sites. Follow up actions removed contaminated soil and product from the site. These efforts allowed remaining groundwater impacts to be included in the countywide, non-groundwater use classification exception area.

Bedrock Fracture Hydraulic Control

While consulting for a municipality, FCE geologist, Roger Edens, conducted a remediation of free phase product impacting groundwater from a fractured shale aquifer. The on-site fueling station at their municipal firehouse had leaked for a long time before losses triggered concerns. The leaking tank system had caused a significant plume of free phase petroleum product and had contaminated groundwater within the fractured shale geology below the site.

To better understand the bedrock fracture network that controlled groundwater flow, Mr. Edens conducted a fracture trace analysis. The analysis identified a prominent fault crossing the site, readily traceable across the cityscape as crests, broken curbs and asphalt, and 2 dead trees in the center of the free phase plume.

A well was placed in-line with the downgradient of the source area. Hydraulic testing indicated good connectivity to other wells within the plume and that strong control of the plume was possible from this highly transmissive location. A groundwater pump and treatment system was connected to this well, and water levels indicated a strong connection with other wells at the site. As concentrations of contaminants dropped with pumping and treating, a second well located at the closed UST was also connected to the system. Together these wells were able to extract the product and remediate the site with simplicity and precision.