Former ARCO Service Station, Tracy

The focus of this project was assessment of complex stratigraphy and hydrogeologic relationships, feasibility testing, and the implementation of a remediation via SVE. Impact at this station presented a unique challenge in characterizing and remediating a relatively large release of petroleum hydrocarbons to the subsurface. This site is located on the east-sloping flank of the Coastal Range, immediately west of Interstate 580. Leaking USTs, product lines and dispensers are estimated to have released over 200,000 pounds of hydrocarbons into the subsurface. There is a domestic water supply well at this site that supplies water for station operations.

Initial site assessment activities, completed in the early 1990s using the hollow stem augers, revealed high concentrations of hydrocarbons in soil onsite and offsite. Only shallow assessment work was performed at this time, as very hard drilling conditions severely limited soil sampling using the standard California-modified split spoon sampling system. By the time Stratus became involved with this site in late 1999, a total of 20 soil vapor wells had been installed. Stratus designed and installed an SVE system and began aggressive remediation efforts.

Subsequent assessment activities by Stratus personnel included detailed logging and sampling of continuous soil cores collected by the mud rotary drilling method, followed by downhole geophysical logging techniques (including resistivity, spontaneous potential, and gamma). This phase of assessment constrained the extent of impact, and identified the presence of groundwater and free product beneath the site. This phase of assessment revealed that while there were two relatively thick permeable zones beneath the site, these permeable zones were not completely saturated. Groundwater and product were present in an interval approximately 1 foot thick of top of the aquitard at the base of both permeable zones (at approximately 65 and 95 feet bgs).

Like the topography in the immediate site vicinity, strata beneath the site were found to be dipping away from the site, a result of orogenic uplift within the adjacent Coast Range. Groundwater flow appears to toward the east, into the Central Valley, generally parallel to the topographic slope. Groundwater flow is also greatly influenced by weather cycles and seasonally rainfall on the adjacent topographic upland. Based on this thin saturated zone and an accurate estimate of the strike and dip of the confining layers, a total of 20 groundwater monitoring wells were installed in an ongoing effort to constrain the lateral extent of dissolved and free-phase hydrocarbon impact off-site to the east.