Solving Contaminated Groundwater Puzzle

In 1986 underground storage tanks were discovered to be leaking at a gas station within the City of Seal Beach. Over the next 23 years, site cleanup was a low priority according to Regional Water Quality Control Board policies. The site’s unsuccessful clean-up efforts included removing the tanks, drilling groundwater monitoring wells, and sporadic remediation using dual-phase extraction and over-pumping of groundwater.

That all changed in 2009 when the station owner, BP/ARCO, requested regulatory approval to close the site — reporting the groundwater plume was stable and the site was low risk. After filing the request, however, sampling required by Orange County Healthcare Agency revealed benzene vapors had migrated beyond the site’s perimeter and beneath residences in the neighborhood causing the temporary evacuation of four homes.

At that point, city officials became involved and sought to re-evaluate closure in light of the potential health risks and community resident concerns including impacts to property values.

Discovering Misleading Groundwater Data

The city hired Dudek’s hydrogeologists, chemists, and environmental engineers to perform an independent review of the reports, vapor monitoring procedures, and remediation processes proposed by BP/ARCO’s consultants following the discovery of benzene vapors in the neighborhood.

“We determined the groundwater monitoring wells were providing misleading data, based on a discrepancy between concentrations of benzene reported in the groundwater and the concentrations in the gas vapors that penetrated the neighborhood,” said Dudek’s Peter Quinlan, R.G. “The monitoring wells’ measurements at the edge of the station had consistently indicated there was no benzene in the groundwater, but the newly-measured vapor concentrations at the same locations were extremely elevated.”

Dudek hydrogeologists looked at the well construction and sampling techniques and determined the well screens were too. Even though the well screens penetrated the contaminated layer, the groundwater that was sampled came from the sandy sediments at the well bottom, so groundwater samples looked falsely clean.

With this finding, the city asked the Orange County Healthcare Agency to direct the drilling of shallower wells, which produced groundwater data consistent with vapor and soil concentrations of benzene. With the contamination confirmed, more appropriate remedial alternatives were implemented and their effectiveness could accurately be evaluated.

Dudek’s review provided the technical basis for the Orange County Department of Environmental Health to require additional site investigations to better define the extent of the contamination. It also identified factors contributing to significant uncertainty regarding the potential effectiveness of previously recommended remediation methodologies using in situ chemical oxidation (ISCO) or electrical heating.

In October 2010, BP/ARCO informed the City and Orange County health officials that it will finally complete the 25-year clean-up at the site by demolishing the station, excavating the underlying contaminated soil, and trucking it to an approved disposal site.