The Long Road to NPL Site Deletion for an Intel South Bay VOC Site

摘要

The Intel Corporation (Intel) Santa Clara-3 site is attempting to be the first National Priority List (NPL) “Superfund” site in the South Bay to be officially deleted from the NPL. There are 23 current NPL Superfund sites in the South Bay, all of which were designated between 1982 and 1986. Santa Clara-3 was ranked and listed in 1986. Similar to many of the other South Bay sites, Santa Clara-3 was affiliated with semicon- ductor operations. Initial subsurface investigations revealed a release of chlorinated sol- vents, most notably trichloroethylene (TCE). Intel’s Santa Clara-3 is typical of other South Bay NPL sites in its remediation progress, from investigation through source re- moval. The U.S. Environmental Protection Agency’s (EPA’s) selected remedy—ground- water extraction and treatment (GTS)—was implemented from 1985 through 1993. The limitations of GTS in achieving Maximum Contaminant Level (MCL) cleanup goals at this site were first recognized in 1990, when numerous attempts began to optimize the performance of the GTS and cyclic pumping trials were conducted. No improvement was observed; however, in mid-1993, after 8 1/2 years of groundwater extraction, volatile or- ganic compound (VOC) concentrations in the groundwater plume were leveling off, and the plume boundaries were holding (based on non-detect levels in “guard” wells). Approximately 45 million gallons of groundwater was extracted over the 8 1/2 years of GTS operation. Approximately 28 pounds of TCE and 10 pounds of Freon-113 were re- moved. The mass removal rate decreased over time due to falling concentrations in groundwater. In 1985, the GTS was removing about 1 pound of VOCs per 1 million gal- lons of groundwater extracted; by 1993, only about 0.1 pound of VOCs was extracted per 1 million gallons of groundwater extracted. Average TCE concentration fell from about 400 micrograms per liter (μg/L) to about 25 μg/L. In 1993, the GTS was decom- missioned, resulting in a de facto remedy change to monitored natural attenuation (MNA). After 10 years of MNA, the residual concentrations in the three wells with de- tectable concentrations showed a diminishing concentration trend, although no sustained contaminant asymptote was achieved. In 2004, the EPA was first approached to consider NPL site deletion under the Technical Impractability (TI) rationale. The California Re- gional Water Quality Control Board had already indicated that it considered the site a candidate for closure under its guidance. The EPA stated that, if Intel implemented a Best Available Technology to attempt to reduce the residual VOCs to their respective MCLs, and if the application of the remedy did not reach MCLs, the EPA would consider a TI waiver. In-situ chemical oxidation was selected and applied through injection at 84 points into the residual plume in late 2006, resulting in a further reduction of TCE to an average of 14 μg/L; this concentration did not, however, fall below the 5-μg/L MCL for TCE (the effectiveness of the chemical oxidation remedy was limited by site geochemistry and lithology). Thereafter, the EPA allowed the application for a TI waiver for NPL site dele- tion to move forward.