Case Studies of 1,4-Dioxane Releases, Treatment, and Drinking Water Contamination - Environmental Investigation and Remediation - page 393

materials. The SLAC data also show the apparently serendipitous removal of 1,4-dioxane on GAC,
possibly by passive biodegradation or chlorine oxidation.
8.5.1 SLACS ITE H ISTORY
SLAC is located in Menlo Park, neighboring the Stanford University Campus. SLAC is a high-en-
ergy physics research facility owned and operated by Stanford University under contract to the U.S.
Department of Energy. The site covers 426 acres. The facility is best known for the 2-mile-long
Stanford Linear Collider, the longest linear accelerator in the world, which is claimed to be “the
world's straightest object.” The linear accelerator can accelerate electrons and positrons up to
50 GeV (giga-electron volts) and has been operational since 1966. Other research projects located at
SLAC include the Positron-Electron Project storage ring, the Stanford Positron-Electron Asymmetric
Ring, and the Stanford Synchrotron Radiation Laboratory. Among numerous other superlative
distinctions, SLAC hosted the world's i rst web site.
Past research and support operations at SLAC included the storage and use of solvents for clean-
ing parts in machining and plating shops and for paint work, as well as other common solvent uses.
A 1984 site investigation discovered contaminated soil and groundwater, and in 1985, the California
Regional Water Quality Control Board issued a cleanup order for a former solvent underground stor-
age tank (FSUST) area at the SLAC facility. Since 1985, SLAC has conducted numerous soil and
groundwater investigations that included the installation of more than 100 groundwater monitoring
wells. Site cleanup activities have included (1) several efforts to remove soil contaminated with
VOCs and (2) extraction and treatment of contaminated groundwater within several small plumes
(Cal EPA, 2005).
8.5.2 G EOLOGIC AND H YDROLOGIC S ETTING
The SLAC facility is located in the rolling foothills of northern California's Santa Cruz Mountains,
above an alluvial plain that borders the western margin of the south end of San Francisco Bay. The
Santa Cruz Mountains were created in an active tectonic province characterized by ongoing strike-
slip and compressional movement of the nearby San Andreas Fault (SLAC, 2006a). The San Andreas
Fault is located 2 miles west of the SLAC site.
The SLAC facility is situated within the San Francisquito Creek Watershed, which encompasses
approximately 40 square miles and extends from the ridge of the Santa Cruz Mountains to the San
Francisco Bay (Cal EPA, 2005). Contaminated groundwater at the eastern end of SLAC l ows pri-
marily within the Ladera Sandstone, a thick sequence of silty marine sandstone that dominates
SLAC's geology. The Ladera Sandstone typically contains about 60% sand and 40% silt and is part
of a 2000-ft-thick sequence of consolidated marine sedimentary rocks of Eocene to Miocene age
(i.e., 55-5 million years old) (SLAC, 2006a). The Ladera is a sandy siltstone with discontinuous
fractures and very low hydraulic conductivity ranging from 10 -4 to 10 -7 cm/s (Sabba and Witebsky,
2003). Nonmarine silts, sands, and gravels of the upper Pliocene to Pleistocene Santa Clara
Formation (about 2 million to 100,000 years old) rest unconformably atop the bedrock (SLAC,
2006a). The bedrock typically has a weak to friable weathered zone that extends to 30 ft bgs.
Groundwater well yields and natural water quality have been evaluated to determine potential
benei cial uses of groundwater at SLAC. Results of the assessment indicate that groundwater in
most areas beneath the facility would not be suitable as a drinking water source on the basis of well
production rates lower than 200 gallons/day and concentrations of total dissolved solids above
3000 ppm (Cal EPA, 2005). Groundwater in the area has naturally high concentrations of total dis-
solved solids ranging from 890 to 8300 mg/L, and well yields measured in extraction wells screened
within the Ladera Sandstone are low yielding. Two SLAC extraction wells completed in the Ladera
Sandstone yielded 50 and 170 gallons/day (SLAC, 2004).