Case Studies of 1,4-Dioxane Releases, Treatment, and Drinking Water Contamination - Environmental Investigation and Remediation - page 394

8.5.3 C ONTAMINANT R ELEASES
There are four small VOC plumes at SLAC, three of which involve release of methyl chloroform and
1,4-dioxane. The plumes emanate from a variety of minor releases that occurred through chemical
handling beginning in the early 1960s. Because the Santa Clara Formation and Ladera Sandstone
have low hydraulic conductivities, the plumes migrate very slowly ( 10 ft/year) and are all less than

200 ft long. The 1,4-dioxane plumes occur at facilities within SLAC known as the Former Solvent
Underground Storage Tank (FSUST), the Plating Shop Area, and the Former Hazardous Waste
Storage Area (FHWSA) (Sabba and Witebsky, 2003). SLAC staff has not found evidence of direct
disposal of solvent wastes, and DNAPLs (dense, nonaqueous-phase liquids) or large masses of sol-
vent have not been encountered at the site. Aerial photographs show that the FHWSA was used as a
drum storage area during the 1970s (SLAC, 2004). In addition to possible drum leaks, other release
mechanisms at SLAC may have included steam-cleaning wastewater, vapor degreasing operations,
l oor sumps, leakage through the l oor of the plating shop, spills and leaks in chemical storage areas,
and spills and leaks at a rinse-water treatment operation. Some combination of these sources con-
tributed to solvent and 1,4-dioxane releases at the Plating Shop, FHWSA, and FSUST.
SLAC reports that vapor degreasing operations switched from trichloroethylene to methyl chloro-
form before 1980; this change provides a time marker for solvent releases bearing trichloroethylene.
Releases including trichloroethylene and its breakdown products are considered to have started
between 1963 and 1968 and stopped prior to 1980 (SLAC, 2003). At the plating shop area, 1,4-diox-
ane is present in plumes having no remaining methyl chloroform, suggesting complete abiotic deg-
radation of methyl chloroform to acetic acid and 1,1-dichloroethylene and biologic degradation to
1,1-dichloroethane and chloroethane.
SLAC staff and consultants cite the DNAPL literature and note that VOC migration in the Ladera
Sandstone depends on the mode of release. A sudden, large-volume spill would be expected to rap-
idly migrate downward and laterally, leaving a relatively large volume of residual VOCs in the
unsaturated zone, whereas a slow leak occurring over long periods of time is expected to migrate
downward along permeable pathways and be less prone to lateral migration. Therefore, daily de
minimis losses from spills, leaks, splashes, and drips are likely to deliver more VOCs to the water
table beneath a thick unsaturated zone (SLAC, 2003). Conditions observed in the Plating Shop Area
at SLAC are consistent with long-term releases of small volumes of solvent.
The local saturated hydraulic conductivity of the Ladera Sandstone at the FHWSA is estimated
to range from 3.3 × 10 -5 to 1.4 × 10 -4 cm/s. Yields from three pumping wells screened in the Ladera
Sandstone at the FHWSA have ranged from approximately 53-170 gallons/day, and groundwater
velocity ranges from 5 to 10 ft/year (SLAC, 2004).
Seasonal water level l uctuations beneath the FHWSA are from 3 to 5 ft. Since 1997, depth to
groundwater has ranged from 10 to 32 ft bgs. The linear accelerator subdrain system drains ground-
water beneath SLAC at 35 ft bgs and exerts an inl uence over local groundwater l ow, reversing the
natural southward and eastward gradients. Groundwater collected by the subdrain system is
discharged to San Francisquito Creek at about 5 gpm (SLAC, 2004). Two other large subterranean
structures used for high-energy particle research at SLAC are equipped with subdrains. Two curv-
ing tunnels were bored through the Ladera Sandstone at depths ranging from 60 to 90 ft bgs, each
several thousand feet long. The tunnels housing the experimental equipment act as groundwater
drains, but discharge only 1.5 and 6.2 gpm over the entire tunnel lengths. The low rate of ground-
water discharge in these two tunnels and in the linear accelerator subdrain support porous media
l ow as the primary mode of groundwater and contaminant transport, although fractures and other
conduits may locally inl uence hydraulic gradients (SLAC, 2004). The complex hydraulics
inl uenced by drains, topography, fracture orientation, and the strikes and dips of bedded subunits
in the sandstone combine to cause a large range of groundwater l ow directions. Three of the small
groundwater plumes at SLAC are migrating toward the linear accelerator subdrain system, which is
monitored for contaminants prior to discharge to San Francisquito Creek (SLAC, 2006b).

<