In descending order, the stratigraphic sequence includes fill, organic marine clay, silty sand alluvium, and fractured Franciscan Formation bedrock. Bedrock is near the surface in the source area (less than 10 feet below ground surface [bgs]) where the overlying unconsolidated layers are thin or pinch out. The alluvium bedrock contact deepens to 200 feet bgs in the southern portion of the study area, where the silty sand that overlies the rock correspondingly thickens. Ground water occurs in the fill, alluvium, and bedrock, with flow generally to the south.

• Dense Non-aqueous Phase Liquids (DNAPLs)
• Fractured Bedrock

Recoverable dense nonaqueous phase liquid (DNAPL) and dissolved-phase chlorinated volatile organic compounds (CVOC) at concentrations greater than 500 milligrams per liter (mg/L) occur in the source area. The plume of dissolved constituents extends south from the source approximately 1,000 feet, where it abruptly attenuates to CVOC concentrations less than 0.01 mg/L. The plume of dissolved constituents outside the source area primarily occurs near the interface of the alluvium and bedrock in the 100 to 200 feet bgs range.

• Halogenated VOCs (0.1 to 500 mg/L)
• 1,1,1-Trichloroethane (0 µg/L)
• 1,1-Dichloroethene (0 µg/L)
• 1,1-Dichloroethane (0 µg/L)

• Borehole Geophysics
  • Natural Gamma
  • Caliper
  • Other (Resistivity)
• Surface Seismic Surveys
  • Reflection
• Other (Slug Tests)

Comments:
The investigation involved drilling wells and performing borehole geophysics, a seismic reflection survey, aquifer testing, and extensive monitoring. Forty-one wells were installed by air and mud rotary technologies with nested and multilevel constructions, resulting in 84 ground water monitoring intervals to depths greater than 600 feet bgs.

Geophysical logging was used to evaluate lithology, map depth and orientation of the fractures, and to assess ground water flow. The natural gamma, caliper, and resistivity (16- and 64-inch normal) logs were used to evaluate lithology and borehole effects. The geophysical logs were used to strategically locate well screen intervals in high-flow intervals and ensure that the plume is conservatively monitored.

The seismic reflection survey was performed to map faults, delineate the topography of the interface of alluvium and bedrock, identify areas of increased fracturing, and guide placement of new wells. The survey identified two primary southeast-dipping normal faults and one primary northeast-dipping reverse fault, each with antithetic faults. The geometry of these features is generally consistent with the orientation of structures in the nearby San Andreas Fault zone. The source area is characterized by high-fracture density and water production where the normal faults and the cross-cutting reverse fault converge. The less faulted area south of the source is characterized by lower fracture density and water production. Broad lithologic changes across the site correspond to structural changes, possibly related to fault movement.

Slug tests were performed in wells and borehole sections that were temporarily isolated with inflatable packers. The results yielded similar hydraulic conductivities for the silty sand and fractured weathered rock (10-4 centimeters per second [cm/sec]), median values for fractured unweathered rock (10-5 cm/sec), and the lowest values for unfractured unweathered rock (10-6 to 10-7 cm/sec).

Ground water monitoring over 3 years indicate that 1,1,1-TCA rapidly degrades to 1,1-DCE and 1,1,-DCA, consistent with studies that indicate abiotic and biological degradation pathways. Generally low concentrations of vinyl chloride, chloroethane, ethene, and ethane indicate that 1,1-DCE and 1,1-DCA are being completely degraded, although at slower rates than they are produced from 1,1,1-TCA. These results suggest that the plume may be in a steady-state condition, although more monitoring is required to evaluate stability of the plume.

DNAPL (1,1,1-TCA) has been periodically recovered from a fractured bedrock well for 3 years. Initially, 29 feet of DNAPL accumulated in the borehole when it was drilled through a fracture zone at 110 feet bgs (alluvium bedrock contact at 30 feet), and approximately 40 gallons were bailed before the well was constructed. DNAPL was periodically recovered by bailing and later by pumping all fluids from the well to increase recovery. To date, 70 gallons of DNAPL have been recovered. Recovery rates decreased to negligible over time. An exception occurred immediately after a magnitude 5.3 earthquake in May 2002, which apparently caused a 3.4-gallon temporary increase in DNAPL flow in the well.

• Other (Monitored Natural Attenuation; DNAPL Recovery)

The pilot test was performed to evaluate the potential to enhance DNAPL recovery.

The vacuum approach increased the recovery rate in the DNAPL well by approximately 25 times over the recovery rates before pilot testing with and without the vacuum. Initially, no DNAPL was recovered after one well volume was purged; DNAPL recovery increased to 6 gallons during the test, which involved removing several well volumes. The vacuum increased water production in the DNAPL well by 10 percent, but did not improve DNAPL recovery. These pilot tests did not induce DNAPL flow to the wells that contained high dissolved concentrations. The results indicate that sustained dewatering is the most significant influence on enhancing DNAPL recovery. Recoverable DNAPL is rare in much of the source area, even though dissolved-phase concentrations that exceed 300 mg/L are common. These observations indicate that much of the DNAPL is unrecoverable, in the residual or diffused phase in the rock matrix or in fractures.

The results of this investigation demonstrate that potential ecological receptors and potable water supplies are not at risk for exposure to site chemicals under current conditions. Based on the present understanding of the site, the remedial strategy may include DNAPL recovery, where practical, and monitored natural attenuation (MNA) for the dissolved plume. Contingency measures  to be implemented if the plume expands or accelerated remediation becomes necessary  may include in situ remediation of the dissolved phase and hydraulic containment.

References:
Warner, Jim; Melinda Truskowski; Lawrence Fieber; Glen Ernstmann; Dan Tisconcik; Bill Henrich. 2004. Strategic Investigation and Remediation of Chlorinated Solvents in Fractured Bedrock. The Fourth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, California, May 24-27.