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Dopaco, Inc. - 2 - 24 July 2013 800 W. Church St., Stockton San Joaquin County or clay with a deeper water bearing sand unit from approximately 30' bgs to 40' bgs. Existing groundwater monitoring wells: DW-1, DW-2 and DW-3 are screened across the sand zone from 30' bgs to 40' bgs. Groundwater flow direction in the sand zone, based on two quarterly monitoring events, was towards the north-northwest. The last round of groundwater monitoring results collected in March 2013 were non-detect for petroleum hydrocarbons and volatile organic compounds (VOCs). GC reported maximum soil concentrations in milligrams per kilogram (mg/kg) as shown in the Table below. Constituent Location/Depth Concentration in mg/kg ESLs Toluene CO- 15' b s 3,070,000 9.2 naphthalene CO- 15' b s 16.8 4.8 xylenes CO- 15' b s. 1,630 110 benzene B1+20' b s 15.2 2 1 =San Francisco Bay Region Water Quality Control Board Environmental Screening Level(ESL)for Commercial or Industrial 2=leaching Eight (8) grab groundwater samples (including one duplicate sample for AB1+) were collected by HydropunchTM or from temporary wells installed at AB1+, D3+, E0, E2+, and E3+ at depths ranging from 30' bgs to 47' bgs. Toluene was detected in all eight water samples, with concentrations ranging from 23.5 ug/L at E2- from 42' to 47' bgs up to 96,400 ug/L at AB1+ from 32.5' to 37.5' bgs. Benzene was detected in sample EO from 30 -35' bgs at 27.1 ug/L. The water quality objective for Toluene is 42 ug/L and for Benzene, 1 ug/L. A Generalized CSM Figure 5, geological cross section, shows the locations of the six product USTs, with a Toluene release from Tank B. In the Report GC provides a Low Threat Closure Policy evaluation and states; "Delineation of the plume vertically and to the north of CPT-13 is required." In addition, soil gas characterization is needed to evaluate potential vapor intrusion to indoor air that exhibited high sub-slab VOCs (toluene and benzene exceeded ESLs or California Human Health Soil Levels) in 2011 prior to building demolition, and additional soil characterization for comparison to LTCP direct contact and outdoor air criteria. GC also states in the Report that heaving sands encountered during the grab groundwater sampling may have caused shallow sediments to be dragged down into the boring to the sampling location, resulting in cross-contamination from shallow sediments in the groundwater data collected in the area of CPT-13. In the Work Plan GC proposed up to nine (9) cone penetrometer testing (CPT) borings offsite across Church Street to the north of CPT-13, with three initial offsite CPTs spaced along the sidewalk on the north side of Church Street (CPT-2013-19 through CPT-2013-21) and five (5) optional (step-out) offsite CPT borings on properties further to the north for lateral delineation. One (1) deep CPT boring (CPT-2013-13D) would be advanced onsite to at least 60' bgs, adjacent to CPT-13, and up to two (2) grab groundwater sample taken for vertical delineation of the groundwater plume.