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Lot B Underground Storage Tank—Monitorin)411 and Additional Delineation Work Plan 216 N.California Street,Stockton,California February 2,2010 Page 2 In a letter dated December 22, 2004, Margaret Lagorio of the SJCEHD directed the Property Owner to submit a work plan to investigate the lateral and vertical extent of contamination at the Site. Condor prepared the Underground Storage Tank Closure Sampling Work Plan dated January 15,2008.The work plan was approved on January 26,2008 by SJCEHD. Condor completed one continuous core boring (SB-1) in the former tank pit area on February 19, 2008 (see Figure 3, Appendix A). Due to saturated soil conditions beyond 25 feet bgs, the drilling equipment could not make efficient progress and the boring was terminated at 50 feet bgs. A groundwater sample was collected using a push-type groundwater sampler. Soil exhibiting excavation backfill characteristics (medium brown silty sand and clayey sand) was encountered from surface to approximately 18 feet bgs. Soil staining was visible as greenish-grey discoloration from 11.5 feet bgs to the total drilling depth of 50 feet bgs. Noticeable petroleum hydrocarbon odors were coincident with stained soil. Silty clay was ` encountered from approximately 16 feet bgs to 26 feet bgs. From 26 feet bgs to 36 feet bgs, a downward- coarsening lithology was encountered (sandy silt to silty sand). Groundwater was encountered at approximately 30 feet bgs and sampled at 34 feet bgs, although groundwater depths in nearby monitoring V wells have recently been reported at approximately 25 feet bgs. Silty sand was encountered from 36 feet bgs to 42 feet bgs, followed by coarse sand with gravel from 42 feet bgs to 44 feet bgs. Sand with silt was encountered from 44 feet bgs to 48 feet bgs,followed by finer-grained silty sand with clay to 50 feet bgs. 60 Laboratory analytical results indicated that concentrations of TPH-D in soil were 2,400 mg/kg and 4,500 mg/kg at depths of 20 and 29.5 feet, respectively. Laboratory analytical results of soil samples collected ` below encountered groundwater indicated that TPH-D in soil was 850 mg/kg and 40 mg/kg at depths of 36 and 48 feet, respectively. TPH-D was detected in the highly turbid grab groundwater sample at a concentration of 590,000 micrograms per liter (gg/L) at 34 feet bgs. Petroleum hydrocarbon-impacted soil was present from the former tank excavation to the depth of groundwater at approximately 30 feet. Petroleum hydrocarbon concentrations in soil decreased from a depth of 30 feet to a depth of 48 feet(see Figure 4, Appendix A). The Site was historically in the channel of Miner Slough which flowed from east to west. The soils encountered indicate that stream channel deposits were present to a depth of 48 feet. Groundwater gradient directions in the vicinity of the Site have been toward the east-northeast, and groundwater depth varies from approximately 25 to 40 feet bgs. Cone Penetration Test (CPT) borings with an Ultra Violet Optical Screening Tool (UVOST) were made at three locations on November 12 and 13, 2008. A pore pressure test conducted in CPT3 at a depth of 98.6 feet indicated the depth to groundwater was approximately 37.3 feet bgs, compared to the 30 feet bgs that was indicated in February of 2008. The highest sorbed phase diesel concentration reported in laboratory analytical data d this phase of the investigation was 4 600 mg/kg in sam 1 CPT2-35 Soil�andab groundwater samples combine with the use of CUVOST systems prove ed sufficient develop a preliminary site conceptual model (see Figure 5, Appendix A). Spatial trends interpreted from available data indicated that heating oil (diesel-range petroleum hydrocarbons) was released from the UST, transported vertically due to gravity/fluctuating ` groundwater elevation, and adsorbed to fine grain soils. The body of diesel appears to thin with distance laterally from the source area and attenuate to lower concentrations both laterally and vertically. The dissolved diesel plume in groundwater appears to be spatially coincident with sorbed diesel, although the lateral and vertical extents of both sorbed and dissolved diesel remain to be delineated. Due to the highly turbid grab groundwater samples collected to date, accurate concentration of dissolved diesel in groundwater has yet to be determined. Diesel appears to be the primary contaminant of concern, as ` previously detected concentrations of benzene, toluene, ethylbenzene, and total xylenes (BTEX) have been less than California Regional Water Quality Control Board water quality goals. 60 i , i� CONDOR