SITE INFORMATION AND CORRESPONDENCE FILE 2

Home Browse Search

• Steve & Gene's Service ti.�►' Page 2 2315 N. Ell Dorado Street 07 March 2008 Stockton, CA feet bsg to collect soil samples and install deep monitoring wells screened across encountered deep sand units, or in the absence of sand units, at approximately 89 feet bsg. Four additional soil borings would then be advanced adjacent to each of the deep wells to install monitoring wells screened approximately 65 to 75 feet bsg; no soil samples were proposed to be collected from the latter four borings. Selected soil samples would be submitted for laboratory analysis for total petroleum hydrocarbons quantified as gasoline (TPH-g) by EPA Method 8015M or 8260, and for benzene, toluene, ethylbenzene, and total xylenes (BTEX) by EPA Method 8021 M or 8260. The new monitoring wells would be added to the existing groundwater monitoring program. The results of the investigation would be incorporated into an updated site conceptual model (SCM). The EHD noted during the review of the site data that the most intensely impacted soil samples were recovered from soil boring SB-B and the boring for monitoring well MW-3, while the soil samples from the boring for monitoring well MW-2 were significantly less impacted than the SB-B soil samples. This suggests to the EHD that proposed boring SB-5 should be located between SB-B and MW-3 and near the former dispenser island to be closer to what appears to the EHD to be the soil plume core to assess the vertical extent of significantly impacted soil. The EHD recommends that sufficient soil samples be collected for laboratory analysis above 55 feet bsg to correlate the analytical data with the data from SB-B and MW-3, and at five-foot intervals from 60 feet bsg to the base of the sand unit inferred to lie between 60 and 80 feet bsg. GTI did not provide a technical justification for continuously sampling soil from 90 to 100 feet bsg; the EHD infers this is to maximize the potential to encounter the sand unit locally sampled at this depth. The EHD approves that sampling proposal. A sand unit commonly encountered between 60 and 80 feet bsg, hereafter referred to as the major sand unit, is inferred to be continuously present beneath the site, based on soil data from MW-1 and MW-101 onsite and from MW-7, MW-8 and MW-9 offsite, the latter wells arranged like the points of a triangle around MW-1 and MW-101. The major sand unit appears to be at least 10 feet and possibly 15 or more feet thick, and locally as much as 25 feet thick (MW-9). Further offsite, the major sand unit may be present in CPT-1, but is less apparent in CPT-2 and CPT-3 and may even be absent in those areas. The EHD considers this sand unit to be the major potential lateral contaminant migration pathway and therefore warrants a high degree of characterization. Onsite, the major sand unit is monitored by MW-1 and extraction well EW-2 is screened across the sand. It is possible that the bottoms of monitoring wells MW-2 and MW-3 are just barely set in the top of the major sand unit, but the boring logs do not support such an interpretation; the two wells may be too shallow to monitor the sand, but may be providing a vertical conduit to within a few feet of the major sand unit. Offsite, the major sand unit is monitored by MW-207, MW-208 and MW-109, though these wells' screens are set in the lower half of the sand, and the MW-207 well screen may actually be just below the sand. Intuitively, one would think that contaminants diffusing into the sand from the overlying fine-grained soil would then migrate laterally by advection with the groundwater and that groundwater in the deeper portions of the sand may be less impacted or even not impacted. Based on the data and interpretations noted above, and on the contaminants detected in the grab groundwater sample collected at 73 feet bsg in CPT-3, the EHD approves the WP Comment Letter 0308