SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Mr. Russell Chapin 1766 W. Monte Diablo Avenue Page 2 of 7 Distribution of Contaminants in Soil — The site geology is described as 6 to 35 feet of fine-grained soil overlying 4 to 31 feet of sandy soil that locally occurs at the surface and is locally underlain by fine-grained soil. Higher concentrations of TPH-g and BTEX in soil extend from 16 feet below surface grade (bsg) downward into the saturated zone in the NE portion of the site. Higher concentrations in soil extend approximately 145 feet toward the SW at 20 plus feet bsg. Utilizing pre-remediation soil data, but omitting more recently acquired data from confirmation borings and borings angled under the building on site, ATC estimates that 10,060 cubic yards of impacted soil underlies the site The impacted soil is estimated to contain 5733 Ib of petroleum hydrocarbons. ATC provided no estimate of the effects of remediation on the sorbed contaminant mass. Distribution of Contaminants in Groundwater — The depth to water has recently been approximately 13.5 to 17 feet bsg. ATC reports the inferred ground water flow direction, based on GW elevations to be predominantly toward the NE and SE, but inferred from the contaminant distribution that it is toward the SW. The simple correlation of the sand occurrences shown on the cross section may not properly indicate the contaminant migration pathway(s). After review of the cross section provided and several of the boring logs, EHD believes that there may be more than one sand unit in the area, and if so, the various sand units may have limited hydrologic communication, causing problems with the inferred flow direction(s). The monitoring histories and contaminant concentration trends for each well are described with an indication made of how the contaminant concentration trend in each well either shows significant contaminant mass reduction or defines the plume margins. In this section, ATC did not mention or include analysis of periods of dynamic remediation and the attendant effects on contaminant concentrations, leaving one to infer that the trends are the result of natural attenuation — which may not have been the intent. The most striking example is the TPH trends in MW-1, installed near the former UST and near the inferred source area, monitored since 1990. ATC notes the concentrations dropped from 200,000 ppb in 1990 to 270 ppb in November 2003. ATC notes this is a 99.8% decrease of contaminant concentrations. Not emphasized in this portion of ATC's presentation is that MW-1 was not monitored from December 1990 (200,000 ppb TPH) to July 1995 (TPH 27,000 ppb). Free product was in the well and removed from August 1993 to September 1993, with no free product reported since 1995. Ground water elevations were generally rising, going over the screen of MW-1 by June 1996 and is still over screen. At this time the possibility cannot excluded that rising ground water elevation may have trapped the free product, preventing it from migrating to the vicinity of MW-1. While natural attenuation has undoubtedly played some role in the reduction of contaminant concentrations, the following table shows the correlation between remedial activities on the site and dissolved TPH-g and benzene concentrations in monitoring well MW-1: