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Former UPRR Service Yard—C�urc Status N./ Page 2 GPE Project No. 376.2 6/6/03 Based on the information gathered during the course of the investigation, especially the above summarized lab data; we believe it would be appropriate to close this site for the following reasons: I. The elevated concentrations of gasoline and diesel hydrocarbons decline dramatically at depth. The lowest depth of vadose zone contamination detectable by field methods was at 30 feet where a very slight gasoline odor was present. The 35-soil sample was subjected to laboratory testing and no detectable petroleum hydrocarbons were present. While there is insufficient data available to complete any mass balance calculations, based on the concentrations and the known vertical distribution, the total contaminant mass in the vadose zone is very low, most likely just a few pounds of total contaminant mass. The contaminant mass dissolved in the groundwater is unlikely to exceed even one pound. 2. The groundwater sample contained only very low levels of gasoline and diesel hydrocarbons. Toluene, ethylbenzene and xylene were present at barely detectable levels and no benzene, MTBE or other oxygenates were present. The character of the gasoline chromatograph was such that it appears that the gasoline is severely degraded by natural attenuation, and the total absence of benzene also supports this. The degraded nature of the gasoline would also appear to suggest, although not prove, that the lateral extent of the groundwater plume is also contraction. The fact that the source of the contamination was removed 15 years ago would support the notion that natural attenuation has probably already resulted in a significant reduction in the size of the plume. At a minimum, it is reasonable to assume that the plume is no longer spreading. 3. The groundwater contaminant levels are so low, that they could be a result of drilling tools carrying them to depth from the shallower vadose zone as the boring was advanced. Even assuming, however, that the lab results are representative of actual groundwater conditions the residual contamination should be considered to represent a very low risk to human health. In a telephone conversation on June 3, Harlin Knoll reported that a California Water Services map showing locations of municipal water supply wells indicates that the nearest well (Station 53) is located 2 blocks west of the site, while the next nearest well (Station 280) is 4 block east of the site. These sites are both sufficiently distant that its is extremely unlikely that the groundwater plume from this release would have impacted either well even if the direction of groundwater flow were directly toward them. In fact, based on groundwater information from a site one block south of the UPRR site, the groundwater flows in a northerly direction, and not toward either of the nearest two wells. Moreover, the shallow groundwater in Stockton is not generally considered potable, and the municipal wells generally pump from depths of greater than 200 feet. 4. Current plans call for construction of a new plant facility on the site. Such a construction would largely prevent stormwater from contacting the soil, and further minimize the 2937 Veneman Ave.,#B240 Geo-Phase Environmental Inc. Phone: (209)569-0293 Modesto, Ca 95356 Fax: (209) 569-0295