SITE INFORMATION AND CORRESPONDENCE FILE 1

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PUBLIC HEALTH SERVIAS f 1O P Y SAN JOAQUIN COUNTY ENVIRONMENTAL HEALTH DIVISION Karen Furst, M.D., M.P.H., Health Officer 304 East Weber Avenue, Third Floor • Stockton, CA 95202 cip; 209/468-3420 GEORGE AND MEI TERNAISHI DEC 0 1 2000 1600 WEST DURHAM FERRY RD TRACY CA 95376 Re: Georges Service 1600 Durham Ferry Road Tracy CA 95376 San Joaquin County Public Health Services, Environmental Health Department(PHS/EHD) has reviewed the report entitled "Work Plan—Supplemental Data Report for George's Service, 1600 Durham Ferry Road, Tracy, CA"(work plan). The work plan was approved on November 16, 2000. However, PHS/EHD has the following additional comments regarding the use of significant digits and hydraulic parameters incorrectly used in the work plan. PHS/EHD does not accept the hydraulic conductivity(K)values used for the groundwater velocity calculations in the work plan. The value for K is presented in the work plan as ranging from 0.10346 to 1.0346 feet per year(ft/yr). The number of significant digits used in the work plan to present K values suggests a degree of precision that is not actually present. These overly precise values are, perhaps unintentionally, misleading. A back-calculation indicates that 0.10346 ft/yr is actually derived from the value of 1 X 10-8 centimeters per second (cm/sec), and 1.0346 ft/yr is derived from the value of 1 X 10-'cm/sec. Moreover, it is unlikely that K values of 1 X 10-7 to 1 X 10-3 cm/sec are accurate for this site. Indeed, these K values are more often encountered in the requirements posted by the State of California for siting Class I and Class II Waste Management Units (landfills). Based on the boring logs included in the work plan, it is unlikely that the permeability of soil underlying the site is so low that the site could be used as a Class I or Class II landfill. Instead, boring logs for this site describe the soil lithology, in the 10 to 25 foot below ground surface (bgs) range as predominantly silty sand to sandy silt to clayey silt, not low permeability clay. A glance at the well-known Table 2.2 in Freeze and Cherry (1979) yields a K value ranging from 1 X 10-5 cm/sec to 1 X 10-' cm/sec for silty sands. Using these values in the seepage velocity equation yields a more reasonable groundwater velocity of 0.05 ft/yr to 500 ft/yr. Therefore, in the ten years since the Proposition 65 report was filed, the contamination in the shallow zone may have migrated anywhere from one half foot to almost one mile from the source. The values used to calculate K for the intermediate aquifer are more reasonable, despite having overly precise values. The values 620.79 ft/yr to 20, 692.92 ft/yr were clearly derived from the values 6 X 104 cm/sec and 2 X 10-2 cm/sec. As shown on Table 2 of the work plan, these are laboratory-derived values and are thus an improvement over purely estimated values, such as those used for the shallow aquifer equations. However, laboratory-derived values measure vertical hydraulic conductivity and it is horizontal hydraulic conductivity that is of interest. An often used rule-of-thumb is to assume that horizontal hydraulic conductivity is one order of magnitude greater than laboratory-derived permeability. Therefore, probable K values for the intermediate water bearing zone range from 2 X 10-' cm/sec to 6 X 10-3 cm/sec. Based on these values, and using an average porosity of 31 percent, groundwater velocity for the intermediate aquifer ranges from about 40 ft/yr to 1300 ft/yr. A Division of San Joaquin County Health Care Services