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i RAMAGE ENVIRONMENTAL 1''"``" Sacramento Office 15 Post Office Box 869 Rancho Murieta, CA 95683 Tel (916) 354-3250 June 15, 1999 Fax (916) 354-1786 Ms. Lori Duncan, R.E.H.S. San Joaquin County PHS/EHD 304 East Weber Avenue, Third Floor Post Office Box 388 Stockton, CA 95201-0388 Re: Residual Hydrocarbon Mass Calculations San Joaquin County Agricultural Commissioner's Office 503 East 101h Street, Tracy, California, PHS/EHD Site Code: 1793 Dear Ms. Duncan: On behalf of the San Joaquin County Facilities Management Department, Ramage Environmental is pleased to submit to the San Joaquin County Public Health Services Environmental Health Division (PHS/EHD)these residual hydrocarbon mass calculations for the above-referenced project. In your letter dated February 4, 1999, PHS/EHD recommended that these calculations be prepared and submitted after evaluating the results of two quarters of groundwater monitoring, sampling and analysis. The majority of the residual hydrocarbon mass, in the form of Total Petroleum Hydrocarbons (TPH)-as-diesel, resides beneath and slightly east of the former UST's, between the depths of 5 and 10 feet below ground surface (bgs). The term"residual"refers to hydrocarbon mass that is bound to soil media. This specifically excludes dissolved hydrocarbons in groundwater and vapor phase hydrocarbons in vadose zone sail pore space. Based on the distribution of residual hydrocarbon mass beneath the site,two zone boundaries have been delineated for evaluation (Figure 1). Zone 1 represents the area within which the highest concentrations of TPH-as-diesel exist beneath and in close proximity to the former USTs. Zone 2 represents the area further from the former USTs within which relatively low concentrations of hydrocarbons exist. Zone 1 is represented by residual TPH-as-diesel concentrations obtained via soil analytical lab data from soil samples associated with the UST removals and lab data obtained from borings SB-lA and SB-3. Within Zone 1, analytical data is available for the sampling depths of 5 and 10 feet bgs. Because the measured hydrocarbon concentrations are significantly lower at the 5-foot depth, as compared to the 10-foot depth, mass calculations were performed separately for these depths. Figure 2 depicts the boundaries of the "5-foot" and "i 0-foot" zones. The zone boundaries account for concentration attenuation for detectable to non-detectable values.