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Wright 'u <br /> Environmental <br /> November 10,2000 Services, Inc. <br /> Project No.4669 <br /> Construction ♦ Engineering * Remediation <br /> M&M Builders Supply <br /> 8111 W.Eleventh Street <br /> Tracy, CA 95378-1107 <br /> RE: Addendum to M&M Builders Workplan; Rationale for Groundwater Definition <br /> M&M Builders Supply 8111 W. Eleventh Street Tracy,CA Site Code#1530 <br /> Cn <br /> ,n <br /> Dear Mr.Kelly, <br /> Wright Environmental Services,Inc.has prepared this addendum to our workplan dated October 16,2000 in <br /> response to the San Joaquin County Public Health Services Environmental Health Division(PHS-EHD)comment <br /> letter dated November 6,2000. PHS-EHD requested a description of how vertical assessment of the on-site <br /> groundwater plume would be defined. The previous site assessment showed that dissolved petroleum hydrocarbons, <br /> depleted in volatile components,were observed in the groundwater. The highest concentrations occurred near the <br /> two former tanks pits. MTBE and fuel oxygenates were not observed in the previous samples. The depth of soil <br /> contamination is defined the deepest of which occurred at the former tank pits. The contaminant sources have been <br /> removed and the contaminant leakage appears to be at least six years old or older. <br /> Technical Approach—Groundwater Definition <br /> The contaminant is a light non-aqueous phase liquid(LNAPL,petroleum fuel). Contaminants migrate to the <br /> capillary fringe where little mixing occurs since the water and fuel are essentially immiscible. Upon reaching the <br /> capillary fringe the contaminant will spread laterally with relatively low concentrations dissolved in the aquifer <br /> water. This continues until the spreading stops as LNAPL source fails to replenish the spill. A stable condition is <br /> reached wherein the dissolved phase attains a residual hydrocarbon saturation throughout the aquifer strata(see <br /> discussion in Testa and Winegardner, 1991). This is the condition in the aquifer at M& M Builders,based upon <br /> previous site data. <br /> Since a residual concentration has been attained,relatively uniform dissolved concentrations occur throughout the <br /> aquifer that diminish as the plume moves from the former source,and as contaminants are absorbed or diluted <br /> through natural aquifer flow. The concentrations are interpreted to be in relative equilibrium depending upon the <br /> distance from the initial source. The exploratory borings will be drilled into the aquifer at the locations shown, and <br /> soil samples will be collected to ascertain capillary fringe residual contaminant presence. Borings will be advanced <br /> to the working aquitard contact so that the aquifer thickness is known. This lower contact location needs to be <br /> known since it defines vertical extent and so monitoring wells can be constructed. A groundwater sample will be <br /> taken from within the aquifer in the borings,below the capillary fringe to ascertain the residual dissolved <br /> concentration. This method is similar to that initially used and observed by PHS-EHD in the April,2000 horings. <br /> In our opinion,this approach is adequate and appropriate for this site investigation. If you have any questions, <br /> please call. <br /> Sincerely, <br /> Wright Environmental Services, Inc. <br /> N"1262 , <br /> t^"'4&n Lynch �� i C2 ?IfiEO <br /> Christopher M. Palmer it E,: <br /> President HG.246;CEG. 1262 <br /> Reference <br /> Testa, S. M.,and Winegardner, D. L., 1991, Restoration of Petroleum-Contaminated Aquifers; Lewis Publishers, <br /> Chelsea,MI,pg. 53-56. <br /> 67 E. 10th Street • Tracy,CA 95376 <br /> 209.833.0758 • Fax 209.832.5152 <br /> wright@inreach.com Lic. #651501 A-BHa-Mat A <br /> websit ht1p:11home.inreach.com/wright Page 1 1W <br />