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Report—Site Closure New Jerusalem School July 22,1999 5. CONTAMINANT MASS ESTIMATE L+ Total contaminant mass of TPH-G in groundwater was estimated from analyses in Table 3, assuming concentric cylindrical volumes centered on P-2 with average concentration decreasing radially away from P-2. The resulting estimated total TPH-G in groundwater is 0.19 kg. The resulting estimated total TPH- i., G in soil is 0.6 kg. Total contaminant mass of benzene is also estimated because benzene is the contaminant of concern, due to its designation as a carcinogen. Benzene has been detected in only one groundwater sample (P-2) centered at the UST location. Benzene was not detected at P-3, 12-feet away where other BTEX constituents were found. This observation is consistent with the greater propensity for benzene to L.. volatilize and biodegrade than other fuel constituents(Hadley and Armstrong, 1991). Total estimated benzene in groundwater is 0.54 grams and in soil is 3.2 grams. Calculation sheets showing assumptions, formulas, and calculations are included in Appendix B. 6. MODELING CONTAMINANT PATHWAY TO RECEPTORS The contaminated soil and groundwater is located at least 7 feet bgs. Two types of human receptors have ` been identified for the site. They are construction workers, digging into the plume, and students or residents drinking water from wells. The contamination is very limited in volume and its depth makes the likelihood that construction workers would encounter the plume,remote. V Two down-gradient wells are potential pathways to receptors. These wells are the agricultural well located approximately 30 feet north of the former UST location and the domestic well located approximately 100 feet northeast of the former UST location. The agricultural well is used for landscape `r irrigation, while the domestic well is used as a water source for residences and the school. 6.1 DOMESTIC WELL u A driller's log of the domestic well is included in the Appendix C. The log shows two twenty-foot thick clay layers separating the upper well screen (120 feet bgs) from the upper aquifer. Contamination from the former UST location was detected 100 feet away from the well. For the sake of easily modeling the potential for contaminant transport, we assume a simplistic distribution of the benzene contamination along a potential pathway to the domestic well. In this model, a maximum possible average concentration of benzene is calculated by redistributing the total estimated mass of benzene in the contaminated zone evenly along a straight-line diagonal path from the former UST location to the well screen, a distance of 156 feet. The resulting concentration of is 4.5 ug/l, which is greater than the California Primary Drinking Water Maximum Contaminant Level for benzene of 1 ug/l. This modeled concentration, however, is overly conservative in terms of health risk assessment for three reasons. First, the true pathway of contamination, if ever completed, would be much longer and ' more tortuous than the 156 feet used in the calculation, causing the benzene to be distributed through a larger volume. Due to the presence of thick horizontal clay sequences noted on the driller's log, benzene- tainted water would have to travel horizontally along a weak gradient and penetrate thick clay layers or a 6a well seal before reaching the well screen. Second, contaminant plumes tend to disperse and decrease in concentration exponentially near the perimeter, so that arrival concentrations would be much less concentrated than an arithmetic average of the whole plume. Third, there is no consideration of plume +••• attenuation from biodegradation or adsorption with time and travel distance. The time of transport, according to earlier estimates of groundwater flow only, would be on the order of centuries. Finally, if any water from the upper aquifer ever reaches the domestic well it will quickly be blended with much i ti CONDOR