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• PHS/EHD has also requested an estimate of the mass of hydrocarbons in groundwater This normally requires constructing a map from which the volume of impacted groundwater can be estimated, however, due to the absence of hydrocarbons in most monitoring wells and the low concentrations that have been detected in others, no such maps have been constructed for this site Hence, only a very rough approximation of the volume of contaminated groundwater can be given here, using the equation volume of groundwater plume= (area of impacted soil) x (thickness of impacted soil) x (porosity) x (H2O volume in gallons/ft) If the analytical results of the very first (February 16, 1996) groundwater samples are used, then the impacted water was restricted to the area around HP-1 and DM-1 (Table 3) A "ballpark" estimate of the area of impacted water is 30 feet by 20 feet Because diesel is lighter than water and therefore tends to float, we assume that hydrocarbons were restricted to the upper 3 feet of the aquifer A typical porosity for moderately sorted to well-sorted sand is 30% Therefore, using the February 16 data, volume= (30 ft x 20 ft)(3 ft)( 30)(7 48 gal/ft) = 4039 gallons In order to convert the volume of contaminated groundwater to the volume of petroleum, it is necessary to multiply the volume by the hydrocarbon concentration The mean TPH-d concentration of the samples from these two wells in February 1996 is unlikely to be representative of the entire volume of contaminated water, but there is no other way of estimating the average concentration in groundwater Hence, using the mean TPH-d concentration of the two February samples, the volume of diesel contained within this 4039 gallons is (71,750 x 10-9)(4039 gal) = 29 gallons If laboratory data from later sampling events are used, the volume of contaminated groundwater might be greater, but the mean TPH-d concentration would be much less, therefore,the calculated volume of residual diesel still present in the groundwater would likely be less than 0 1 gallon 5.0 POTENTIAL FOR CONTAMINANT MIGRATION Two lines of evidence indicate that the potential for contaminant migration is low The first comes from the site-specific monitoring data When the borings were drilled, no soil contamination was detected in the borings that were drilled outside the limits of the dispenser island and tank cavity, which indicates that the contaminants had not migrated very far prior to the tank closure and removal In the ensuing four years, groundwater monitoring has demonstrated that the contaminants have not reached TT-1 (which is 30 5