ARCHIVED REPORTS XR0010089 - Laserfiche WebLink

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Contaminant Distribution An estimate of remaining contamination has been calculated It must be noted that calculations are estimates only in the area of the observed sampling Estimates for TPHG and Benzene were calculated since these components of the most interest The calculations are estimates for the on-site problem not for the plume migrating toward the site from south of Eleventh Street The calculations indicate the following, Contaminant Soil Capillary Fringe 3 ft thick A uifer ave 7 ft thick Estimated TPHG in gallons 151 0 92 Estimated Benzene in gallons 0 0069 0 105 These numbers are indicative of the highly degraded condition of the remaining contaminants terms of both quantities and mobility A relatively high degraded TPHG in soil is expected since laboratory column tests show that up to 9% to 25% of organic liquids may absorb onto soil matrix The Benzene is in soil is highly depleted compared to TPHG due to more oxygenation in the soil These data again strongly indicated that the plume is in a condition of biodegradation and has achieved a `steady-state" condition wherein the mobility is contained within the observed area Overall fluctuations of observed well monitoring data are due to long term degradation, and water level changes which affect the area of monitoring well screen and aquifer for desorbtion Discussion - Monitoring Approach and Review of Potential Adverse Effects The preceding discussion includes the rationale of taking a "monitoring only" approach for this site The source (tanks and soil) were removed from the site The exploratory boring and • monitoring well program have defined the soil and dissolved plumes and monitoring since 19 9 6 shows that the plume has not moved beyond the mapped areas presented on the Figures The aquifer is very thin and is underlain by an extensive aquitard Historic groundwater motion is northerly and has remained consistent through the monitoring period A well search has revealed that wells in the area are either upgradient, or of sufficient distance that they are not expected to be affected by the relatively small site plume Further, well seals on active wells must be at least 50-ft thick for City use and thus are sealed through the aquifer of interest Calculations of remaining contaminant mass indicate that small quantities of TPHG and Benzene remain, and are in a state of depleted volatile content The overall plume position has remained constant and suggests that the more aerobic conditions at the plume perimeter are degrading the contaminants at a rate roughly similar to groundwater motion The inner core of the plume suggests more anaerobic conditions where plume degradation has slowed The more elevated concentrations in the interior monitoring wells show that this is a probable occurrence The calculated quantities of contaminant are very low and cannot be cost-effectively removed Since this plume is highly depleted in volatile constituents, it poses a very low threat to the nearby receptors and underlying aquifers The edge of the plume is lust at the property line of Tracy High School however, the high school on-site monitoring well shows that dissolved contaminants are not present Given the low contaminant concentrations, slow to no plume movement, biodegradation process and source removal, it is Wright's opinion that the remaining contaminants cannot be cost-effectively removed even using existing best available • technology Page 9