ARCHIVED REPORTS_PHASE II ENVIRONMENTAL SITE ASSESSMENT

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W1GIK . Southwest, line. Solubility of metals is related to a variety of factors particularly the site-specific soil and groundwater chemistry. 11.3 Fate and Transport Mechanisms The boring logs included as Appendix A show that the shallow soils underlying the site were confirmed to consist predominantly of silty clays and clayey silts. Most of the interbedded sandy zones were noted to contain a significant clay faction. These soil types typically have slow infiltration rates that impede the downward flow of surface water, inhibiting the leaching process whereby contaminants migrate to greater depths with infiltrating water. Leaching is highly dependent on the solubility of a contaminant and its ability to bind to soil particles. Both of these variables are influenced by the specific geochemistry of the soil and water. Downward migration is also restricted by the upward hydrostatic pressure of the shallow groundwater. The rate of migration of contaminants in groundwater is also dependent on the groundwater gradient and flow rate as well as degradation and attenuation. PCBs are typically only slightly soluble in water, particularly those with a relatively high percentage of chlorine by weight such as the Aroclor congeners identified in the grab • groundwater samples. This gives them a low tendency to leach. However, in the presence of organic solvents, e.g., petroleum hydrocarbons, PCBs become highly soluble and can leach rapidly. In all of the groundwater samples with detectable PCBs, TPH-mo and/or TPH-d were also detected. PCBs also have a high affinity for particulate matter especially that with high surface area such as the clays and silts underlying the site and are readily adsorbed and absorbed onto fine soil particles. Soil sorption is more pronounced in the heavily chlorinated PCBs, such as those identified on the Learner site. Degradation of some of the contaminants of concern is likely on this site. These include the petroleum hydrocarbons, as well as the limited SVOCs and VOCs identified. PCBs tend to be resistant to biodegradation and to be chemically and physically stable. Metals will not be affected by degradation processes,but will be subject to erosion, in particular erosion resulting from wind. PCBs also tend to be dispersed into the atmosphere and transported by wind. Because the site is unpaved and soil particles are fine-textured, predominantly clays and silts, wind erosion has the potential to result in localized redistribution of surficial contaminants. Significant differences in the levels of soil and groundwater contamination were not apparent in the samples collected from former depressed areas where storm water was concentrated, • indicating this has not been a significant factor in distributing contaminants across the site. 704. PRI.05 28 July 21, 2008