SITE INFORMATION AND CORRESPONDENCE

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u A R and M E Glover 7675 W. Eleventh Street Page 2of3 xylenes. Several of these compounds exceed the Maximum Contaminant Level (MCL) established by the Department of Health Services for public drinking water. ATC interprets the data to indicate that contaminants from the UST release have migrated down-gradient between the two obliquely down-gradient monitoring wells MW-4 and MW-5. EHD concurs with that interpretation. ATC further interprets the contaminant concentration trends in the existing monitoring wells to indicate a stable plume, and, citing the lack of sensitive receptors in the down-gradient direction, considers the site a low risk case and recommends proceeding with site closure. EHD cannot concur with this interpretation and recommendation. The lack of sensitive receptors in the down-gradient direction does not make a site a low-risk case; the plume must be shown to pose no risk to human health and safety or to the environment, which includes additional ground water supplies. The contaminant concentration data from the monitoring events is highly variable over time, the concentration trends are obscured by the variability of the data, compounded by the lack of consistent quarterly monitoring since 1999. To concur with ATC's recommendation, EHD must rely on a sound technical demonstration, based on adequate and reliable data, that the plume of contaminants poses no further risk to human health and safety or . to the environment, and that natural processes will return the site to background conditions within a reasonable time frame. To this end, EHD directs you to continue the assessment and characterization of impacted ground water in the down-gradient direction and resume quarterly monitoring of your site. EHD believes two additional down-gradient wells may be adequate to address the assessment and characterization directive if the site hydrogeological conditions hold and the plume extent is as ATC has modeled it, with one shallow monitoring well in the vicinity of GP-1 and GP-2, and the other near the shed at the south end of the canopy. If the plume is characterized, your consultant can utilize the contaminant mass and distribution and the hydrogeological model to prepare a fate and transport model or other demonstration that the contaminant plume poses no further risk and will degrade to background conditions. To demonstrate the occurrence of natural attenuation through biological breakdown of the contaminants, you should have your ground water samples analyzed for dissolved oxygen, carbon dioxide, dissolved iron, nitrates, sulfates, total alkalinity and the redox potential. ATC has demonstrated that 1,2-dichloroethane (1,2-DCA), ethylene dibromide (EDB), di-isopropyl ether (DIPE), ethyl tertiary ether (ETBE), tertiary amylmethyl ether (TAME), and tertiary butanol (TBA) are not chemicals of concern at this site, therefore, you may discontinue analyzing for these compounds.