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Preliminary Endangerment Assessment Page 2 Former Haley's Flying Service GPE Project 474.2 The groundwater surface at the time these new soil boring were installed was found at a depth of about 8 to 9 feet below grade. This is two to three feet shallower than when soil borings B-1 and B-2 were installed for the original PEA. Several of the groundwater samples contained detectable concentrations of organo-chlorine pesticides, primarily of the Endosulfan sub-group and Toxaphene. The most severely impacted sample contained 22 parts per billion (ppb) of Toxaphene. The boring that produced this sample (B-6) was located west of the washout pit near the east hangar(see figure 2). Human Health Hazard Assessment Based on the information contained in the original PEA, there are no nearby water production wells save the on-site well, which was sampled and did not contain pesticides. This means there are no likely receptors for pesticide-contaminated groundwater. This part of Tracy has been zoned for industrial development, and the future water supply will rely on the municipal water system rather than shallow domestic wells. Moreover, since pesticides are non-volatile, vapor will not be exuded that can penetrate up through the soil and be inhaled. It thus appears that there is little likelihood that future development of land in this area will produce new human receptors capable of contacting the groundwater. Based on the absence of exposure pathways, the groundwater contamination should not result in any net increased risk to human health above and beyond the risk associated with the previously documented shallow soil contamination. General Observations 1. The groundwater in the vicinity of the pesticide release has been impacted with very low concentrations of organo-chlorine pesticides believed to have been leached from the shallow soil. The largest source of leaching is suspected to have been the washout pit. The Chemicals of Potential Concern (COPCs) in groundwater are identified as Toxaphene, Endrin, and the Endosulfans. 2. The pesticides in groundwater represent an impermissible degradation of groundwater quality, but do not appear to constitute a human health risk. Recommended Actions 1. As a part of the RI/FS investigation of this site, groundwater monitoring wells should be drilled and constructed to define the extent of the groundwater contamination. 2. Additional groundwater samples should be collected in the assumed down-gradient direction (north) to obtain information on the possible presence of pesticides in the groundwater. Sample locations should include sites on the agricultural parcel lying north of the irrigation ditch. 2937 Veneman Avenue,Suite B240 Geo-Phase Environmental Inc. Phone: (209)569-0293 Modesto,Ca 95356 Fax: (209)569-0295