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• • Mr. Ron Rowe Page 2 of 4 and unrelated to releases on the subject property. Any additional efforts to define the limits and nature of the increasing contamination beneath the southern portion of the site should be bome by the party responsible for its release. Insignificant Threat to Human Health or the Environment Average benzene concentrations in groundwater appear to be below the level that would cause a significant health risk to occupants of the property. The stability of the plume suggests that no significant threat to beneficial uses of groundwater at point and time of use exists. These circumstances suggest that natural attenuation may be the most appropriate remedy to complete the restoration of groundwater. In order to evaluate this, Ground Zero proposes to perform the following scope of work: TASK 1 QUARTERLY GROUNDWATER MONITORING AND SAMPLING Ground Zero will perform quarterly groundwater monitoring and sampling at the site during the fourth quarter of 1999. As requested, sampling during this event will include oxygenate analyses for all monitoring wells. TASK 2 CONFIRMATION SOIL BORINGS AND VERTICAL DELINEATION The purpose of this task is to document the existing concentrations of contaminants in the vadose zone where soil vapor extraction has been operating. Additionally, this task will complete the vertical definition of contamination in the saturated zone. Ground Zero will obtain permits from the San Joaquin County Public Health Services/Environmental Health Division(PHS/EHD)to install four soil borings. Ground Zero will observe the drilling of three soil borings (CB 1, CB2 and CB3) to depths of approximately 60 feet below ground surface (bgs), and one soil boring (CB4) to an estimated maximum depth of approximately 85 feet bgs. The soil borings will be advanced using a drill rig equipped with 6-inch diameter hollow stem augers. If feasible, however, the shallower borings may be advanced with direct-push equipment. The proposed i locations of the confirmation and vertical delineation borings are presented on Figure 4. We anticipate encountering groundwater at a depth of approximately 38 to 40 feet bgs at the time of drilling. Upon completion the soil borings will be tremie grouted with a cement slurry to the ground surface. Soil samples will be collected during drilling from the surface to the groundwater interface at approximately five foot intervals for logging and analytical purposes. Selected soil samples from each boring will be submitted t Argon Laboratory of Ceres, California under chain-of- ro k'slosme\closwp.doc