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1 Corrective Action Plan MW-7-P at a depth of 20 feet bgs. No hydrocarbons above the laboratory reporting limits were ' detected in any of the other five samples analyzed(see Table 1). The water table elevation at the time these wells were installed was approximately 17 feet bgs. Both wells were completed at a depth of 30 feet bgs. The wells were screened in the interval from approximately 8 feet bgs to t the bottom. The size for both wells are 4-inch-diameter casing installed in a 12-inch-borehole. Well construction details are presented in Table 2. Wells MW-7-P and MW-8-P are generally upgradient from the source of the contamination. ' On May 16, 1997, groundwater elevation measurements and sampling were conducted at the site by OST. Due to site access restrictions, the off-site monitoring wells, MW-7-P and MW-8-P, ' were sampled by Park on May 30, 1997. The groundwater analytical results of the most recent monitoring are presented in Table 3 and on Figure 3. The groundwater elevations are presented in Table 4, and the elevations and gradient are plotted on Figure 4. To provide a summary of the results of all previous groundwater monitoring events, a copy of Park's April 1996 Groundwater Monitoring Report is included in Appendix C. Certified Analytical Reports and field data sheets are included in Appendix D. ' 3.1 Impacted Soil ' Based upon careful review and consideration of the results of soils analyses derived from the previous investigations cited above, OST estimates the radius of impacted soil in the vadose zone M to be less than 15 feet surrounding the former UST. The average hydrocarbon concentration in this zone is estimated to range between 2,500 mg/kg and 5,000 mg/kg. The soil in the intermittently saturated"smear zone" (vertical interval of water table fluctuation) is estimated to ' have been impacted to some extent over an area roughly approximating the impacted groundwater plume. This zone is approximately 10 feet thick ranging from around 15 feet bgs to approximately 25 feet bgs. The average hydrocarbon concentration in this zone is estimated to be within 100 mg/kg to 200 mg/kg. Based upon the highest concentrations detected in the soil w at the site,with various assumptions as to their lateral and vertical extent, OST estimates as little Al� as 3,500 pounds (lbs) of petroleum hydrocarbons to as much as 8,500 lbs to be present. The ' most likely total estimated amount is 4,500 lbs. 3.2 Impacted Groundwater Plume ' The plume of impacted groundwater is estimated from the reported record of all previous site ' investigation and groundwater monitoring events. The plume is estimated to cover an area approximately 190 feet long by 90 feet wide and oriented to the northwest as illustrated on Figure 5. Although the illustrated interpretation is not the only one reasonable from the cited data, ' alternative possible interpretations would not alter the proposed plan to remediate the soil and groundwater at the site. The highest concentrations of impacted groundwater have been found in monitoring well BEI-MW3, presumed to be located at the source. In fact, the most recent monitoring event by OST reported TPHg and benzene concentrations in BEI-MW3 at their highest level ever at 110,000 micrograms per liter(µg/l) and 11,000 µg11, respectively. The next ' OST 701-1-2.cap Page 6