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Coucctive Action Plan MW-7-P at a depth of 20 feet bgs No hydrocarbons above the laboratory reporting limits were detected in ani of the other fi-,e samples anal zed (see Table 1) The water table elevation at the time these wells were installed was approximately 17 feet bas Both wells were completed at a depth of 30 feet bgs The wells were screened in the intev�al from approximately 8 feet bgs to 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 31 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 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 V be within 100 mg/kg to 200 mg/kg Based upon the highest concentrations detected in the soil at the site, with various assumptions as to their lateral and vertical extent, OST estimates as little 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 4,5001bs 3.2 Impacted Groundwater Plume The plume of impacted groundwater is estimated from the reported record of all preti sous 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 die illustrated inteipietation is not the only one icasonablC fiorn 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 pg/1, respectively The next OST 701-1-2 cap Pace 6