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IT CORPORATION Mr. J. Kelly Williamson Kayo Oil Company April 17, 1985 Page 3 volume to . the .assumed concentration of . hydrocarbons (sum of xylene, benzene and toluene) in the soil . The co centration of hydrocarbons is the., parameter.. with the greatest uncertai.nt since only a few concentra- =tions. are. known for- each site. Therefore the amount of product in the soil at any particular site will tend to be overstated, especially if the volume of soil involved is large. The graph shows hypothetical volumes of 20,000 and 40,000 cubic feet of contaminated soil. Note that a small difference in the assumed concentration of hydrocarbons will ' dramatically effect the predicted volume of hydrocarbons present. The larger the volume of soil involved, the more dramatic this relationship r will be. REMEDIAL ALTERNATIVES The majority of contamination is too deep for excavation to be cost- effective, especially -since_ some contami ation appears to exist below both highways adjacent to the station, Since the water table has been found with free hydrocarbon present, a pumped recovery system is appro- priate for preventing the further spread of the plume and recovering contaminated water from under the site. There does not appear to be + sufficient gasoline' in the soils . to require a second pump for skimming gasoline from the recovery well . If gasoline accumulates in the recov- ery well , it can be removed via manual umping. We have preliminary data that indicates that there are bacteria in the soil that will con- sume petroleum ..hydrocarbons. These bac eria could be stimulated by recirculating the water into the tank ba kfill and adding appropriate k amounts of oxygen, phosphorous, and nitrog n to the water. RECOMMENDATION The tank inlets should be equipped with spill prevention devices to prevent the further introduction of hydrocarbons into the subsurface. If the tanks are to be replaced in the near future, the excavation plan should include the removal of contaminated soil under the tanks to a depth of 20 feet and extended laterally to include the contamination at MW-3 at the I5-foot level . IT Corporation recommends that a recove y well be installed between monitor wells MW-6 and MW-7 and a cone o depression established under the site that will prevent the further spread of contamination. Three additional monitor wells should be installed in the locations shown in Figure 5. The well adjacent to the tanks will verify that the contami- nation is moving straight down. The two wells in the adjacent highways will verify that the plume has not migrated a significant distance beyond the edge of the station property. The estimated cost for construction of the recovery well and the associ- ated water handling system is $63,000, assuming that the water can be discharged to a sanitary sewer. If a surface discharge permit is required, the cost of the water treatment (carbon adsorption) system is