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- 1 which is just above the current groundwater level indicates degradation of the groundwater has probably occurred. As ti such, a subsurface investigation will be required to define the extent of soil and groundwater contamination. To provide for preliminary definition of groundwater flow and quality conditions a minimum of three groundwater monitoring wells will be required. These wells should be placed in a triangular pattern around the former tank pit with two wells in the down groundwater gradient direction (westerly) as shown on Figure 3. To provide for immediate source control the contaminated soils to a depth of 32 feet could be excavated and treated on site. Due to the proximity of a building, the lateral extent and vertical depth of soil removal may be restricted. The presence of gasoline hydrocarbons in the 15 foot sample from the north side of Tank B indicates leakage and/or spillage of product has occurred. The non-detectable BTEX and TPH-G concentrations in the 15 foot sample collected on the south side of the tank pit suggest the lateral extent of spreading is limited to a radius of approximately 15 feet. The non--detectable levels of gasoline hydrocarbons in three samples collected between a depth of 20.5 to 30.5 feet indicate the contamination is limited to the upper 15 feet of soil. Although field inspection noted slight paint thinner odors below 15 feet the absence of TPH-PT in the 30.5 foot sample indicates levels, if present, are below method detection limits of the analysis. Based on this data, a volume of approximately 454 cubic yards of soil have been impacted by varying concentrations of gasoline hydrocarbons. The limited extent of sail ! contamination around Tank B indicates no immediate threat to groundwater resources exists. The 32 foot depth to water provides a small buffer (17 feet uncontaminated soil) which reduces the potential for the shallow soil. contamination E ' from adversely affecting groundwater quality. The more permeable sandy soil types occurring below a depth of 21 feet reduce the effectiveness of this soil interval from attenuating vertical migration of contaminants. 6.0 RECOMMENDATIONS Based on the data collected to date from Tank B, there does _-not--appear- -to-have been significant -leakage of waste oil product from this tank. Assuch we recommend no further assessment for the Tank B area. To prevent the limited area of gasoline hydrocarbon contamination associated with the north end of Tank B from adversely affecting groundwater quality,we recommend that the soils to a depth of approximately 15 feet be excava-ed. ' Excavation of the shallow soil contamination and treatment on-site (aeration) or disposal at a certified waste i 5