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Field observations and laboratory analyses of the selected soil samples also indicate the lateral extent of the impacted soil is contained within the area between soil borings TH-2, TH-4b, TH-5, and TH-6b Exhibits 5, 6, 7, and 8 are graphical interpretations of the laboratory data showing the TPHg concentrations in soil in profile and map view at depths of approximately 25 and 30 feet (bsg) The petroleum impacted soil at depth of 25 feet (bsg) is projected to cover an area of approximately 1 ,590 square feet, centered around the former excavation (Exhibit 7) At a depth of 30 feet (bsg) the soils become water saturated and the northwestern limit of impaction extends beyond boring TH-5 (Exhibit 8) However, based on the concentration of TPHg at TH-5, the limit of lateral extent is believed to be within the vicinity of the boring Presumably, once the downward migration of the hydrocarbons encountered groundwater the impacted area spread laterally, covering an area of approximately 3,850 square feet Laboratory analyses indicate groundwater is impacted with petroleum hydrocarbons in the vicinities of monitoring wells MW-2 and MW-3 The highest concentrations were encountered in MW-3 resulting in 210,000 parts per billion (ppb) for TPHg and 850 ppb for benzene Groundwater elevation measurements show a gradient of 0 021 feet/ foot to the southeast, in contrast to the western regional groundwater gradient Neither gradient, however, provides a satisfactory explanation for the distribution of hydrocarbons impacting groundwater observed at the site The high concentrations of hydrocarbons • detected in MW-3 does not lend itself to any readily assessable explanation The soil at TH-6b/MW-3 was not impacted with hydrocarbons, even at a depth of 30 feet (bsg) Nonetheless, groundwater was highly impacted at MW-3, suggesting that hydrocarbons have migrated by way of lateral spreading through groundwater However, neither the westerly regional gradient, nor the southeasterly site gradient are sufficiently aligned to transport high levels of hydrocarbons from the vicinity of the former tank cluster to the northeast corner of the site In addition, the site gradient is seemingly too steep to facilitate lateral spreading normal to the gradient towards MW- 3, or up gradient towards MW-2 Consequently, the distribution of hydrocarbon concentrations observed in groundwater beneath the site may be a result of offsite contamination or seasonal variations in gradient direction thus accounting for the anomalous distribution of hydrocarbon concentrations impacting groundwater at the site. 9 0 RECOMMENDATIONS The area containing the former underground tanks was excavated for the removal of the tanks and backfilled to surface with a mixture of pea gravel and earthen matrix A cap of non-porous material, such as asphalt, is recommended as a cover for the backfill material to prevent the introduction of water into the subsurface and to further diminish the downward migration of hydrocarbons to groundwater . 16 of 17