ARCHIVED REPORTS_XR0009348 - Laserfiche WebLink

Home Browse Search

GFEDRALATRIX gasoline did not contain lead, EDB, 12DCA, MtBE, or other oxygenates The maximum ' extent of PHC-affected soil and groundwater at the site is limited to about 13 to 35 feet bgs vertically and about 200 feet laterally ' The concentration distribution of PHCs in soil and groundwater is controlled by the site stratigraphy, groundwater occurrence, and the nature of the former release The explored portion of the site subsurface consists of three stratigraphic layers (Figures 3 and 4) As previously described, the shallow and deep layers are mostly coarse-grained sediment, while the middle layer is mostly fine-grained sediment However, lenses of fine-grained sediment are common in the coarse-grained stratigraphic units and lenses of coarse-grained sediment are common in the fine-grained stratigraphic unit(Ross, 1999 and Geomatnx, 2002a) These sediments were deposited in a slow-moving, meandering river environment, which produced the lenticular and abundant fine-grained nature of the stratigraphic layers below the site Groundwater occurs under confined to semi-confined conditions at the site The shallow coarse-grained layer is typically unsaturated The middle fine-grained layer is a confining layer, but is partially saturated and yields water slowly Groundwater will rise to the potentiometnc surface in a borehole when the lower coarse-grained layer is encountered. Groundwater levels measured in three site monitoring wells over the past 4 years have fluctuated between about 9 and 13 feet bgs Groundwater elevation data for these three wells indicates that the groundwater flow direction beneath the site has vaned substantially, with a range that encompasses nearly all directions (Table 2) The most current groundwater elevation data includes the new wells and indicates a southwest flow direction with a gradient of 0.004 ' Based on the groundwater gradient, a hydraulic conductivity of 10-5 to 10-3 centimeters per ' second (cm/sec) for silty sand, and an effective porosity of 30 percent, the apparent average groundwater flow velocity in the coarser sediments is estimated to be 2 to 200 feet per year ' The apparent average groundwater flow velocity in the middle fine-grained unit (based on a hydraulic conductivity of 10-8 to 10 -5 cm/sec) is 0 002 to 2 feet per year ' Before the USTs were removed in 1988, an unknown volume of unleaded gasoline was released to sediments in the middle fine-grained stratigraphic layer below the tanks Free ' product has not been observed in the monitoring wells at the site The shallowest impacted • soil sample was collected from 13 feet bgs Because there is no evidence of PHCs in the 11PRolECTW000s16437 00016437-08 doc 12