2007-11-20_REVISION - M1982015 (12)

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SHT. In areas where the SHT does not exceed approximately 5 feet in depth, vertical leaching of contaminants to groundwater is restricted. There are some areas near the release point where the SHT is expected to exceed 5 feet. Sandy and gravelly deposits are present beneath the surficial soils. These areas would tend to have a higher vertical permeability and conditions would be more conducive to leaching of gasoline to groundwater. ' Contaminant Mobility in Groundwater and Vadose Zone ' The chief contaminant is gasoline. The contaminants of concern are benzene, toluene, ethyl benzene and total xylenes. The release originated from a pipeline located approximately 17 feet bgs in sand and gravel alluvium associated with the St. Vrain River Valley. Groundwater in the area is approximately 12 feet bgs. The area of the spill has been substantially filled and reformed to channelize the St. Vrain River and dewater for quarry operations. The near surface soils consist of silt, sand and clay with various amounts of gravel and clayey fill material to a ' depth of 6 to 13 feet bgs. Native alluvium consisting of fine to gravelly sands and cobbles is present beneath the surficial alluvium. Bedrock consists of the Denver Shale which is a regional aquitard. ' Based on the January 23, 2007 data, the groundwater gradient is 0.026 ft /ft measured from St. Vrain Gauge Station #1 to MW-6. Hydraulic conductivity ' testing has not been completed to date, however, based on field observations in the saturated zone, the hydraulic conductivity is estimated at 1.0 x 10 a feet/second for fine sands (USGS Water Supply Paper 2220, 1989). Assuming the porosity is 0.20, the estimated groundwater flow velocity is: V = (IVn) x (dh/dl) = 1.0 x 10-4 ft/sec / 0.20 x 0.026 ' = 1.3 x 10"5 ft/sec = 4.7 ft/day ' Benzene has the highest mobility of the contaminants and will almost match the prevailing groundwater velocity (not accounting for factors that retard chemical transport such as dispersion, adsorption or volatilization). The prevailing ' groundwater flow direction is to the north or away from the St. Vrain River and towards the quarry ponds. This is somewhat atypical for alluvial valley systems. However, since the quarry ponds are being drained, the St. Vrain River is "losing" ' water to the quarry ponds as they recharge. With continuous pumping, this condition is being maintained. When the slurry wall is completed, the groundwater velocity and flow direction will be significantly altered in the spill ' area. Groundwater adjacent to the river should resume a more typical flow direction, that being toward and generally in the direction of the river flow. The groundwater to the north of the slurry wall will resume to be toward the ponds but will be slowed and eventually stopped when the quarry ponds are filled. The slurry wall will bisect the groundwater plume if it is completed in the near future. Thus, consideration will be taken to evaluate the groundwater characteristics after the slurry wall is completed. C. The applicant should then provide, using the information contained in the site assessment, a risk based analysis of all exposure pathways which details how the proposed remediation will obtain acceptable risk levels. This analysis must show 18