2012-09-06_REVISION - M2008070 (25)

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Design and Operations Plan October 24, 2011 Western Gravel E & P Waste Disposal Facility Page 18 of 51 2.4.4 Fluid Transport Modeling of potential migration and transportation of contaminants through native soils and/or bedrock materials beneath the facility was conducted in order to predict the travel time that potential contaminants may impact certain down- gradient receptors. The following three migration pathways were evaluated. • Model 1: Downward percolation from the landfill through Wasatch Formation bedrock into White River alluvium. • Model 2: Downward percolation through Wasatch Formation bedrock into White River alluvium and then laterally to the nearest downstream permitted alluvial water well. • Model 3: Lateral fluid migration through Wasatch Formation bedrock to the nearest permitted domestic water supply well. Darcy's Law for saturated flow was used to obtain very conservative travel time estimates. Permeability of unsaturated material is a function of the water content. When using Darcy's Law, it has been assumed that the water content of soil and bedrock is saturated and at a maximum, resulting in a maximum rate of flow. Parameters used to calculate travel time include Wasatch Formation bedrock permeabilities ran ging from 1.0 x 10 -6 cm/s to 8.0 x 10 -9 cm/s and porosities ranging from 5 % to 30 % and White River alluvium permeability ranging from 5.4 x 10 -1 cm/s to 2.5 x 10 -2 cm/s and porosities ranging from 35% to 45 %. Additional factors including adsorption, dispersion, diffusion, and degradation were not considered but would be expected to further reduce fluid migration within the unsaturated zone. An additional factor not fully considered using Darcy's Law for the intended purpose is fractured flow in bedrock. Fracture flow could increase the fluid flow rate. These factors have not been accounted for because of the complexity of site geology and hydrogeology and limited information regarding the deeper subsurface and unsaturated conditions. Travel time of direct flow of fluid from the source (waste cell) to potential receptors has been evaluated based upon a worse case scenario (fastest travel time) to best case scenario (slowest travel time). Travel time was estimated for fluid migration from a waste cel 1 through bedrock to White River alluvium (Model 1), through bedrock to White River alluvium and to the nearest alluvial water supply down stream (Model 2), and through bedrock to the nearest domestic bedrock water supply well (Model 3). Maximum, minimum, and median flow velocities through bedrock and alluvium materials and subsequent travel times were calculated for each of the three scenarios as discussed below. • Model 1: The closest receptor to a proposed cell is the White River alluvium. Based upon m odel results, the most conservative and quickest travel time to reach the alluvium via fluid migration through saturated bedrock would be a minimum of 43 years. The White River alluvial gravels are located approximately 205 feet from the nearest cell. The prediction assumes 1 x 10 -6 cm/sec permeability, 5% effective porosity, and a 0.225 slope. The maximum estimated travel time was over 26,032 years and the median travel time is 1,562 years. • Model 2: Model 2 assumes fluid migration through bedrock and then transport through White River alluvium to the receptor. Therefore, fluid migration travel time estimates through bedrock was taken from Model 1. Taking travel time through bedrock and adding travel time through alluvium, 44 years is the earliest that fluids are projected to reach the nearest permitted alluvial well. Estimated travel time through alluvium to the well ranges from 1 to 20 years. Lateral flow through alluvial gravels assuming the fastest flow rate assume 5.4x10 -1 cm/sec permeability, 35% porosity, and average slope of 0.005 foot per foot (ft/ft) for a distance of 6,000 feet. The maximum estimated travel time is 26,052 years and the median travel time is 1,564 years. NWCC, Inc.