WORK PLANS - Laserfiche WebLink

Home Browse Search

Geofigiwf 7ec&iks Inc. N.01 Page 2 McGranahan(Gawne Road) Work Plan-Fate&Transport Model Project#251.2 March 21, 1997 The conceptual model will be the basis of modeling and discussion pertaining to the agencies' concern as described in Section 1.0. A complete discussion of the modeling, its results, and appropriate references will be documented in the final modeling report_ The conceptual model shown in Figure 3 shows the vadose zone is divided into four layers with each one composed of both course grained and fine grained sediments. The model presented below has been refined slightly from the draft presented to Mr. Ron Rowe last month. These for layers are defined as follows: Layer 1: Silt, Silty Sand, & Sand; 45 feet (13.72 m)thick. Most of the gasoline is contained in this zone. To better emphasize the distribution of the gasoline, this layer is sub-divided into three zones or sub- layers that have nearly equal thickness. Based on the available information this layer contains the highest concentration of contamination with Benzene in excess of 100,000 ppb and is the primary layer for loading of hydrocarbons into underlying layers. Layer 2: Clay/Silt, Sand & Gravel; 20 feet (6.10 m)thick. The silt/clay unit is the second fine grained layer. It has acted as a deterrent to vertical migration, but has not stopped it downward movement. Gasoline is still present at elevated concentrations in this zone. The geology of this layer is subdivided into three sub-layers of varying thickness. Layer 3: Clay/Silt& Sand; 17 feet (5.18 m)thick; The silt/clay unit is the third fine grained layer. It has acted as a deterrent to vertical migration and the available information suggests that the gasoline has ,not migrated through it yet. This layer is subdivided into two sub-layers due to its importance in tracking the vertical movement of contaminants in time. Layer 4: Silty/Clay; 18 feet(5.49 m)thick; The silt/clay unit is the fourth fine grained Iayer. It has not been impacted by the downward migration of the gasoline. The ranges of the various physical and chemical properties for each layer and sub-layer along with their rationale are presented in the following sections. 3.0 SOIL CHARACTERISTICS A number of soil borings have been drilled at the site in the past. Geological Technics has selected samples that were analyzed to measure specific soil characteristics. The attached data tables presents a summary of the lab test results. A complete report of the subsurface investigation was prepared in October 1996. The physical characteristics examined includes: porosity, moisture, grain size distribution, bulk density, total organic carbon(TOC), cation exchange capacity (CEC), and pH.