SITE INFORMATION AND CORRESPONDENCE_2001-CURRENT

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Mr.Bright July 17,2001 ENVIRONMENTAL AUDIT,INC. 094.01902 Page 2 Model Setup The FLOTRANS model was setup with a finite element grid of 40 rows and 80 columns. The horizontal distance modeled was set at an arbitrary 500 feet and the vertical depth of the model was 230 feet below the groundwater table of 30 feet. The stratigraphy was further simplified into six layers consisting of clay and sandy clay, sand, and clayey sand (Model Setup - Attachment A). The stratigraphic depths are identified as depths below the groundwater table because the model only examines saturated material. Hydraulic conductivity values were assigned to each layer based on published values for similar material(Table 1). Constant head boundaries were established at the top and bottom of the model to simulate the hydraulic effects of 20 feet of drawdown at the pumping well. As soon as the water supply well is pumped, drawdown will occur and the potential for vertical flow through the saturated material exists. From evaluation of production tests conducted at similar wells in the area,it appears that approximately 10 feet of drawdown would occur from a pumping rate between 600 and 1,000 gallons per minute. The modeled drawdown of 20 feet was selected to represent a possible worse case. The transport parameters such as retardation,dispersivity,decay rate,and diffusion coefficient used by the model were estimated from published documents for similar geologic materials(e.g. sand or clay). A summary of the transport parameters is presented in Table 2. Transport parameters were also selected relative to benzene as the constituent of concern. Benzene was selected because it moves readily in water and has available transport parameters. To incorporate existing benzene-impacted groundwater into the model, benzene concentrations from nearby monitoring wells MW-21S, MW- 21D, MW-22S, MW-2213,MW-24S, and MW-241) for the March 2001 monitoring event were used. Benzene concentrations,at 20,30,and 60 feet below groundwater,of 0.29, 1.8,and 2.7 milligrams/liter (mg/L), respectively, were set in the model (Attachment A). In addition, a monitoring well was simulated that is screened in the middle of the lower sand aquifer. This well will detect any benzene impacts at that depth. Model Results The model was run for three time steps of 365,2,000,and 6,000 days(1,5.5,and 16.4 years). For each of these time steps,the extent of benzene migration from the source areas was calculated and plotted as the model output(Attachment A). The results indicate that under conditions of pumping at a discharge rate of approximately 600 gallons per minute from the deeper aquifer and sealing off the upper petroleum hydrocarbon-impacted aquifer, the benzene impacted groundwater should not migrate any deeper than 130 feet below ground surface (bgs). Because of the degradation of benzene, the overall extent of the impacted benzene should decrease within 5 to 10 years. In addition, no benzene was detected in the monitoring well simulated at the center of the deeper(180 feet bgs) sand. The model results should be considered approximate because no site-specific hydraulic properties or transport parameters were available as input to the model. However, worse-case conditions were simulated where ever possible. Well Design As a result of the modeling, SECOR recommends that casing should be securely grouted in place from the surface to a depth of 260 feet. This will seal off the upper petroleum-impacted aquifers from the lower zones, and seal off the lower portion of the cased zone that may be impacted by saline water in the near future. It would be advisable to conduct at least a cement bond geophysical log on this casing to assure its proper sealing with the formation. SECOR is recommending the same well design as in EAClimt ReponslDelta College�D UaFlo.doo-00:jes:l