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E C 0R . 7-Eleven Store #14117 Additional Site Assessment Report and Fate and Transport Model February 27, 2004 Page 6 grid Based on groundwater elevation data collected on September 18, 2003, the groundwater flow direction was assumed to be to the northeast, although historic groundwater flow directions have varied greatly over time Four model simulations were performed • The first model used a hydraulic conductivity (K) of 90 feet/day (ft/day) and a hydraulic gradient (i) of 0 0001 to the northeast The results of this model are presented in Attachment G, Plate 1 • The second model used a hydraulic conductivity of 5 ft/day and a hydraulic conductivity of 0 001 to the northeast (Attachment G, Plate 2) An estimate for a hydraulic conductivity of 5 feet/day for silty to fine sand described in the aquifer modeled was obtained from a 1989 United States Geological Publication (Williamson, A K , Prudic, D E , and Swain, L A , 1989 Ground-Water Flow in the Central Valley, California United States Geological Survey Professional Paper 1401-D 127p) This value for the hydraulic conductivity was supported by previous testing of similar aquifer materials in the site vicinity (Upgradient Environmental Consultants, 2002) • The third and fourth models assumed a net zero hydraulic gradient over time Two models were constructed with a "flat" hydraulic gradient using hydraulic conductivities of 5 feet/day . and 90 feet/day (Attachment G, Plates 3 and 4) In these models, the primary methods of contaminant reduction are decay, dispersion, and diffusion For each model, linear head boundaries were established along each side of the modeled area MtBE was the contaminant selected for modeling The initial concentration of MtBE of 36,000 dig/L was used because this historically represents the highest detected concentration of MtBE in the site wells (MW-2, September 25, 2001) Other parameters used in the model include 1) retardation factor, 1 0, 2) first-order decay constant 0 001 1/day (based on a published MtBE half-life of 720 days), 3� longitudinal dispersivity of 10 feet, 4) effective porosity 0 2, and 5) diffusion coefficient, 9 8 x 10-4 ft /day Each model was run for a period of 1,800 days or approximately 5 years Model Results i Results of the first (K = 90 ft/day, i = 0 0001) and second (K = 5 ft/day, i = 0 001) modeling runs are very similar (Attachment G, Plates 1 and 2) and indicate that MtBE will migrate approximately 120 to 180 feet from the source area to the northeast Results of the third and fourth modeling runs representing a near-"Flat' hydraulic gradient with values for K of 90 ft/day and 5 ft/day (Attachment G, Plates 3 and 4) These models indicate that the MtBE contamination would remain in the immediate vicinity of the UST area and concentrations will decrease with time due to diffusion and natural decay The migration is mitigated by dispersion/diffusion and the natural degradation of MtBE, which has an average half-life of 720 days Overall, it is difficult to accurately model the migration of contaminate plumes at this site because of the relatively flat groundwater table and the highly variable groundwater flow direction in the area SUMMARY AND CONCLUSIONS Groundwater monitoring well MW-8 and soil boring SB-1 were installed on August 14, 2003 Sod samples collected from MW-8 and SB-1 indicate that the vertical limits of TPHg, benzene, and MtBE 14117 Vertical Assessment and Fate and Transport Model doc $ECOR International Inco or p rp ated