SITE INFORMATION AND CORRESPONDENCE

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Mr Peter MacNicholl February 21, 2009 Page 2 area is mainly open space, but is bounded by a rail spur and also is the site of a groundwater treatment plant, GWTP 2. Specific Comments 1 . Section 2.4 Vertical groundwater gradients should also be discussed. The section should be revised. 2. Section 2.3 and 2.4 contain sufficient data to calculate local transmissivity and groundwater velocities, as well as calculate the injection mound's upgradient and lateral extent. This calculation should be added to the discussion, because it is relevant to analysis and interpretation of the hydrologic data. Using the Theis equation and the data in the preceding section, the injection mound height (s) was 5-16 feet high, the injection rate (Q) 10 gpm, the radial distance between injection and observation wells about 5 feet (r), and for about 365 days of injection and a storativity of 10%, resulting in a Transmissivity (T) of about 2000 gpd/ft^2, and a conductivity (k) of 5.9 ft/day. For a regional gradient of 0.0001 and saturated thickness of 45 feet, the upgradient extent is nearly 10000 feet and the plume width about 5000 feet. While obviously high, the exercise shows that the width and length of the plume are consistent with the extent of the injection mound. The section should be revised. 3. Section 2.4 Using the data in section 2.3 and figures 2.4 and 2.5, the plume velocity can be back-calculated and this velocity can be compared with the k calculated above to calculate retardation of the plume. The plume is about 200 feet long and 14 years old, so moved about 0.039 May under a gradient of about 0.0001, giving a k of about 3 ft/day. This suggests little or no retardation of the plume, which suggests the dieldrin is not dissolved. Furthermore, figure 2-4 shows that the dieldrin concentration at CPT258, 300 feet from the well, is basically the same as at the source, also suggesting that no retardation is occurring. The section should be revised. 4. Section 2.4 The radial velocities on the injection mound were calculated fwo different ways, first using the average gradient between the injection well and the observation point, and second using the bulk velocity based on the radius and the injection rate. The bulk velocity is considered more accurate because the injection rate is probably better constrained than hydraulic conductivity. This calculation shows that even 200 feet from the well, groundwater velocities are fairly high. Assuming the extent of the plume is constrained by a limiting groundwater velocity, then the dieldrin stops or starts moving at a velocity of about 0.03 ft/day. The section should be revised.