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Mr Peter MacNicholl February 21, 2009 Page 3 5. Figure 2.4 Water levels at the screened intervals should be added to the figure. The figure should be revised. 6. Section 3.0 introduction. The dieldrin plume is apparently only stable if groundwater velocities stay below some threshold value. This is not equivalent to stable under all conditions. The section should be revised. 7. Section 3.1 The radius of influence was not correctly calculated. Using the Theis method and the same assumptions above, for 3 gpm the threshold velocity is reached at about 60 feet. The section should be revised. 8. Figure 2-3 The word lenses should be removed, and the sands and gravels should be described as channels, consistent with the depositional environment. Figure 2-4 shows that the deposits are not actually continuous, but more likely represent a high-energy braided stream environment, with discontinuities formed by channel shifting. This both allows for a better conceptual model of the geologic environment, and suggests strategic approaches to remediation. The stratigraphic description should be revised to better portray the depositional environment. 9. Section 3.1 Figure 2-4 shows that LM74AU encountered a gravel 'lens' (actually a probable channel deposit) which coincidentally has unattenuated dieldrin concentrations. The shape.of the plume, both in plan and cross section, strongly suggests it is being influenced by a high-conductivity channel.. The section should be revised. 10.Section 3-1 The above analysis does not support the conclusions of the natural attenuation study. There is no convincing evidence that dieldrin is being adsorbed, because dieldrin has traveled at ambient groundwater velocities for over 300 feet. The evidence presented shows that dieldrin prefers to travel in the path of least resistance, or highest conductance. This matches the pattern for non adsorbed and nondegraded tracers, and is quite atypical for dieldrin. The assertion that the bulk of the dieldrin is adsorbed is not supported by the data. The section should be revised. 11.Section 3.1 The mass of dieldrin in the plume should be estimated. GSU's estimate using the supplied data is that for a volume of 400' x 325' x 50' x 10% x 0.1/1000000000, and converting cubic feet to liters, is 0.002 grams, which suggests that the bulk of the dieldrin is not adsorbed, but merely filtered out in transit. The section should be revised.