SITE INFORMATION AND CORRESPONDENCE

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Mr Peter MacNicholl February 21, 2009 Page 6 upgradient direction (the exact inverse of the downgradient capture distance for extraction) 27.Section 4.3 The lack of dieldrin in the lower zone appears to be a lucky accident caused by clogging/ inadequate development of the lower screen intervals of the injection wells and mechanical filtration of the dieldrin in fine-grained units. No adsorption is required to explain its distribution. 28.Section 4.4 GSU agrees that increasing the groundwater gradient will remobilized the dieldrin. GSU observes that groundwater velocities, while currently stable, cannot be assumed to remain so indefinitely. The section should be revised. GSU can envision situations where pumping or injection in the shallow zone can remobilize the dieldrin. 29.Section 5.3 In addition to its ordinary toxicity, dieldirin's high partition coefficient causes it to sorb to human fat. Dieldrin is thus associated with (and even found in)tumors mainly in fatty tissues (skin, breast, mesenteric, and brain). It is also a potent neurotoxin, and is implicated in Parkinson's disease, resulting from its strong affinity for brain tissue. Oral acute toxicity is actually only part of the picture. While GSU defers to HERD in these matters, GSU does not feel that the threat of exposure is sufficiently controlled and that the risk levels should be raised. 30.Section 7.0 The obvious high risk of remobilization of the plume by pumping cannot be lightly dismissed. 31.Section 8.2 The MNA alternative approach assumes that attenuation does not occur, which tends to contradict earlier discussions saying that (irreversible) sorption is a factor in plume migration. 32.Section 8.2 In situ treatment has not been sufficiently explored. Many features of this release suggest that in situ treatment might actually work: the migration in a channel of limited extent, the retention of particles along the original flow path, the stability under low gradients, all suggests that injecting a highly soluble oxidizer at the original injection sites would likely follow the same pathways and reach the resting-phase dieldrin. Injecting in other locations would likely be futile. Darcy's law requires that, for a given geometry and distribution of units, flow will follow the same pathways regardless of the gradient, because the relative velocities in different units are fixed by conductivity and area. The original injection points are known, so the flow paths are basically fixed by the geology. Treatment at the injection points is all that is needed to chase the dieldrin. The volume of oxidizer need not be much more than enough to allow for organic