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If the post mining land use is to be rangeland, there is little reason to take measures other <br />than those that would avoid ponded water. However, if construction is anticipated over <br />the site, the limestone mass could have a low compressive strength relative to other <br />materials, and that could require special considerations for building construction. <br />For CKD to set up into a competent, concrete-like mass, it needs a chance to dry out <br />after wetting. If kept saturated, it could continue to release alkali and produce high pH <br />indefinitely. This is something CEMEX needs to work out with CDPHE. <br />With regazd to monitoring the Dakota formation: If C-pit is impermeable, then all, most, <br />and/or enough of the dissolved Se would remain in the pit azea or would have migrated <br />only a short distance, and likely would not have affected an aquifer. The Dakota well on <br />the eastern edge of the target azea is artesian. Considering the local topography, <br />geology, and hydrology, that means C-pit water could not reach the Dakota via fractures <br />lest those same fractures would bleed water from the Dakota upwazds into the pit via <br />artesian flow. If CEMEX's explanation provides enough of the whole geohydrology to <br />satisfy our concerns about pollution in the Dakota aquifer, then the Division will <br />reevaluate the requirement for a Dakota monitoring well in addition to the exiting Hall <br />well. <br />6. With regazd to the in-progress water treatment proposal: The Division will not give a <br />pass on the elevated Se and high pH in the C-pit for any time other than this mitigation <br />period. In this event; provided C-pit water has migrated into adjacent formations only <br />minunally, and provided the Division has no reason to require CEMEX to remediate the <br />adjacent formations, then treating C-pit water to neutralize alkalinity and lower Se levels <br />would only be necessary to protect wildlife. <br />If CEMEX is willing to cover the pit water surface with bird balls, the Division could <br />accept the minimal seepage of high alkaline high Se water into adjacent formation as <br />minimizing impacts. The definition of minimization in this case will have to be worked <br />out between CEMEX and the Division. <br />If you have any questions, please do not hesitate to call me at 303-866-4943. <br />Sincerely, <br />~"~.~~,~ <br />Erica S. Crosby <br />Environmental Protection Sp' <br />cc: Bruce Humphries; DMG <br />Cazl Mount; DMG <br />Harry Posey; DMG <br />Kate Pickford; DMG <br />Paul Banks; Banks & Gesso <br />