Laserfiche WebLink
Information to Support Denison BLM Environmental Assessment Remand <br />Denison Sunday Mines Group <br />mile can show any air transport contamination that may or may not have <br />occurred to the surrounding environment (i.e.- surface soils). <br />Discussion CDM disagrees with this comment that no data is available for knowing <br />what conditions exist below the mine dumps. As discussed in the <br />Response to Comment 1, both site - specific geochemical and geotechnical <br />data were collected from each of the Sunday Mines Complex <br />development rock piles in accordance with a data collection work plan <br />provided to BLM (CDM 2008). This site - specific data were used to <br />support a transport model to assess the potential for leachate from the <br />rock piles to impact subsurface soil and groundwater. As previously <br />discussed, the transport model indicated that it is unlikely for leachate to <br />have significant impacts to subsurface soil and groundwater. BLM's <br />comment that the use or site - specific geotechnical and climate data and <br />development of the UNSAT -H model is only a theoretical approach and <br />does not represent a robust assessment of current conditions is unclear. <br />Transport models are commonly used to assess current and future <br />environmental conditions using a limited data set. Other empirical <br />methods for assessing deep percolation through development rock piles <br />(e.g., collection of pore -water or groundwater via wells /lysimeters) have <br />several disadvantages; for example, it will be difficult to assess <br />background water quality conditions (e.g. pre - construction condition) for <br />pore water and groundwater beneath the rock piles. It's also unclear <br />which criteria BLM would propose to use in evaluating pore water or <br />groundwater collected beneath the rock piles. Available literature <br />indicates that ambient groundwater quality in the area of the rock pile is <br />poor; accordingly, the use of Colorado Basic Groundwater Standards <br />(CBGS) or EPA Maximum Contaminant Levels (MCL) would not likely <br />be appropriate criteria. Other disadvantages of these types of assessment <br />approaches include the extensive time required to assess percolation <br />through the rock piles, and very significant expenses for installation, <br />monitoring and data evaluation. <br />Additional site - specific data and observations also support that historic <br />placement of the development rock piles is unlikely to have significantly <br />affected soil or groundwater beneath the dumps. For example, paste pH <br />data from the rock piles indicate that development rock material is not <br />acid generating nor will it likely become acid generating in the future. <br />Historic monitoring and observation of surface water runoff from the <br />rock piles also supports that it is unlikely that precipitation coming into <br />contact with the development rock piles will impact the surrounding <br />environment; for example, there are not large denuded areas <br />downgradient of the rock piles. In addition, in the 30 years that the <br />development rock piles have been present at the sites, there have not been <br />any visual observations of leachate at the toe of the piles. <br />Final Sunday Mines BLM Remand Letter.doc <br />