2005-01-31_REVISION - M1977208 (2)

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Attachment 2 - Point by Point Response to Adequacy Letters Page 1 of 4 November 22, 2004 Adequacy Letter from the Division of Minerals and Geology Adequacv Item No. 1: Groundwater Mitigation Plan Cemex does not believe a groundwater mitigation plan is necessary at this time based on our findings concerning the geology and hydrology of the site as well as the results of MYRGT model. At this point, a mitigation plan is impractical because we cannot characterize the problem (e.g. the mitigation for elevated pH would be different than the mitigation for elevated selenium). Since C-Pit poses no immediate threat to groundwater resources. Cemex proposes to continue pumping from C-Pit to reduce the pond size, install a detection well (as described in the cover letter) and revisit this issue with the Division once the pond has reached its steady state level and conditions. Adequacv Item No. 2: Quantification of Ditch Seepage West of C-Pit are the Swede Ditch and Boulder Feeder Canal. An estimate of the rate of seepage from these features into C-Pit has been prepared using Darcy's Law and is presented in Attachment 3. It should be noted that the Swede Ditch is contained in a sealed pipe for its entire length adjacent to C-Pit. Also, these ditches flow intermittently for 6-8 months per year. The results of the calculation suggests that the total amount of seepage from the Boulder Feeder Canal that could potentially reach C-Pit ranges between 0.6 and 6 gallons per minute (gpm) for hydraulic conductivity values of 1 x 10-6 and 1 x 10-5 cm/s, respectively. Considering the canal operates for about 180 days, this daily seepage amount is equivalent to an annual seepage volume ranging between 160,000 and 1,600,000 gallons. The total seepage volume probably represents an upper bound as visual examination of the Boulder Feeder Canal, which is excavated in bedrock, indicated few visible fractures that seepage through which seepage might occur. The actual seepage rate is likely to be less than 1 gpm since visual examination of the western C-Pit highwall has only shown temporary dampness along the exposed highwall after precipitation events. Adequacv Item No. 3: Thallium Cemex accepts the Division's request for more data before requesting the removal of thallium from the list of analytes. This issue will be revisited no later than December 2005. Adequacv Item No. 4: Well in the Dakota Formation As discussed in our December 17, 2004 meeting and in the cover letter to this response, Cemex does not believe a Dakota well is technically justified. Cemex proposes to install a detection well approximately 300 feet east of C-Pit to be screened in the Fort Hays limestone and 10 feet above and below this geologic unit. This well will be sampled quarterly for the proposed analytes. The purpose of this well is to confirm Cemex's evaluation of the geology and hydrology in the vicinity of C-Pit and confirm the results of the MYRGT model previously submitted to the Division. Such confirmation would be evidenced by a lack of groundwater in the well and/or low background water quality concentrations of the proposed analytes. ~~ ~~~ v. E i ~~l ~l' tti ~~ ~~ ~~ ~ {~', ~t ~,?1"~ ~Z