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volume of flow into the volume of rock, this computes to an average drainable porosity <br />of the diatremal rocks between the Roosevelt Tunnel level and the Carlton Tunnel level <br />of about 17%. While this may be a high estimate, due to inclusion of flow resulting from <br />some desaturation of the Precambrian rock and residual drainage from rock above the <br />Roosevelt Tunnel level, the result is characteristic of predominantly primary (void) <br />porosity, rather than of fracture porosity alone. Thus, it is likely that there is a <br />multiplicity of pathways for water flow through the rock as well as the fracture, and a <br />multiplicity of opportunities for contact between the seepage water and the carbonate <br />contained within the rock matrix. <br />With respect to seepage from surface mining that may reach the regional ground water <br />table intersected by the Carlton Tunnel, CC&V agrees with DRMS that it is unlikely that <br />any significant quantity of sulfide oxidation constituents carried by such seepage has <br />reached the regional ground water table at the level of the Carlton Tunnel. As stated in <br />the MLE application, the placement of mined overburden on the surface (in overburden <br />storage facilities or as backfill in mines) has the effect of delaying the passage of <br />moisture through the overburden until the moisture content reaches the field capacity, a <br />process that is estimated to take an average of 12 years for a 400 foot thick overburden <br />deposition.. As mining associated with the Cresson Project commenced in <br />approximately 1993 and the majority of the site continues to receive additional <br />overburden material placement, there are limited locations (e.g., Arequa Gulch <br />Overburden Storage Area) wherein moisture content throughout an area may have <br />reached field capacity. Thus, seepage through CC&V placed overburden associated with <br />the Cresson Project is in general only now starting to enter the underlying bedrock, and it <br />will be between 1 and 50 years more before it appears at the regional ground water table <br />intersected by the Carlton Tunnel, due to the time that it takes to transit 3,000 feet <br />vertically and as much as 15,000 feet laterally through the diatreme. For this reason, <br />CC&V has not asserted or speculated that the diatreme carbonate is currently <br />neutralizing significant quantities of overburden leachate that may have been produced <br />by material mined by CC&V. <br />DRMS maintains in its comment that CC&V assumes that the Carlton Tunnel water <br />quality will remain unchanged when potential acid leachate from the overburden mined <br />by CC&V passes through the diatreme and arrives at the regional ground water table <br />intersected by the Carlton Tunnel, and that such an assumption is risky. CC&V has not <br />made this or any other substantial assumption in its evaluation. CC&V does conclude, <br />supported by exhaustive investigation, experimentation, testing, and analysis presented <br />in detail in the MLE application (and consistent with what has been provided in past <br />amendment applications as approved by DRMS), that the diatreme carbonates will in the <br />future neutralize acidic leachate that may be produced from CC&V overburden <br />associated with the Cresson Project, and that as a result no acid leachate from mined <br />rock will ever report to the regional ground water table intersected by the Carlton <br />Tunnel. <br />Specifically, this evaluation comprised the following: <br />Detailed measurement and block modeling of the sulfide and carbonate that will <br />be contained in the mined overburden using tens of thousands of laboratory tests <br />taken on a 20 foot basis from essentially all exploration holes. <br />Detailed testing of the sulfide oxidation behavior of the mined overburden over <br />the full range of the sulfide content of the overburden, and for both principal <br />geological rock types (volcanic and granitic), involving 42 humidity cell (HCT) <br />tests of up to two years duration, and over 400 net acid generation (NAG) tests. <br />Comprehensive evaluation of the quantity and rate of release of the products of <br />sulfide oxidation from the mined overburden proposed in the MLE application