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COLORADO DEPARTMENT OF PUBLIC HEALTH & ENV/RONMENT -Water Quality Control Division <br />Rationale -Page I3 Permit No. COR-040000 <br />V//. CHANGES AFTER PUBLIC NOTICE (cont.) <br />modified to specify that waste products include any acid generating material exposed by the mining activity, <br />and the vegetative cover criteria addresses sedimem production. The Division believes that this will address <br />the concerns of both the Division and the permittees. <br />Reeardine isolation or removal of mine waste: The regulations clearly require that srormwater discharges <br />from mine and mill sites that are contaminated by the contact list of materials, must be authorized by a <br />permit. Permittees must decide whether isolation or removal of the material is practical, feasible or <br />economically possible. Continued permit coverage is availa6[e if permittees decide not to attempt to isolate or <br />remove the material. <br />s. The Division's burden of proof reeardine water ouality impacts. <br />Many commenters objected to the latitude that is reserved to the Division regarding such things as requiring <br />subsequent monitoring, requiring an individual permit, and deciding whether or not to terminate a <br />certification. In general, commenters repressed the feeling that the Division should have a greater burden of <br />proof than "has reason to believe" that there is a water quality problem. Commenters assert that several of <br />the Division's decisions are too subjective and the vague language does not provide pernrittees with the kind of <br />certainty that they need for decision making. <br />Response The Division is aware that several of the decisions can be subjective. Once greater experience is <br />gained by the Division and permittees, the subjectivity can probably be reduced. At this time, however, the <br />Division believes the permittees will benefit since we have the latitude to consider site-specific factors. Had <br />the Division elected to eliminate the subjectivity to a greater extent, the necessarity conservative nature of the <br />criteria would have greatly reduced the flexibility for the permittee and the Division, and imposed <br />requirements on all permittees that may not be appropriate in all cases. <br />6. Disposal of water stared within mine waste <br />A few commenters had questions or comments regarding the restrictions on discharge of water stored within <br />the mine waste. "Disposal of water stored within the mine waste" is a phrase that is used in Pan I. C.S. e. <br />This is a section that described the additional SWMP items for sites seeking permit termination. General <br />clarification is necessary. <br />Response: This general permit only authorizes stormwater discharges; srormwater is precipitation-induced <br />runoff. Water stored in tailings impoundments or other areas of mine waste is not runoff The characteristics <br />of these stored waters can vary dramatically on a site-by-site basis and can have deleterious impacts if <br />discharged into receiving waters. Because of the nature of these waters (not runoff) and the potential water <br />quality impacts, discharge is not authorized under this permit. An individual permit is required which allows <br />jot site-specific assessment of the quality, quantity and impacts of the stored water. <br />Veeetative termination criteria <br />Several commemors questioned the vegetative cover criteria in Pan /.F.1.6. Some commenters thought that, <br />as written, the criteria are too onerous, may take too long, and require importing a soil matrix suitable for <br />plant growth. One commenter suggested that it be changed from a numeric criteria to a narrative criteria and <br />require "adequate" cover. One commemor suggested that the term "vegetative cover" be defined. <br />Response: The vegetative criteria stares that "a minimum of 40 percent vegetative cover, or 70 percent of the <br />vegetative cover of a similar undisturbed site, whichever is higher' is required. That means that 40 percent <br />cover is the lowest Level. This number was determined through assessment of the relationship between cover <br />and soil erosion for a variety ojconditions using the Universal Soil Loss Equation. Below 40 percent, a small <br />increase in vegetative cover makes a dramatic decrease in soil erosion; above 40 percent, a small increase <br />does nor have so large an impact. Long-term stabilization is the goal and self-sustaining vegetative cover is <br />