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COLORADO DEPART7KENT OF PUBLIC HEALTH & ENVIRONMENT -Water Quality Comrol Division <br />Rationale -Page 13 Permit No. COR-040000 <br />V/I. CHANGES AFTER PUBLIC NOTICE (cont.) <br />modified to spedfy that waste products include any acid generating material exposed by the mining activity, <br />and the vegetative cover criteria addresses sediment 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 stormwater discharges <br />from mine and mil[ sites that are contaminated by the contact list of materials, must be authorized by a <br />permi[. Permittees must decide whether isolation or removal of the material is practical, feasible or <br />economically possible. Cominued permit coverage is available if permitrees decide not ro attempt ro isolate or <br />remove the material. <br />5. T7te Division's burden of proof reeardine water ouality impacts. <br />Many commentors objected to the latitude that is reserved to the Division regarding such things as requiring <br />subsequent monitoring, requiring an individual permit, and deriding whether or nor to terminate a <br />certification. In general, commemors expressed 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. Commemors assert that several of <br />the Division's decisions are too subjective and the vague language doer not provide permittees with the kind of <br />certainty that they need for decision malting. <br />Response the Division is aware that several of the decisions can 6e 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 permitrees 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 necessarily con.;ervative Harare 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 stored within mine waste <br />A few commemors had questions ar comments regarding the restrictions on discharge of water stored within <br />the mine waste. "Disposal of water stored within the mint waste" is a phrase that is used in Part I.C.S.e. <br />This is a section that described the additional SWMP items for sits seeking permit termination. General <br />clarification is necessary. <br />Response: This genera[ permit only authorizes stormwater discharges; stormwater is precipitation-induced <br />runoff. Water stored in tailings impoundments or other areas of mine waste is nor runoff. The characteristics <br />of these stored waters can vary dramatirnlly 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 />for site-specific assessment of the quality, quantity and impacts of the stored water. <br />7. Veeetative termination criteria <br />Several commentors questioned the vegetative cover criteria in Part /.F.l.b. Some commemors thought that, <br />as written, the criteria are too onerous, may rake too long, and require importing a soil matrix suitable for <br />plant growth. One commentor suggested that it be changed from a wmeric 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 states that "a minimum of 40 percent vegetative cover, or 70 percent of the <br />vegetative cover of a similar undisturbed sire, whichever is higher' is required. That means that 40 percent <br />cover is the lowest level. This camber was determined through assessmem of the relationship between cover <br />and soil erosion for a variety of conditions using the Universal Soi! 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 not have so large an impact. Long-term stabilization is the goal and self-sustaining vegetative cover is <br />