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.COLORADO DEPARTMENT OF PUBLIC HEALTH & ENVIRONMENT -Water Quality Control Division Rationale -Page 13 Permit No. COR-040000 VII. CHANGES AFTER PUBLIC NOTICE (cont.) modified to specify that waste products include airy acid generating material exposed by the mining activity, and the vegetative cover criteria addressu sedimem production. The Division believes that this will address the concerns of both the Division and the permitteu. Reeardine isolation or removal of mine waste: The regulations clearly require that stormwater discharges from mine and mill situ that are contaminated by the contact fist of materials, must be authorized by a permit. Ptrnittees must decide whether isolation or removal of the material is practical, feasible or economically possible. Continued permit coverage is available if permittees decide not to attempt to isolate or remove the material. 5. The Division's burden of proof reeardine watcr ouality impacts. Many rnmmtmors objected to the latitude that is ruerved to the Division regarding such things as requiring subsequent monitoring, requiring an individual permit, and deciding whether or wt to terminate a cemfication. In genera[, commemors expressed the feeling that the Division should have a greater burden of proof than 'has reason to believe" that there is a water quality problem. Commemors risen that several of the Division's decisions are too subjective and the vague language does not provide permittees with the kind of certainty that they need for decision making. Response The Division is aware that several of the decisions can 6e subjective. Once greater experience is gained by the Division and permittees, the subjectivity can probably be reduced. At this time, however, the Division believes the permlttees will benefit since we have the latitude to consider site-specific factors. Had the Division elected to eliminate the subjectivity to a greater extent, the necusari[y conservative nature of the criteria would have greatly reduced the flexibility for the pennittee and the Division, and imposed requirements on all pernittees that may not be appropriate in all cases. 6. Disposal of water stored within mine waste A few commeruors had questions or comments regarding the rurrictions on discharge of water stored within the mine waste. "Disposal of water stored within the mine waste' is a phrase that is used in Parr I.C.S.e. This is a section that described the additional SWMP items for sites seeking permit termination. General clarification is necusary. Response: This general permit only authorizes stormwater dischargu; stormwater is precipitation-induced runoff'. Water stored in tailings impoundments or other areas of mine waste is not runoff. The characteristics of these stored waters can vary dramatically on a site-by-site basis and can have deleterious impacts if discharged into receiving waters. Because of the nature of these waters (not ntno~ and the potential water quality impacts, discharge is not authorized under this permit. An individual permit is required which allows for site-specific assessmem of the quality, quantity and impacts of the stored water. 7. Veeetative termination criteria Several commentors questioned the vegetative cover criteria in Pan I.F.I.b. Some commemors thought that, as written, the criteria are too onerous, may take too long, and require importing a soil matrix suitable for plant growth. One commeraor suggested that it be changed from a numeric criteria to a narrative criteria and require "adequate" cover. One commemor suggested that the term "vegetative cover" be defined. Response: The vegetative criteria states that "a minimum of 40 perceru vegetative cover, or 70 percent of the vegetative cover of a similar undisturbed site, whichever is higher" is required. That means that 40 percem cover is the lowest level. This number was determined through assessmem of the relationship between cover and soil erosion for a variety of conditions using the Universal Soi[ Loss Equation. Below 40 percent, a small increase in vegetative cover makes a dramatic decrease in soil erosion; above 40 percent, a small increase does nor have so large an impact. Long-term stabilization is the goal and selj-sustaining vegetative cover is