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Mr. Robert Hagen - 2 - June 12, 1991 The Colorado Rules require that all disturbed area drainage be passed through a sedimentation pond, a series of sedimentation ponds or a treatment facility before leaving the permit area. CMLRD ma.y grant an exemption of this provision, pursuant to Rule 4.05.2(3), if a demonstration is provided which shows that sedimentation ponds and treatment facilities are not necessary for drainage to meet the effluent limitations of 4.05.2. It is also necessary to demonstrate that applicable state and federal water aualit.y requirements for downstream receiving waters are met. Such an exemption ma.y allow an operator to avoid installation of a sediment control structure. However, compliance with Rule 4.05.2(3) and the ability to achieve the applicable effluent limitations ma.y be enhanced by installation of some alternate structure, as allowed by Rule 4.05.5 (sediment control measures). Rule 4.05.5 acknowledges that the best technology currently available to control sediment ma.y be some method other than a sedimentation pond. This concept is substantiated at 30 CFR 816.45 (Hydrologic balance: sediment control measures). Both Colorado Rule 4.05.5 and 30 CFR 816.45(b)(6) indicate that check dams, dugout ponds or sediment traps are appropriate means by which sediment control and effluent standards may be achieved. The OSM Directive TSR-3 (November 2, 1987) further clarifies that such alternate sediment control methods may be implemented. 30 CFR 701.5 states, in part, that within the constraints of the permanent program, the regulatory authority shall have the discretion to determine the best technology currently available on a case-by-case basis, as authorized b.y the Act. The assignment of state authority is reiterated within Directive TSR-3. Sediment traps or dugout ponds, such as those found at the New Elk Mine, can be considered proper sediment control measures, if the traps are designed to the standards specified at Rule 4.05.5. Compliance with Rule 4.05.5, would be an integral component of the demonstration required to obtain the small area exemption. The CMLRD has historically approved designs which contain the anticipated 10-year, 24-hour precipitation event, thus ensuring compliance with applicable effluent standards. Specifically, the applicable effluent standards are those of the NPDES program. Under the NPDES Drogram, flows resulting from precipitation events greater than the 10-year, 24-hour event, are exempt. Contrary to our definition of sedimentation pond at Rule 1.04(115), the alternate structures in question are not designed to slow runoff, prior to passage off site. Rather, the alternate structures are designed to contain water flows and sediment. Thus, applicable effluent standards are met. The traps are considered to be a proper application of the best technology cur rentl.y available and they are intended to ensure compliance with the small area exemption provisions of Rule 4.05.2(3). In summary, the structures identified at the New Elk Mine are sediment control measures as allowed at Rule 4.05.5, implemented to ensure compliance with Rule 4.05.2(3). The areas adjacent to the structures in question should be categorized as small area exemptions. Alternative sediment control, as evidenced by the dug out ponds, enhance the requirements of Rule 4.05.2(3). Therefore, we have required the permittee to revise the approved permit such that the appropriate small area exemption criteria are met.