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Mr. Robert Hagen - 2 - January 29, 1993 Colorado contends that the structures in question are incised temporary impoundments which are properly designed, permitted and constructed to meet the requirements of Colorado Rules 4.05.2(3), 4.05.6 and 4.05.9. Thus, our response should have been evaluated within the regulatory framework for this particular situation. Regarding the question of whether the facilities in question are sediment ponds or impoundments, Colorado believes that the AFO must defer to the permitting actions which have been previously taken by the State. As noted in the AFO inspection report, these facilities were permitted as temporary impoundments via Technical Revision 13. Both facilities meet the regulatory definitions of impoundments as well as the specific design and performance requirements for impoundments. The Colorado Rules at Section 4.05.2(3) allow operators an exemption from the requirement to pass affected area water through a designed treatment system or sediment pond prior to discharging from the site. This Rule allows the exemption on a limited basis, and requires the operator to demonstrate that the discharge from these small areas meet applicable effluent limitations. Historically, the Division has required that operators provide some type of BTCA treatment, such as a siltation structure, to ensure that the discharge meets the water quality effluent limitations. In this context, Small Area Exemptions may be viewed as a limited alternative sediment control requirement. The Division's interpretation and application of the Small Area Exemption rule in this manner ensures that water quality limitations are achieved by requiring additional environmental protection. Historically, non-discharging structures have been permitted as a means of achieving compliance with Rule 4.05.2(3). The utilization of an appropriately permitted and constructed non-discharging impoundment, such as those in question, is an acceptable approach to achieving compliance with the water quality effluent limitations. This approach does not in any manner conflict with the Small Area Exemption rule, and is consistent with previous interpretations and applications of the Rule. Colorado believes that this practice is compatible with its Rules, the requirements of SMCRA and with those of the Clean Water Act. Rule 4.05.9(2) addresses temporary impoundments. Technical Revision 13 was approved in the context of Rule 4.05.9. This Rules establishes impoundment design and construction criteria, and requires that specific portions of Rule 4.05.6, sediment ponds, be applied to impoundments with dams (emphasis added). Section 4.05.9(6) states that only the applicable requirements of Rule 4.05.9 need be addressed for impoundments which are excavated. The impoundments in question are incised into the ground, and neither have dams or embankments. Therefore, the provisions of 4.05.9(6) are effective. During the permitting of the temporary impoundments, the Division required that the operator comply with Rule 4.05.9 as it applies to incised ponds.