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COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale Page 6, COG-0500000 Sand and Gravel General Permit exempted from an antidegradation review. Based on the information and data in the application, the permit writer will make an assessment of the low flow dilution ratio of the discharge to determine if antidegradation applies. Under this general permit, an antidegradation (AD) limit will be calculated as 15% of the Water Quality Standard. The permittee would then have the choice of this AD limit, or of a non-impact limitation (NIL). The NIL is either the limitation contained as of September 2000, or may be determined by the use of an implicit limitation if a previous limit did not exist. The implicit limit is determined as the maximum effluent concentration in the years prior to September 2000 (later data may be substituted on a case b) case basis if data is unavailable from this time period). Alternately, if data does not exist, a compliance schedule may be added to the permit to obtain such data, to determine the implicit limitation and the final antidegradation-based effluent limit. An individual permit will be required where the permittee requests consideration of dilution and ambient water quality. In addition, the permittee may elect to perform an alternatives analysis. As this may be subject to public notice requirements, an individual permit will be required. See Regulation 31.8(3)(d) and the Division's Antidegradation Guidance document for more information regarding an alternatives analysis. g. Whole Effluent Toxicity (WED - The Water Quality Control Division has established the use of WET testing as a method for identifying and controlling toxic discharges from wastewater treatment facilities. WET testing is being utilized as a means to ensure that there are no discharges ofpollutants "in amounts, concentrations or combinations which are harmful to the beneficial uses or toxic to humans, animals, plants, or aquatic life" as required by Section 31.11 (1) of the Basic Standards and Methodologies for Surface Waters. Some discharges covered under this general permit may exhibit whole effluent toxicity based on the potential pollutant concentrations in the discharge. Thus, WET testing may be incorporated into the permit on a case-by-case basis. If it is determined that WET testing is required, Chronic WET testing will normally be incorporated into the permit. However, Acute testing may be applied where the receiving stream has a zero low flow in all months and the discharge is intermittent, or when deemed appropriate for any other site-specific reason. h. Threatened and Endangered Species- The US Fish and Wildlife Service and the Division have entered into a Memorandum of Agreement (MOA) regarding discharges to federal T&E waters. In this MOA, a permittee that discharges to a T&E water may have additional constraints placed upon the discharge. These constraints may include accepting end-of-pipe limitations (no . dilution), moving the discharge point to a different location, or using a dyer to obtain instantaneous mixing of the effluent and the receiving water. This last option may allow for a portion of the available assimilative capacity (dilution) to be incorporated intt the permit. However, as this is a general permit, all limitations are imposed as end-of--pipe limits, and therefore, the first option is met. Generally, it will be assumed that the end-of-pipe limitations will satisfy the MOA, and no further consideration is needed. O? a case-by-case basis, additional constraints may need to be evaluated. h. Mixing Zones -Under this general permit mixing zone regulations do not apply, as the water quality standards are applied as the effluent limits(i.e., no dilution is allowed.). i. Discharges to 3030 Listed Waters - Where the receiving water is included on the state's 303(d) list, total maximum daily loads (TMDLs) are being developed in accordance with the Division's schedule for TMDL completion. Once a TMDL has been established, if it is necessary to incorporate limitations for a facility certified under this general permit, based upon the outcomes of the TMDL or the results of the data analysis, a modification to the certification to discharge, or an individual permit may be required to regulate discharges from a facility. The facility would continue to be covered under this general permit certification until the individual permit is issued. For example, the industrial process ofsand and gravel operations typically involves dischargingslluvial water. When the alluvium is dewatered in an area ofshale deposits (e.g. Mancos), selenium is likely to be present in elevated concentrations in the discharge and is automatically considered a pollutant of concern. Preliminary monitoring data indicates that operations ii areas of shale are likely contributing to exceedances of stream standards for selenium, and therefore, monitoring requirements will be included for data collection for a future reasonable potential analysis. New discharges to these segments will be required to meet the water quality limitations for selenium upon commencement of discharge. The basic standards for selenium are found in Table III of The Basic Standards and Methodologies or Surface Water (5 CCR 1002-31). There are currently ongoing national discussions between the USEPA and the USFWS on appropriate aquatic life standards for selenium. Should the USEPA adopt new water quality standards for selenium, the division's standards unit will begin incorporating federal standards into Colorado's water quality standards during basin triennial reviews. In the future, treatment of effluent to remove selenium may be required to attain water quality standards.