2025-07-09_PERMIT FILE - C2009087 (10)

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Technical Memo  Page 7 August 16, 2024 Peabody – CDPS Permit Supplement Use of the EPA TSD is consistent with Colorado Regulation 61. Section 61.8(2)(b)(i) says, “Effluent limitations designed to meet water quality standards shall be based on application of appropriate physical, chemical, and biological factors reasonably necessary to achieve the levels of protection required by the standards. Such determination shall be made on a case- by-case basis.” Section 61.8(2)(b)(i)(B) says, “When determining whether a discharge causes, has the reasonable potential to cause, or measurably contributes to an in-stream excursion above a narrative or numeric water quality standard, the Division shall use procedures, including appropriate water quality modeling, which account for existing controls on point and nonpoint sources of pollution, the variability of the pollutant or pollutant parameter in the effluent, the sensitivity of the species to toxicity testing (when evaluating whole effluent toxicity), and where appropriate, the dilution of the effluent in the receiving water.” The EPA TSD provides guidance for consideration of appropriate physical, chemical, and biological factors reasonably necessary to achieve the levels of protection required by the selenium standards. In addition, the EPA TSD provides procedures that account for variability of selenium in the effluent and dilution of the effluent in the receiving water, as well as the biological uptake of selenium. 3.2.1 Definition of Steady State The TSDs define an organism being at “steady-state” as when the rates of chemical uptake and depuration within the organism are equal and the tissue concentrations remain constant over time (EPA 2024a). Steady-state refers to “conditions where sufficient time has passed after the introduction of a new or increased discharge of selenium into a waterbody so that fish tissue concentrations of selenium are no longer increasing”. Steady-state is also referenced as taking “months” to reach in lotic systems which is one reason why the selenium studies for Cow Camp and Bond creeks were conducted across multiple seasons and across multiple years. 3.2.2 Methods for Determining RP In EPA 2024c, it is stated that “regardless of the specific sample type (composite samples vs. individual samples), the maximum central tendency (e.g., mean, median, etc.) selenium data from fish tissue at each sampling event should be used in RP determinations and evaluating compliance with WQBELs”. Additionally, EPA notes that when data from different samples of fish are collected during a single sampling event the maximum fish tissue value based on the maximum composite value or maximum central tendency individual fish concentration obtained from the different samples of fish should be used as the basis for an RP determination. If there is more than one sampling event, the fish tissue concentration, either the maximum composite value or maximum central tendency individual fish concentration, observed across all sampling events should be used as the basis for RP determinations and to