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At the January 2007 Temperature Standards Rulemaking Hearing, the Colorado Water Quality Control Commission (WQCC) adopted new temperature criteria for Colorado's surface waters. As part of this ruling, the WQCC adopted numerical temperature criteria, effective December 31, 2007, in Regulation 31, The Basic Standards and Methodologies for Surface Water (5 CCR 1002-31). The new rulemaking sets revised temperature table values for cold and warm water streams, lakes, and reservoirs, requiring that "temperature maintain a normal pattern of diel and seasonal fluctuation and spatial diversity with no abrupt changes and shall have no increase in temperature of a magnitude, rate, and duration deleterious to the resident aquatic life." Regulation 31 states that these numerical table values will be used as the starting point for establishing segment-specific numerical standards, while providing an opportunity for a demonstration that alternative site-specific standards are appropriate. The numerical criteria include distinct summer and winter values to protect reproductive functions of the aquatic species and their normal patterns of seasonal fluctuation, as well as a set of more protective values to apply where thermally sensitive species either occur or are expected to occur.

Consistent with the Colorado Water Quality Control Division (WQCD) 2006 temperature implementation guidance (Policy Statement 06-1 Temperature criteria methodology), the temperature table value standards presented in Regulation 31 are used in combination with effluent design flow, upstream flow, and upstream temperature conditions to calculate the final monthly acute and chronic temperature criteria used in compliance evaluations. These calculated criteria are the acute Daily Maximum Effluent Temperature (DMET) criteria and the chronic Weekly Average Effluent Temperature (WAET) criteria. To achieve compliance, the Maximum Weekly Average Temperature (MWAT) of a water body cannot exceed the chronic temperature criterion (WAET) and the Daily Maximum (DM) cannot exceed the acute temperature criterion (DMET) more than once in three years. The chronic and acute standards are applied so that there is attainment at the edge of chronic and acute regulatory mixing zone, respectively. These temperature effluent limits do not apply, however, to effluent-dependent streams or to discharge of water from natural hot springs. In addition, temperature excursions associated with extreme ambient air temperatures or critical low flow volumes are not considered to be a breach of the criteria. The WQCD is currently developing additional guidance to facilitate implementation of the new temperature criteria.

To evaluate whether temperature effluent limits are necessary for a particular discharge, it is necessary to assess the likelihood of attaining the temperature standard by (1) evaluating the fish species expected to be present in the receiving water body; (2) calculating the DMET (acute criteria) and WAET (chronic criteria) based on site-specific data, appropriate fish species, and the Regulation 31 table value standards; (3) determining the potential for the DM and MWAT of the discharge to exceed the temperature criteria; (4) conducting an excursion analysis to determine whether effluent temperatures are influenced by extreme conditions of flow or ambient air temperatures; and (5) evaluating spatial and temporal patterns of temperature changes upstream and downstream of the discharge.

As stated at the January 2007 Rulemaking Hearing, the WQCC anticipates that it will be necessary to make refinements to the new temperature regulatory system in the future. In particular, in the course of the 2010 Basic Standards Rulemaking, the WQCC expects to review and revise the temperature criteria in recognition of newly available data; discuss numeric thermal shock provisions in addition to the current criteria; and consider applying the temperature standards to ephemeral and intermittent streams.

Posted on September 15, 2008.

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