5 Colo. Code Regs. § 1002-38.86
The provisions of C.R S. 25-8-202(1)(a), (b) and (2); 25-8-203; 25-8-204; and 25-8-402; provide the specific statutory authority for adoption of these regulatory amendments. The Commission also adopted the following statement of basis and purpose pursuant to C.R.S 24-4-103(4).
BASIS AND PURPOSE
The Commission considered the selenium and mercury standards for Sand Creek, segment 16a of the Upper South Platte Basin. Based on evidence presented in this hearing, the Commission divided the segment and adopted site-specific ambient-based selenium standards and a temporary modification of the mercury standard as follows.
Selenium and Resegmentation:
As determined in an earlier hearing (December 2008), the Toll Gate Creek system, a tributary to Sand Creek, has elevated levels of selenium caused by natural or irreversible human-induced sources. Ambient-based selenium standards were adopted for Toll Gate and its tributaries in that hearing. In this hearing, evidence was presented that the effects of Toll Gate Creek's contribution change the water quality characteristics of Sand Creek below its confluence with Toll Gate Creek, to the extent that the selenium levels exceed the relevant table value criteria. In this hearing, the Commission split the mainstem of Sand Creek into two segments, retaining the segment number (16a) and the table value selenium criteria above the confluence with Toll Gate Creek.
The Commission created a new segment, segment 16i, and adopted ambient quality-based site-specific standards for selenium applicable to the portion of Sand Creek below Toll Gate Creek. The ambient quality-based standards are based on the 85th percentile (chronic) and the 95th percentile (acute) of the selenium data collected at two specific instream monitoring locations: Sand Creek at the Peoria Street crossing (which has a station identifier of SWA) and Sand Creek just upstream of the Union Pacific Railroad crossing (which has a station identifier of SW1); each is upstream of a wastewater outfall. The most recent five years of data were used since there has been a persistent and significant increase in the concentration of selenium since 2008. Two assessment locations are appropriate since the selenium concentrations consistently decline along Sand Creek, probably due to influx of lower concentration groundwater, however the mechanism has not been identified. It is the Commission's intent to maintain this natural or human-induced irreversible pattern of water quality, and not to inadvertently create assimilative capacity.
The Commission added assessment locations to section 38.6 to record the assessment strategy for ambient quality-based site-specific standards for selenium applicable to Sand Creek segment 16i. It is the Commission's intent that attainment of the standard is to be assessed separately with data from two specific monitoring locations (SWA and SW1). Further, it is the intent of the Commission that selenium effluent limits for any permitted discharge be calculated to assure attainment of the criteria only at the assessment location (SWA or SW1) closest to the discharge, even if the closest assessment location is upstream.
Mercury:
The Commission adopted a temporary modification of the mercury standard for the new segment 16i (the mainstem of Sand Creek from Toll Gate Creek to the confluence with the South Platte River). There have been several instances of total mercury concentrations in Sand Creek below Suncor's outfall exceeding the water quality standard. In addition, Suncor presented evidence that it will have a compliance problem with the water quality-based effluent limit based on the existing standard.
Mercury is a bioaccumulative pollutant and fish tissue is the endpoint of concern. The rate of bioaccumulation is variable, so there is uncertainty regarding the total mercury water column standard necessary to maintain fish tissue concentrations below the human health criteria of 0.3 mg/kg.
Suncor agreed to undertake a study to resolve the uncertainty, with the following conditions, to ensure that the fish tissue data collected is representative of the potential human health exposure to mercury:
* Fish tissue will be sampled multiple times per year, during variable flow conditions and seasons.
* Appropriate sampling methods will be used for capturing the larger fish individuals.
* The largest individuals caught will be sampled for each species at site SW2-1.
* Fish tissue samples will be collected as skinless filets, where possible.
* Suncor will submit an annual progress report with fish tissue data to the Division every year beginning in December 2013.
* Suncor will continue to collect monthly water quality samples and analyze them for total mercury at SW2-1 using the low-level detection method.
* Suncor will work with the Division and EPA to calculate the bioaccumulation factor for Sand Creek and to develop a site-specific standard.
The Temporary Modification is set to expire on 6/30/2017. This anticipates that Suncor will report progress to the Commission in the December 2015 annual Temporary Modification hearing, and that the uncertainty will be resolved during the December 2016 annual hearing. The Commission is adopting the mercury temporary modification for the newly created Segment 16i with the notation of "current condition" rather than a numeric value. It is the Commission's intent that this will preserve the status quo during the term of the temporary modification. The Commission does not intend that this temporary modification will apply to new facilities or in Preliminary Effluent Limitations.
PARTIES TO THE RULEMAKING HEARING
5 CCR 1002-38.86