2020-06-08_REVISION - C1982057 (20)

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The method detection limit for the sulfide analysis (MDL: 0.02 mg/L) conducted by SCC's lab exceeds the CDPHE Yampa Segment 13d water quality standard for un-ionized sulfide (0.002 mg/L).This analytical method detects both dissolved sulfides and acid-soluble metallic sulfides that are present in suspended matter and provides a single cumulative concentration that includes both the ionized (HS-) and un- ionized forms of hydrogen sulfide (HzS).The un-ionized hydrogen sulfide is the potentially toxic form which CDPHE based the water quality standard on.The distribution of sulfide between the un-ionized hydrogen sulfide and ionized form is dependent on the temperature and pH. At low pH most of the total dissolved sulfide exists as un-ionized hydrogen sulfide. In alkaline waters, like those present at Seneca II- W, most of the total dissolved sulfide exists as non-toxic ionized sulfide. A procedure for calculating the un-ionized form from the sulfide data can be found in the American Public Health Standard Methods for the Examination of Water and Wastewater.The results of this calculation indicate that un-ionized hydrogen sulfide will not exceed the water quality standard when the non-detect sulfide concentration is equal to 0.02 mg/L. None of the sulfide samples collected at the Hubberson Gulch/ Dry Creek stream points exceeded the detection limit. Mercury The method detection limit for mercury(0.02 ug/L) used by SCC's lab is above the 0.01 ug/L aquatic life standard for mercury. None of the samples collected during the last five years exceeded the labs method detection limit.The CDPHE performed a reasonable potential analysis for the Seneca NPDES outfalls and mercury monitoring was dropped from all outfalls except Outfall 005, which did not have enough sample data for CDPHE to complete the analysis due to its intermittent flow regime. Mercury has not exceeded the Yampa Segment 13d aquatic life water quality standard in any of the effluent samples collected from Outfall 005 during the last five years. Selenium Yampa Segment 13d currently has temporary modification of the chronic aquatic criterion table value standard of 4.6 ug/L to current conditions.This modification will continue until 12/31/2022 unless extended by the Colorado Water Quality Control Commission. Between 2015 and 2019 dissolved selenium exceeded the CDPHE Yampa Segment 13d acute selenium standard once at WSD5 (SW-S2W- SG5).The sample was collected on April 20, 2016 and had a dissolved selenium concentration of 26.8 ug/L. Samples were not collected from the NPDES outfalls during the monitoring event however selenium concentrations from samples collected at stream points WSH7, which receives drainage from Outfalls 016 and 017, and WSH1, which receives drainage from Outfall 006, were 1 ug/L and 1.2 ug/L, respectively.The sample result from WSD5 appears to be an anomaly. Dissolved selenium did not exceed the acute standard at any of the other stream points between 2015 and 2019. Although the 4.6 ug/L chronic selenium table value standard is temporarily suspended for Yampa Segment 13d, its important to note that none of the stream samples, except for the April 2016 WSD5 sample, exceeded the selenium table value standard either. Selenium was monitored nearly two hundred times at our NPDES outfalls between 2015 and 2019 and there were no excursions of the NPDES selenium limits, Segment 13d acute aquatic standard, or temporarily suspended 13d chronic selenium table value standard. Spoil Springs Several spoil springs have been identified within the permit area. Five of these are actively monitored. Spoil Spring 1 (WSSPGI) is located above NPDES Outfall 005. Spoil Spring 2 (WSSPG2), Spoil Spring 3 (WSSPG3), and Spoil Spring 4 (WSSPG4) are located upstream of NPDES Outfall 006, and Spoil Spring