2013-04-18_PERMIT FILE - C2009087A

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RULE 2 - PERMITS • CDPHE surface water agricultural use standard of 0.2 mg/L at sites SSG1 (0.32 mg/L), SSG2 (0.25 mg/L), YSG5 (0.24 mg/L), and YSGF5 (0.25 mg/L). Peak concentrations additionally exceeded the agricultural standard at all three NPDES outfalls, and NPDES3's average concentration (0.68 mg/L) exceeded the standard. However, as stated previously, this standard does not apply, since soil pH's in this area are generally above 6.0. The trend of increasing manganese concentration in Grassy Creek from upstream (YSGF5) to just above the Little Grassy Creek confluence (SSG1), followed by a decrease at SSG2, located below Little Grassy Creek and NPDES3, suggests that naturally elevated levels in Grassy Creek are diluted by those in Little Grassy Creek, despite the additional load from the existing NPDES outfalls. Little Grassy Creek, at site SSLG5, exhibited slightly elevated levels of nitrite and selenium not observed at other monitoring sites. These are due to natural conditions. Average nitrite concentrations exceeded the 0.05 mg/L CDPHE Yampa River Segment Be (Grassy Creek) aquatic life standard at SSLG5 (0.06 mg/L average, 0.09 mg/L peak), peak concentrations slightly exceeded the standard once at SSG1 (0.06 mg/L). Exceedances at SSLG5 are suspected to be due to naturally occurring upstream conditions. The effects were diluted at SSG2, which had uniformly low levels. Peak selenium concentrations (8.1 ug/L) were also elevated above the Segment Be (Grassy Creek) aquatic life chronic standard (4.6 ug/L) at SSLG5, but did not exceed the acute (18.4 ug/L) or agricultural use (20 ug/L) standards. Average sulfide concentrations reported in the tables are influenced by the use of the detection limit (0.02 mg/L) in cases where the laboratory reported (<0.02 mg/L). Hence, any apparent exceedance of Yampa River water quality standards (0.002 mg/L un- ionized; set to the 0.02 mg/L detection limit) is • spurious when comparing average values. In addition, values up to 0.06 mg/L, while reported at some stations, were flagged by the laboratory as uncertain. The only elevated sulfide measurements considered reliable were two at NPDES4, measured as 0.12 and 0.33 mg/L. The analytical method available to Seneca Coal Company detects both dissolved sulfides and acid - soluble metallic sulfides present in suspended matter. It is suspected that a large portion of the sulfides detected are of the latter type, which would bias the un- ionized results high. Any dissolved sulfides present in surface water should eventually oxidize to sulfates. Mercury levels were influenced by detection limits in a similar fashion to sulfide. While the Yampa River standard is 0.01 ug/L, the detection limit used in this report is 0.2 ug/L or greater for springs and streams. All stream samples were reported as less than 0.2 ug/L, except for 4/5/07 sample at SSG2, which was reported as less than 1.0 ug/L, and the 4/23/09 sample at YSGF5, which was reported as less than 0.4 ug/L. Thus, all of the reported mercury concentrations are spurious for stream sampling points. Impoundments are subject to different, and lower, detection limits. Like streams, all but one of the reported impoundment mercury concentrations were "less than" values, and thus do not actually represent exceedance of a standard. One sample, 2/5/07 at NPDES4, was reported as 0.023 ug/L, which does represent a single exceedance. The trilinear diagrams for Grassy Creek and Little Grassy Creek are shown on Figures 2.04.7 -F16 through Figure 2.04.7 -F19.1 YSG5 was historically only monitored for TDS, field params and occasionally for iron, but has recently (3 samples) been sampled for the full suite of water quality params. YSG5 therefore has a limited period of record as compared to the other stations. Three of the • five sites (SSG1, SSG2, and SSLG5) are of a calcium- sodium cation type, with high magnesium and mixed anions (chloride - sulfate- bicarbonate). Water at YSGF5, the most upstream station on Grassy Creek, and at YSG5, the most downstream station on Grassy Creek, alternates between PSCM Permit App. 2.04 -76 Revision 12/17/09