2019-09-10_REVISION - C1994082 - Laserfiche WebLink

Home Browse Search

standard and there is no reason to believe the total mercury within Grassy Creek is above the aquatic life standard(Table 7). Selenium As noted in CWQCC Regulation 33, a temporary modification of the dissolved selenium chronic standard is in place for Yampa Segment 13i and 13j while additional information is collected to further evaluate the limited aquatic use of these segments and the extent of ambient sources of selenium within this watershed. Several of the tributaries flow over soils derived from the Lewis Shale,which naturally contains elevated concentrations of selenium. Therefore, only the dissolved selenium acute standard for Segment 13i and 13j and the Regulation 31 agricultural use water quality standards were compared to the selenium data collected from the Grassy Creek surface water points.No exceedances of the Segment 13i acute or CWQCC Regulation 31 agricultural use standards occurred at either YSGF5 or YSG5 between 2014 and 2018 (Tables 1 and 2). Selenium was monitored 59 times at Outfalls 010 and six times at Outfall 011 during this period(Table 10 and Table 11). Potentially dissolved selenium exceeded the Segment 13i acute selenium standard and the agricultural use standard once at Outfall 010. The sample collected on February 3, 2016 had a potentially dissolved selenium of 149 µg/L. However,the total recoverable selenium for this sample was measured as non-detect(<0.5 µg/L). The total recoverable selenium analysis includes a measurement of both the metals that are dissolved in the water and the metals that are present in the particulates in the water after its been treated with acid preservative. The potentially dissolved metals analysis measures the metals present in the filtrate of the water that was first treated with acid preservative and allowed to stand for several hours before being filtered through a membrane filter. The potentially dissolved selenium can not be greater than the total recoverable selenium as the potentially dissolved form is a subset of the selenium that is measured as a part of the total recoverable analysis. This suggests the elevated potentially dissolved selenium results may have been the result of an ICP-MS matrix interference which can result in overestimation of selenium concentrations (Smith and Compton, 2004). There were no exceedances of the Segment 13i acute or agricultural use selenium standard at Outfall 011. Although the CWQCC Regulation 33 chronic selenium table value standard (4.6 ug/L) is not currently applicable to Yampa Segment 13i and 13j it is worth noting that only two exceedances SL-7 - 38