2009-05-12_REVISION - C1981025 (8)

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determine the relationship of current downstream concentrations of mercury with the Numeric Standard. Mercury was undetected in an Outfall 001 sample of July 17, 2001, the last time it was checked. The Numeric Standard for mercury is 0.01 ug/1. Mercury was detected at less than 0.1 ug/l in baseline upstream and downstream measurements. Nickel (Ni) There has been no continuous or recent monitoring for nickel at the downstream monitoring location on North Thompson Creek, thus there is no direct information to determine the relationship of downstream concentrations of nickel with the Numeric Standard. The Numeric Standard for nickel is 83 ug/l (chronic). Nickel was undetected in Outfall 001 discharges of July 17, 2001, the last time it was checked. Selenium (Se) There has been no continuous or recent monitoring for selenium at the downstream monitoring location on North Thompson Creek, thus there is no direct information to determine the relationship of current downstream concentrations of selenium with the Numeric Standard. The Numeric Standard for selenium is 4.6 ug/l (chronic). Selenium was undetected in upstream baseline monitoring, ranged from undetected to 6 ug/1 in baseline downstream samples, and was undetected in the Outfall discharge of July 17, 2001, the last time it was checked. Silver (Ag) Silver was undetected in the July 17, 2001 sample of Outfall 001 discharge. There has been no other testing for silver. The Numeric Standard for silver is 0.84 ug/l (chronic). Zinc (Zn) Monitoring results indicate that zinc concentrations in North Thompson Creek are not effected by Outfall 001 discharges, and that the Numeric Standard for zinc is not exceeded in downstream flows. The Numeric Standard for zinc is 190 ug/l (chronic). The measured concentrations at outfall 001 range from 20 ug/l to 320 ug/l, with an average of 141 ug/l. Other Effects Conductivity There is no conductivity receiving stream standard. However, the Division has established a material damage suspect level for irrigation water in northwest Colorado of 1,500 umhos/cm. The suspect level of 1,500 umhos/cm is based on technical literature that indicates irrigation water of this salinity could reduce a farm's crop yield by at least 3%. Monitoring suggests that there may be an increase in the conductivity of North Thompson Creek from Outfall 001 discharges, but that it is not significant. The measured North Thompson Creek C-1981-025 Permit Renewal 05 12 May 2009 28