Water Quality Assessment Components for Permit # CO00##### Receiving water Facility Name 1. Multiple Discharger Consideration (From Table 1) Yes, Modeled with XXXXXX WWTF (if applicable) 2. Receiving Water/Segmentation Information (From Table 1) Receiving Stream Name

Segment ID

Designation

Classification(s)

COGUSM04b:

San Miguel River

“Mainstem of the San Miguel River from a point immediately below the CC ditch to a point immediately below the confluence of Naturita Creek.”

Undesignated

Aquatic Life Warm 1 Recreation Class E Agriculture Water Supply

3. WQ Standards (Table A-3) In-stream Standards for Stream Segment COGUSM04b Physical and Biological Dissolved Oxygen (DO) = 5 mg/l, minimum pH = 6.5 - 9 su E. coli chronic = 126 colonies/100 ml Temperature March-Oct. = MWAT = 23.3 ° C and DM = 30.9 ° C Temperature Nov.-Feb. = MWAT = 9.0 ° C and DM = 13.0 ° Inorganic Total Ammonia acute and chronic = TVS Chlorine acute = 0.019 mg/l Chlorine chronic = 0.011 mg/l Free Cyanide acute = 0.005 mg/l Sulfide chronic = 0.002 mg/l Boron chronic = 0.75 mg/l Nitrite acute = 0.5 mg/l Nitrate acute = 10 mg/l Chloride chronic = 250 mg/l Sulfate chronic = For WS, the greater of ambient water quality as of January 1, 2000 or 250 mg/l Metals Dissolved Arsenic acute = 340 µg/l Total Recoverable Arsenic chronic = 0.02 µg/l Temporary Modification: As(ch)=hybrid. Expiration date of 12/31/21 Dissolved Cadmium acute and chronic = TVS Total Recoverable Trivalent Chromium acute = 50 µg/l Dissolved Trivalent Chromium chronic = TVS Dissolved Hexavalent Chromium acute and chronic = TVS

Dissolved Copper acute and chronic = TVS Dissolved Iron chronic = For WS, the greater of ambient water quality as of January 1, 2000, or 300 µg/l Total Recoverable Iron chronic = 1000 µg/l Dissolved Lead acute and chronic = TVS Dissolved Manganese chronic = For WS, the greater of ambient water quality as of January 1, 2000, or 50 µg/l Dissolved Manganese acute and chronic = TVS Total Mercury chronic = 0.01 µg/l Total Recoverable Molybdenum chronic = 160 µg/l Dissolved Nickel acute and chronic = TVS Dissolved Selenium acute and chronic = TVS Dissolved Silver acute and chronic = TVS Dissolved Zinc acute and chronic = TVS 4. Ambient Water Quality Methodology (Station(s), Comparable Watershed) (Copy/Paste from WQA) To conduct an assessment of the ambient water quality upstream of the Nucla Station WWTF, data were gathered primarily from Riverwatch sampling location 100 (San Miguel River at Power Plant Bridge), located almost ½ mile upstream of the facility’s discharge. Data from this location were available for a period of record from January 2004 through September 2009. EC data was gathered from USGS station 09714600. Data were available from March 2009 through October 2004. Data for MWAT and DM were collected by Tri-State Generation and Transmission Association, from the San Miguel River immediately upstream of the facility’s discharge. Temperature data were available from a period of record from October 2010 through October 2011. 5. Hardness Methodology (Copy/Paste from WQA with small changes in wording) Hardness data for the San Miguel River near the point of discharge of the Nucla WWTF were insufficient to conduct a regression analysis based on the low flow. Therefore, the Division’s alternative approach to calculating hardness was used, which involves computing a mean hardness. Sampling data from Riverwatch Station 100, located on the San Miguel River immediately upstream from the Nucla Station were used. (add dates if possible) 6. Low Flow methodology (Station(s), Watershed Comparison) (Copy/Paste from WQA) To determine the low flows available to the Nucla Station, USGS gage station 09174600 (San Miguel River at Brooks Bridge near Nucla, CO) was used. Data were available from 2/11/2002 through 12/2/2012. This flow gage provides a representative measurement of upstream flow because it is located immediately upstream of the Nucla Station’s discharge. However, the Nucla Station water intake, which according to the facility’s flow balance accompanying its permit application draws water from the San Miguel River prior to the discharge location, therefore the water withdrawn must be considered. This information (daily withdrawal) was received from the facility for the same time period and deducted from the daily gage records. From these data, the annual 1E3, 7E3 and 30E3 low flows were calculated using U.S. Environmental Protection Agency (EPA) DFLOW software.