PERMFILE119809

• Permit No: C1981022A
• IBM Index Class Name: Permit File
• Doc Date: 3/21/2006
• Doc Name: NPDES Permit ‎(CO-000132)‎
• Section_Exhibit Name: Exhibit 2.03-E4 Part 3c
• Media Type: D
• Archive: Yes

COLORADO DEPARTMENT OFPUBLIC HEALTHAND ENG'IRONMENT, Water Quality Control Division
Rationale-Page 6, Permit No. CO-0000132
Outfa11018: Previous evaluations have determined that there is no reasonable potential for a conventional water
treatment plant backwash discharge to contribute to an ezceedence of any water quality standard pollutant other
than total residual chlorine. On this basis, a further reasonable potential analysis for ourfa![ 018 is not necessary.
e. Antidegradation: Since the receiving water is Use Protected an antidegradation review is not required pursuant to
Section 31.8(2)(6) of The Basic Standards and Methodologies for Surface Water.
f. Colorado Mixing Zone Regulations: Pursuant to section 31.10 of The Basic Standards and Methodologies for
Surface Water. a mixing zone determination is required for this permitting action. The Colorado Mixing Zone
Implementation Guidance, dated Apri12002, idenhf:es the process for determining the meaningful limit on the area
impacted by a discharge to surface water where standards may be exceeded (i.e., regulatory mixing zone). This
guidance document provides for certain exclusions from further analysis under the regulation, based on site-specific
conditions.
The guidance document provides a mandatory, stepwise decision-making process for determining if the permit limits
will not be affected by this regulation. Exclusion, based on Extreme Mixing Ratios, may be granted ifthe ratio of
the design flow to the chronic low flow (30E3) is greater than Z:1 or if the ratio ojthe chronic low flow to the design
flow is greater than 20:1. Since the ratio of the design flow to the lowflow is greater than Z.•1, the permittee is
eligiblefor an exclusion from further analysis under the regulation.
g. Salinity Regulations: In compliance with the Colorado River Salinity Standards and the Colorado Discharge
Permit System Regulations, the permittee shall monitor for total dissolved solids on a ouarterly basis. Samples shall
be taken at all authorized effluent discharge points.
h. Whole Effluent Toxicity (WET) Testing: For outfall 01 S, acute WET testing is required. (See Part LA of the
permit.)
i.. Pose of WET Testing: The Water Quality Control Division has established the use of WET resting as a
method for identifying and conh•o1[ing toxic discharges from wastewater treatmentfacilities. WET testing is
being utilized as a means to ensure that there are no discharges afpollutants "in amounts, concentrations or
combinations which are harmful to the beneficial uses or toxic to humans, animals, plants, or aquatic life" as
required by Section 31.11 (1) of the Basic Standards and Methodologies for Surface Waters.
ii. In-Stream Wnste Concentration /IWC): Where monitoring or limitations for WET are deemed appropriate by
the Division, chronic in-stream dilution as represented by the chronic IWC is critical in determining whether
acute or chronic conditions shall apply. According to the Colorado Water Quality Control Division
Biomoniroring Guidance Document, dated July 1, 1993, for those discharges where the chronic IWC is greater
than 9.1 % and the receiving steam has a Class I Aquatic Life use or Class 2 Aquatic Life use with all of the
appropriate aquatic life numeric standards, chronic conditions apply. Where the chronic IWC is less than or
equal to 9.1, or the stream is not classified as described above, acute conditions apply. The chronic IWC is
determined using the following equation:
IWC = [Facility Flow (FF)/(Stream Chronic Low Flow (annual) + FF)J X 100%
The flows and corresponding IWCfor the appropriate discharge point are:
Discharge Point Chronic Low Flow, Facility Design Flow, ~,C (%)
30E3, (cfs) (cfs)
015 0 unknown I00%
The IWCfor this permit is 100% which represents a wastewater concentration of 100% effluent to 0%
receiving stream. However, if discharge occurs, it is expected to be intermittent and very short term (typically,
for less than one day), as was the discharge from previous outfall 011, which originated from the same general
source. Thus, the actual impact of such discharge would be acute. Additionally, because any discharge that
occurs is not expected to last for three days, it would not be possible to collect the three composite samples
required for the chronic WET test. Thus, acute WET testing will be required far outfall 015. If in the future,
discharge occurs chronically, the WET criteria may be reevaluated and chronic WET testing may be imposed.
Such a permit requirement change would be subject to public notice provisions. The petminee will be required
to submit docuntentatiort whenever discharge that is subject to WET testing occurs regarding the duration of the
actual discharge event.
Lnst ReviseA.~ 4/28/2004