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COLORADO DEPARTMENT OF PU6 HE.ILTN AND ENVIRONMENT, Water Qa y Control Division Rationale -Page 9, Permit No. CO-0042161 The Division performed an antidegradation evaluation in 1992. This evaluation determined that a total recoverable limitation of 0.5 mg/P would not resale in degradation. The Division eurremly reviewed this prior evaluation and found that this limitation should be continued. eJ Salinirv Regulations: In rompliance with the Reeulations For Imolemenration of the Colorado River Salinirv Standards 7hroueh the NPDES Permit Proeram. the permittee shall monitorjor total dissolved solids on a quarterly basis. Samp/cs shall be taken at the ejfluem discharge point 001. Salinity requiremerus are included in Part !, Section B.3 of the permit. Bared on the average of the past sampling data srtbmiued, this entity's told dircfiarge does not meet the regulatory limitation of a rnaximurrt of one ton per day (or 350 tour per year]. The permittee mart submit a report which documems whether itrs feasible to treat to these levels. The Salinity Regulations allow for the waiver ojany limitations upon submittal oja report. Report requirements are roreained in Parr I.A.3. of the permit. ~ Whole Effluent Toxicity /WE77 Testine: For orufa/l 001, chronic WET testing is required. (See Parts I.A and LB of the permit.) l) Puroose of WET Testine: The Water Quality Control Division has establrshed the use of WET testing as a method jot idemifying and controlling taric discharges from wastewater treatmem facilities. WET testing is being uriliud as a means to ensure that there are no disdtarges of pollutants 'in amounts, ronantrations or combinations whidr art harmful to the bentfirial user or taric ro humans, animals, plates, or aquatic life' as required by Section 3.1.11 (1) ojtht Basic Standards and Methodoloeies for Surface Waters. 2J Insrrenm Waste Concentration /IWC): Where monitoring or limitations for WET are deemed appropriate by the Division, chronic intercom dilution as repraemed by the chronic IWC is critical in determining whether acute or chronic conditions shall apply. For those discharges when tlu chronic /WC is greater than (>J • 9.1 %, chronic ronditions apply, where the iWC u lens than or equal to (5) 9.1 acute ronditions apply. The chronic JWCu determined raring the joilowing equation: IWC =(Facility Flow (FF)/(Stream Chronic Low Flaw (annual] + FFJJ X !OD% The Jlows and corresponding IWC jot the appropriate disdrarge point are: ,.. ,. ..Discharge :Point': ;: : Clirotuc low: Flow; <;; : ; ::.: Faiality`DsigR'Tlop; ; c j 7iS!C, :(`k):.. r ; ;::: 001 1.4 1.22 47 The IWCfor this permit is 47%, which represents a wastewater roncentration of 47% effluem to 53% receiving stream. 3) Chronic WET Limitations: Bemuse of demonstrated toxttiry in several permittees' roa! mint drainage, the Division believes then is nasonabk poreruial for the discharge to interfere with attainment of applirobk water quality classifications or standards. Because of this condition, the chronic limit has been incorporated into the permit and because the ptrmiaee hat been performing WET testing jot about five years the limit beromes effective immediately. The permittee is required to ronduct routine monitoring. the results of the testing are to be reported on Division approved forms. The permittee veil! be required to conduct a statistical derivation on the data, looking for any statistically signifrcam difference in toxicity between the rontral and the effluent ronantrarions. This set of rolertlations will look at the full range of toxicity (lethality, growth and reproduction). Ija level of chronic toxicity occurs, such that there is a • statrstically sign~cam difference in the lethality (at the 95% ronfrdence level) between the rontrol and any effluent concentration less than or equal to the lnstream Waste Concentration (IWC), the permiate veil! be required to follow the automatic compliance schedule tdentified in Part LB of the permit, ijthe observed tonicity is due to organism lethality. Only exceedanee ojthe limitation specked in Part I.A.I. will trigger the rryuirrment for corutucting the automatic compliance schedule ident~ed in Part LB. of the permit.