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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division Rationale - Page 7. permit No. CO-0000221. ~,I. TERMS AND CONDITIONS OF PERMIT Table VI-4 -- Effluent Limits - 008 Parameter Limit Flow, MGD Report a/ TSS, mg/1 35/70 b/ Total Recoverable iron, mg/1 6.0 d/ Total Recoverable Iron, mg/1 1.0 a/ pH, s.u. 6.5-9.0 c/ Oil and Grease, mg/1 10 d/ Total Dissolved Solids, mg/1 Report d/ Whole Eff. Toxicity, Acute Report Whole Eff. Toxicity, Ch ronic Report Table VI-5 -- Effluent Limits - 009, 010 Parameter Limit Rationale Flow, MGD Report a/ TSS, mg/1 35/70 b/ Total Iron, mg/1 3.0/6.0 b/ it and Grease, mg/1 1C d/ settleable Solids, ml/1 0.5 e/ Total Dissolved Solids, mg/1 Report d/ Rationale NA New Source Performance Standards New Source Performance Standards Water Quality Standards Water Quality Standards State Effluent Regulations Salinity Regulations See Discussion, VI.A.S See Discussion, VI.A.S NA New Source Performance Standards New Source Performance Standards Water Quality Standards state Effluent Regulations Eff. Limits for Precip. Events Salinity Reoulati.ons a/ 30 Day Average b/ 30 Day Average/Daily maximum c/ Minimum-Maximum d/ Daily Maximum e/ Effective during periods of precipitation less precipitation event for this facility, subject described in permit. than or equal to the 10-year, 24-hour to burden of proof requirements as f/ The acute toxicity limit is exceeded if (1) a statistically significant difference in mortality (at the 959 confidence level) is observed for either species between the control and the identified IWC, or (2) a species mortality in any dilution of effluent (including 1009 effluent) exceeds 509. 1. Water Ouality Standard Based Effluent Limitations: a) pH: For pH, the limits ace set directly from stream standards. b) Iron: Amass balance equation is usually used to determine the effluent concentrations for those parameters whose Limitations are based on the Water Quality Standards (WQS). However, the mass balance equation relies on a low flow for the receiving waters in the calculation. Since discharge is expected from this facility only in the event of a precipitation event, there is insufficient data to uae a mass balance equation. Therefore, any applicable Water Quality Standards (WQS) are applied directly to the effluent. For this facility, discharge points 004 and 008 discharge to a stream with WQS for Iron, and eo at these two points the wQ5 for Total Recoverable Iron of 1.0 mg/1 ie applied, as a 30 day average. The daily maximum limit for Iron ie based on HAT.