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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division <br />Rationale - Page 6. Permit No. CO-0000213 ~` <br />DISCUSSIO[d OF PERMIT LIMITATIONS AND CONDITIONS: <br />C. Snowmeelt Greater than 10-year 24-hour Event <br />Section 40 CFR 434.63(d)(2) states that the exemption from TSS, total 9.ron <br />and settleable solids limitations is available only for discharges "caused <br />by precipitation within any 24-hour period greater than the 10-year <br />24-hour precipitation event (or snowmelt of equivalent volume)." Thus for <br />snowmelt, pond inflow volume must be greater than the 10-year, 24-hour <br />event equivalent volume (regardless of effluent volume). Thus, the <br />permittee must be able to submit documentation skiowing that the total <br />volume over a 24-hour period into a pond is greater tha;i the 10-year, <br />24-hour event equivalent volume. Continuous inflow vol;ime monitoring is <br />typically required for this demonstration. The Liivisio;i allows this <br />exemption for an additional 48 hours to allow the: pond to stabilise. <br />D. WQS-Based Parameters Other than pH; Flow <br />Surface runoff discharges (Discharge Points 002, 003, O(14 and 005) from <br />coal mining facilities are intermittent and normally occur during periods <br />when receiving waters are well above low flow. The Colorado Mined Land <br />Reclam:atioa Division and the State Engineers Office typically require . <br />ponds ito be dewatered within 24 to 36 hours after a storm event. For <br />these reasons no WQS-based limitations other than pH are: deemed necessary <br />at thi;a time to protect instream beneficial uses for the surface <br />stormwater discharge points. Also, for the mine water discharge points, <br />the only water quality standard-based limitations are for pH sad total <br />recoverable iron. A comparison of analytical data submitted by the <br />permitt:ee (see data submitted on March 13, 1989) .Eor discharge point 001 <br />directly against the receiving stream standards indicates that the only <br />parameters with a potential to violate stream standards for the receiving <br />stream are Total Recoverable Iron and Total Recoverable ManganQSe. All <br />other parameters are at levels below the stream standard for the San <br />Miguel River, Segment 5. When mass balances are performed for these <br />parameters, the effluent level is well below the resulting limitation. <br />See Appeadia A for the specific calculations. In addition, since there <br />are no WQS-based limitations other than for pH, tkie Division has <br />determined that no flow limitations are necessary for th:Ls permit; <br />however, flow is required to be monitored on a re€;ular bees is. <br />E. Total Suspended Solids <br />The State Effluent Limitation Regulations dictate that w}iere Federal <br />Effluent Guidelines exist they shall take precedence over SES. This is <br />the reason for selection of BPT-based TSS limitations. <br />F. Salinity (Total Dissolved Solids) <br />Regulation 3.10.0, Regulations for Implementation of the Colorado River , <br />Salinity Standards Through the Colorado Discharge Permit Program, addresses <br />the discharge of total dissolved solids (T DS) to the Colorado River Basin. <br />