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COLORADO DEPARTMENT OF HEATH, Wnler Qunliry Dirisiort Rntimmle - Pnge 10. Permit No. CO-0043648 improvement is well documented and has been described in the UAAs presented to the Commnission in support ofsite specific water quality stnrtdar•ds for Arequn Gulch. The Division Itas compared the data at AG 1.0 with the assimilative capacity of Cripple Creek for the parameters listed in Table A-9 of the nssessntent and, with the exception ofar anomalous value for mercury at the 0.2 pg/1 detection limit, the mnximurn measured value for each parameter is less than or signficmnly less that the available assimilative capnciryfor Cripple Creek. This fact, in combination with thestrongpossibiliry that theflow atAG 1.5 used to determine the assimilative capacity in Cripple Creek is dinviraished between AG 1.5 and Cripple Creek, leads the Division to find [hat, with dte exceptimt of mercury, there is no reasonable potential for the Outfall OOIA to cause or contribute to an exceedance ofa water quality standard in Cripple G•eek. In order to assure that the improvement in water quality between AG 1.5 mrd AG 2.0 continues to protect the standards in Cripple Creek, quarterly monitoringwill berequired for total recoverable iron and zinc. The metals will be used as irzdicntorparnrneters since they have maximum values that are closest to the assimilative capacity of Cripple Creek. Monthly monitoring will be required for mercury to assess whether t/tere are concentrations of mercu~ypresent at AG 2.0 that could cause ar contribute to an exceedance ofthe water quality standard for Cripple Creek. i) Aluminum MnnQanese. Zinc, and pH In 1998, CC&V requested site-specific water quality standards for Aregua Gulch based on a UAA. The Water Quality Control Convnission adopted water quality standards for aluminum (recalculation procedure), manganese (ambient WQS), and zinc (ambient WQS) based on the UAA. In 2001, based on a subsequent UAA, the Commission adopted a Temporary Modification of the standard for pH of 5.6 s. u. and a final pH stmrdard of 6.0 s.u with the intention of reviewing thatproposed pHstandard after additional data are collected. In July of 2002, the Cornrnission set n revised site-specific chronic water quality standard for zinc of 600 pg/l based on the 85ib percentile of a five-year record of darn and revised the manganese standard based on the recalculation procedure. Pursuant to 5 CCR 1002-61.8(2)(b)(iv), permit limitations are to be written in a manner that respects the methods by which the water quality standards were derived. The curreru chronic water quality standard for mmrganese and zinc for Arequn Gulclt, as well as the chronic standard for zinc adopted by the Commission at theirJuly 2002 hearing, are based on the 85`n percentile ofdatn that were presented at the 1998 and 2002 hearings, respectively. As noted in 5 CCR 1002- 61.8(2)(b)(iv), "the existence of water quality standards, pnrttcularly where based orr ambient stream data, does not necessarily prohibit at all times discharges that may result in pollutimr ofthe receiving waters in excess ofthe applicable water quality standards. " More specifically, when the permit limitation for n metal parameter equals the water quality standard because the permit applies to all of the flow in the stream, as it does in the case ofArequa Gulch, one may expect the water quality standard, to be exceeded by 15 percent of the darn, if the statistics on which the stnndnrds are based continue to be representative. The Division has analyzed thelatestfiveyearsofdntaforzincatOutfnll001Aandthe85"' percentile ofthnt data is 590 pg/1 which is 98.3% of the 600 pg/1 water quality statdard. Therefore, there is a reasonable potential that the disdmrge front the Sedimentation Pond will cause or contribute to an exceedance of this water quality standard and a limit of600 pg// will be set in thepermit. 77ie limit, with respect to the method by which the water quality standard was derived for adoption by the Commission, will be set based on a "moving 85i° percentile " offive years of darn including the »tortth for which reporting is required. Based orr those same data there is no reasonable potential for the discharge to cause or contribute to mr exceedance of the current 800 pg/l water quality standard for zinc and monitoring using this method will be required until the 600 pg/l limit takes effect on February 1, 2003. Similarly, there is no reasonable potential for the discharge to cause or contribute to nn exceedance of the current acute or chronic water quality stnndnrds for manganese based on the magr:itude of the maximum reported 30-day average curd reported daily maximum values being significantly less than the identified assimilative capacities. These scone darn show that there is reasonable potential far the discharge to cause or contribute to an exceedance of the chronic manganese standard that takes effect on January 20, 2003 and, in recognition of methodology ofthe recalculation procedure used to calculate the standard for manganese, a 30-day average limit equivalent to the water quality standard will be set in the permit. Discharge data for dissolved aluminum have exceeded the acute water quality standard of 11,000 pg/1 and there is reasonable potential for the discharge to cause or contribute to an exceedance of the standard. A daily maximum limit, equivalent to the acute water quality standard, will be set in the permit in recognition of the methodology of the recalculation procedure used to calculate the standards for aluminum. Finally, there is also reasonable potential for the discharge to cause ar contribute to an exceedance of the minimum pH standard as reported data have been nt the level ofthe standard. Since the standard was based on the minimum value in [he three year data set presented to the Commission, a daily minimum limit ofS. tf s. u. will be set in thepennit. Monitoring for these parameters will be maintained at a once per week frequency until consistent compliance at a level reasonably