2001-12-11_PERMIT FILE - C1980007A (4)

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COLORADO DEPARTMENIOF PUBLlC HE-ILTH,4ND E~WIRONMEM, Water Quality Control Division Amendment ,Vo. 4 - Rariowle. Page 4, Permit No. CO-0038776. • This receiving water is within segment 2. Ibis segment is not G'se Protected, however, the Division has determined that no further review is necessary because no additional impact is expected since this new outfall consists of nvo previortsly reviewed aurfalls and all flow has previatrsly been evaluated jot impact. V. EVAL UATION OF EFFL LENT LL41IT.4TIONS .9ND OTHER CO,VDIT/ONS A. Effluent Limitations and ,4lonitoring Requirements These changes. especially the addition of the netiv ,411YIF with a design capacity of ?, 000 gpm, have the potential to have a substantial impact on the receiving waters. For this reason, it is necessary to perform a more encompassing evaluation of discharges from this facility to the receiving waters. The primary impact is to the ,North Fork of the Gunnison River. The majority of the outfalls discharge to the ,North Fork, the others discharge to tributaries to the North Fork that have chronic low flows of zero. Also, segment 6 has rto total recoverable iron standard and flue mine water outfalls discharging to this segment are permitted upon issuance of this amendment. On this basis, this evaluation will (ook at [he overall potential impacts on the North Fork and is applicable to all outfalls except outfail 012, which will be discussed at the end of this section. Tl:e chronic low flow or 30E3 in the Norr/t Fork is 41 cfs. Many of the discharges are surface runoff flotivs, which are intermittent and occur when stream flow volume is well above the law flow. These intermittent flows cannot be evaluated fully because they are unpredictable but they would not be expected to have an appreciable impact on the receiving water. Far this reason, the focus here tivill be on mare quantifiable flows -the mine water flows. The permittee has stated that they believe the 2, 000 gpm (4.5 cfs) figure [Frey are using for the design of the ,41WTF is representative of the m¢cimum mine water flow from all locations. This appears to be valid based an a review of the three most recent years effluent monitoring data. This value would result in a dilution ratio of 4! cfs to 4.5 cfs, or 9.1:1. This does not indicate a great amount of dilution, so a mass balance equation will be utilized to further evaluate the need for a water quality based total recoverable iron limitation. Zhe mass balance equation is: M, = hl~Qt - M,Qr Q: Where.• Qr = Upstream lorv flow (41 cfs) Q, = Macimrtm discharge flow {4.5 cfs) Qr = Combined downstream flotiv (Qr + Q) hl, = Upstream background pollutant concentration (unknown -assume zero) M. = Un,Cmown; ,llatimum allowable effluent pollutant concentration calculated using mass balance equation hll = ,4laximum downstream allowable pollutant concentration (WQS for TR iron of 1.0 mg/Q) The resulting limitation is 10.1 mg/Q. This value is higher than the federal effluent guideline based limitations of 3.5/7.0 mg/Q that currently apply to discharges from coal mining facilities. On this basis, rto change in the permit limitations is necessary to accommodate a rata! mine water effluent flow from this new treatment facility of 2, 000 gpm. The total iron limitations will remain at 3.x/70 mg/Q for all outfalls except 012. Outfall 012 discharges to :4/innesota Creek. This receiving water is within segment 5. The segment has a total recoverable iron standard oj' !.0 mg/P. The chronic lativ flow (30E3) for ,4finnesota Creek is zero. On this basis. tl:e • total recoverable iron limitation equals the standard of 1.0 mg/D. Because the situation for outfaf! 012 is unique from all other mine water outfalls, the iron limitation is different from the other ourfalls. This results in no change from the current limitation for this outfall.