PERMFILE105530

• Permit No: C1982056
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: Exhibit 44 NPDES Permit No. CO-0042161
• Media Type: D
• Archive: No

Twentymile Coal Company Water Quality Assessment 000042161
Using the equation provided in the beginning of Section IV, the chronic low flows set out in Table
• A-5, the mean ammonia background concentration contained in Table A-4, and the in-stream
standards found in the Colorado Total Maximum Daily Load and Wasteload Allocation Guidance,
assimilative capacities for chronic ammonia were calculated. The data used and the resulting
calculations are contained in Table A-8.
Table A-8
Chronic Assimilative Capacities for Total Ammonia (mg/1)
'Month Q, (cfs) Ql (cfs) Q, (cfs) M, (mg/!) M3 (mg/!) M, (mg/!)
January 3.1 1.2 4.3 0.031 0.7 1.4
February 3.5 ] .2 4.7 0.031 0.6 1.3
Mazch 4.1 1.2 5.3 0.031 0.4 1.7
April 4.4 1.2 5.6 0.031 0.4 1.8
May 2.5 1.2 3.7 0.031 0.3 0.86
June 1.9 1.2 3.1 0.031 0.2 0.47
July 1.3 1.2 2.5 0.031 0.2 0.38
August 1.3 1.2 2.5 0.031 0.2 0.38
September 1.3 1.2 2.5 0.031 0.2 0.38
October 1.3 1.2 2.5 0.031 0.2 0.38
November 1.6 1.2 2.8 0.031 0.3 0.66
December 3.1 1.2 4.3 0.031 0.5 1.7
. V. Antidegradation Review
As set out in the Colorado Regulation No. 31 contained in The Basic S[andards and Methodologies
of Surface Water, Section 31.8(2)(b), an antidegradation analysis is required except in cases where
the receiving water is designated as "Use Protected." Note that "Use Protected" waters aze waters
"that the Commission has determined do not warrant the special protection provided by the
outstanding waters designation or the antidegradation review process" as set out in Section
31.8(2)(b). This antidegradation section of the regulation became effective on December 20, 2000,
and therefore antidegradation considerations are applicable to the current Twentymile Coal Company
permit issuance.
According to the Colorado Regulation No. 33, the Upper Colorado River Basin and North Platte
River (Planning Region 12), Stream Segment 136 is Undesignated and thus an antidegradation
review must be conducted on this segment.
Consistent with current WQCD procedures, the baseline water quality (BWQ) for pollutants of
concern should be established so that the baseline can be used as part of antidegradation reviews.
Calculations of the BWQ for pollutants of concern, specifically total recoverable iron, dissolved
manganese, and ammonia are thus set forth in the paragraphs that follow.
BWQ is defined by the WQCD as the condition of the water quality as of September 30, 2000.
• Furthermore, the WQCD specifies that B WQ will include the influence of the discharger ifit was in
Appendix A Page 10 of 12