HYDRO20139

• Permit No: C1980007
• IBM Index Class Name: Hydrology
• Doc Date: 12/12/2005
• Doc Name: Discharge Permit CO-0038776
• From: Colorado Department of Public Health
• To: Mountain Coal Company, LLC
• Permit Index Doc Type: NPDES
• Media Type: D
• Archive: No

Mountan Coat and tieaz l:oal Water lluauty vssessment ~;~-vv~a t io anc ~,v-vvwv~ i ~
Table A-8
Low Flows for Sylvester Gulch, West Sylvester Gulch, the Dry Fork of Minnesota Creek,
and Lone Pine Gulch at the Mountain Coal WWTFs
lE3
Acute
30E3
Chronic
IV. Technical Analysis
In-stream background data and low flows evaluated in Sections II and III aze ultimately used to
determine the assimilative capacity of the North Fork ofthe Gunnison River, Sylvester Gulch, West
"- - Sylvester Gulch, the Dry Fork of Minnesota Creek and Lone Pine Gulch neaz the Mountain Coal and
Beaz Coal W WTFs for pollutants of concern. For all pazameters except ammonia, it is the WQCD's
approach to conduct a technical analysis of stream assimilation capacity using the lowest of the
monthly low flows (referred to as the annual low flow) as calculated in the low flow analysis.
For ammonia, it is the standazd procedure ofthe WQCD to detemrine assimilative capacities for each ~ '
month using the monthly low flows calculated in the low flow analysis, as the regulations allow the ;
use of seasonal flows when establishing assimilative capacities.
The WQCD's standazd analysis consists of steady-state, mass-balance calculations for most
pollutants and modeling for pollutants such as ammonia. The mass-balance equation is used by the
WQCD to calculate the maximum allowable concentration ofpollutants in the effluent, and accounts
for the upstream concentration of a pollutant at the existing quality, critical low flow (minimal
dilution), effluent flow and the water quality standazd. The mass-balance equation is expressed as:
M3Q3 - M1Q1
Mz=
Qz
Where,
Qt =Upstream low flow (lE3 or 30E3)
QZ =Average daily effluent flow (design capacity)
Q3 =Downstream flow (QI + Q2)
M1= In-stream background pollutant concentrations at the existing quality
MZ =Calculated maximum allowable effluent pollutant concentration
Mj =Maximum allowable in-stream pollutant concentration (water quality standazds)
When Q~ equals zero, QZ equals Q3, and the following results:
Mz = Ms
Appendix A Page 11 of 20 Last Revised 3/9/2004