HYDRO24004

• Permit No: C1981022
• IBM Index Class Name: Hydrology
• Doc Date: 3/30/2004
• Doc Name: Public Notice & Draft Permit ‎(CO-0000132)‎
• From: CDHO WQCD
• To: DMG
• Permit Index Doc Type: NPDES
• Media Type: D
• Archive: No

Oxbow Mining W WTF Water Quality Assessment CO-0000132
Table A-4
Low Flows for Sanborn Creek and Elk Creek at the Oxbow Minin ,LLC WWTF
Low Flow Annual Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
(cjs)
lE3 0 0 0 0 0 0 0 0 0 0 0 0 0
Acute
30E3 0 0 0 0 0 0 0 0 0 0 0 0 0
Chronic
N. Technical Analysis
In-stream background data and low flows evaluated in Sections II and III aze ultimately used to
determine the assimilative capacities of Sanbom Creek and Elk Creek neaz the Oxbow Mining, LLC
W WTF 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 (refen•ed to as the annual low flow) as calculated in the low flow analysis. For ammonia, it is
the standard procedure of the WQCD to determine 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 standard 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 (minunal
dilution), effluent flow and the water quality standazd. The mass-balance equation is expressed as:
M3Qs -MiQ~
Mz=
Qz
Where,
Q, =Upstream low flow (lE3 or 30E3)
Q2 =Average daily effluent flow (design capacity)
Q3 =Downstream flow (Q, + Q2)
M~ = In-stream background pollutant concentrations at the existing quality
MZ =Calculated maximum allowable effluent pollutant concentration
M3 =Maximum allowable in-stream pollutant concentration (water quality standazds)
When Q~ equals zero, Qz equals Q3, and the following results:
Mz = Ms
Because the low flow (QI) for Sanbom Creek and Elk Creek is zero, the assimilative capacity of
Sanbom Creek and Elk Creek for the pollutants of concern is equal to the in-stream water quality
standazds.
Appendix A Page 6 of 10 Draft