PERMFILE44465

• Permit No: M1980244
• IBM Index Class Name: Permit File
• Doc Date: 8/15/2002
• Doc Name: Draft NPDES Permit
• From: CDPHE
• To: DMG
• Media Type: D
• Archive: No

Table A-6
Ambient Water Quality for Arequa Gulch AG2.0
eriod of Record =1/1/1996 throw h 5/3/2000
Parameter Number
of
Sam les 15th
Percentile 50th
Percentile 85th
Percentile Min/Mean/Max Chronic
Stream
Standard Notes
As, Dis (ug/l) No data available for this parameter in data set. 100
(trec)
Cd, Dis (u 1) 37 0 0 0.2 0 / 0.216 / 2 6.2
Cu, Dis (u 37 0 0 3 0 / 0.89 / 6 29.28
Cn, Free 37 0 0 0 0/ 0/ 0 0.005
Fe, Trec (u ) 37 0 78 213 0 / 143.5/ 1060 1000
Pb,Dis(u ) 37 0 0 0 0/0/0 10.9
Mn, Dis (u ) 37 0 0 249 0 / 96.5 / 1090 6300
Ni, Dis (u ) 37 0 0 0 0 / 1.48 / 20 168
H ,Trec (u I 37 0 0 0 0 / 0.008 / 0.2 0.01 tot
Se, Dis u 37 0 0 0 0/ 0.62 / 9 4.6
A ,Dis(u 1 37 0 0 0 0/0/0
Zn, Dis (u 1 37 0 0 62.6 0 / 22.24 / 168 800
Nitrite + Nitrate
(m 37 5.2 7.4 10.8 2.63 / 7.97 / 15 NA
NH;, Tot (mg/1) 37 0 0 0 0 / 2.0 / 75 NA
NH3,Unionized
(m 1) No data available for this pazameter in data set. 0.02
III. Water Quantity
Tlme Colorado Regulations specify the use of low flow conditions when establishing water quality based effluent
limitations, specifically the acute and chronic low flows. The acute low flow, referred to as 1E3, represents the
one-day low flow recurring in a three-year interval. The chronic low flow, 30E3, represents the 30-day average
low flow recurring in a three-yeaz interval.
Low Flow Analysis
To best determine the low flows available for dilution in Cripple Creek above Arequa Gulch, a flow gage
measurement inmmediately upstream of the confluence should be used. This data was not available, so the flows
from CC-03, Cripple Creek below Arequa Gulch, were used in this assessment. Even though station CC-03 is
located below Arequa Gulch the low flows are the same above and below Arequa Gulch because the addition of
Arequa Gulch, a zero low flow stream, adds very little flow to Cripple Creek in these critical times. Therefore, use
of 0003 low flows is acceptable.
The low flows were determined from a limited data set, therefore the EPA DFLOW program was not used.
Records of individual flows measurements were sorted by magnitude, and selected the fourth lowest record as the
acute low flow. The chronic low flow was determined from estimates of montlily average flows (developed from
the single day record). The third lowest monthly average flow was selected as the chronic low flow.
The lowest daily flows (.09 and .25 cfs) occurred in July. The critical acute low flow value (.4 cfs) occurred in
both October and November. Also due to the fact that there was very little available flow data for this segment,
low flows were estimated to be the same for every month. This is adequate information because it will not be
necessary to detemtine unionized ammonia linmits, so monthly low flows aze not necessary.
Appendix A Page 8 of 13