HYDRO30334

• Permit No: C1981022
• IBM Index Class Name: Hydrology
• Doc Date: 7/12/1999
• Doc Name: OXBOW MINING INC WORST-CASE WQ EVALUATION
• Permit Index Doc Type: CORRESPONDENCE
• Media Type: D
• Archive: No

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MONTGOMERY WATSON
147 Pine Grove Rd Suite 109
PO Box 774018
Steamboat Springs, Colorado 80477
Tel: [phone]
Fax: [phone]
To: Joseph J. Dudash
Colorado Division of
Minerals & Geology
Date: 7/7/99
JUL 121999
1313 Sherman Street -Room 215
Denver, Colorado 80203
[phone]
Subject: Oxbow Mining, Inc. -
Worst-Case WQ Evaluation
Division of Minerals 8 Geology
Reference: 1342821.0.1210800
Via: Fed X UPS US Mail
Joe -Enclosed are three copies of the mass balance calculations for worst-case water quality
discharge conditions for OMI's proposed 2,000 gpm mine water discharge to the North Fork. As
indicated by the table, worst-case cumulative discharge levels would exceed the DMG's material
damage guidelines for domestic water supply and approach guideline levels for agricultural use
for TDS only. For all of the other parameters of interest, cumulative dischazge levels are below
water quality thresholds and mine discharge would actually tend to reduce naturally high iron
levels in the North Fork.
While these numbers may have some value relative to bracketing the potential for any adverse
water quality impacts, it is important to recognize the following to evaluate these numbers and
put them in the proper perspective:
1) The numbers reflect continuous mine discharge at the maximum 2,000 gpm rate, minimum
historical low flows, and are being compared against guideline levels (DMG Material
Damage Suspect Levels) that are very conservative and below corresponding EPA effluent
levels for similar uses. The probabilities of maximum continuous discharge and minimal
stream flows are both low, so the cumulative probability of these two conditions occurring
concurrently are similarly low
2) If stream flows are at such a low level that discharge from Paonia Reservoir is used to
maintain minimum in-stream flows, there would probably be some natural water quality
degradation (below the average quality values used) due to high TSS levels in the reservoir
and, as a short-term consideration, water availability would probably be more of a concern to
down-stream users than the limited potential changes in water quality indicated by [he worst-
case mass balance calculations