GENERAL44216

• Permit No: C1981044
• IBM Index Class Name: General Documents
• Doc Date: 12/11/2001
• Doc Name: Page 33 through OSM Mine Plan Approval
• Permit Index Doc Type: Other Permits
• Media Type: D
• Archive: No

conductivity are elevated levels of chloride and sulfate. No monitoring wells
• are located in the Williams Fork alluvium below the point of No. 5 and No. 6
Mine water discharge. However, as the mine water is pumped directly to the
Williams Fork River, and because no significant increase in chemical
constituents has been detected in the downstream Williams Fork River
monitoring site, there should also be no degradation of water quality in the
Williams Fork alluvium from this mine pumpage.
Another potential impact which was identified during this review was the
potential for seepage of water from the flooded mines of No. 5 and No. 6 to
degrade the surface water alluvial system and river system. Using Darcy's
Law, the applicant has predicted seepage rates from the E and F seam subcrop
into the Williams Fork alluvium. The maximum possible head for the mines in
the subcrop area was predicted at approximately 100 feet above ground surface,
a permeability of 2.5 feet per day, an aquifer thickness of 12 feet, and
subcrop width of 1000 feet. This calculation produces seepage rates for the E
and F seam subcrops of 19.5 gpm and 13.0 gpm respectively. This seepage is
projected to be in direct communication with the alluvium of the Williams
Fork. Other factors included in the loading equation are a recharge from
direct precipitation and irrigation of fresh water of 3 inches per year and
Williams Fork River recharge to the alluvium in the spring based upon an
average water level rise in the alluvium each year of approximately 2 feet.
The net impacts of the seepages were estimated using the balance technique as
shown in the permit application on page 2.05.6-17 Ra. The average water
• quality of the No. 5 Mine discharge was used to load into the equation. The
estimated net effect to the Williams Fork alluvial water was an increase in
the SAR from 1.9 to 5.2. As the applicant states, this would not have a
significant impact on the usefulness of the water for subirrigation. As is
further documented in the section on Alluvial Valley Floors of this document,
the primary function for the Williams Fork alluvial valley is flood
irrigation. This practice will further mitigate any impacts that could occur
from increased sodium content in the subirrigated portion of water available
to the alluvium of the Williams Fork valley.
The combined projected seepage rate from the E and F seam subcrops are
predicted to be .072 cfs. The 7-day/10-year low flow event supplied by the
Colorado Department of Health for the Williams Fork River is 25.9 cfs. The
total predicted seepage rate is less than .3% of this flow. Therefore, no
significant change is levels of chemical constituents in the Williams Fork
River is projected. This dilution would make any impacts to the quality of
the Williams Fork River during low flow undetectable.
In the permit application, the operator has characterized the impact of
discharge from mine water and the No. 1 Strip Pit on the quality of water in
the Williams Fork and Yampa Rivers during a 7-day, 10-year low flow event.
The operator has used the maximum discharge permitted through the NPDES, with
average levels of total dissolved solids and SAR data from mine discharge.
Seasonal fluctuation in quality of the mine water is not significant.
•
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