GENERAL33297

• Permit No: M1983194
• IBM Index Class Name: General Documents
• Doc Name: COVER SHEET 3 TEXT CHANGES TO DRAFT EIS
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3. TEXT CHANGES
In a report prepared for WRC, Weston (1985) predicted
a mixing of the upper and lower aquifers due to a high
degree of fracturing from subsidence. Based upon the high
degree of fracturing portrayed in Wesson's report, the
following impacts would be expected. Mixing of the perched
aquifer zone with the confined portion of the upper aquifer
would be considered a significant impact, because the water
quality in the perched zone usually meets state drinking
water standards, whereas the confined portion of the upper
aquifer more often ericeeds these standards. Thus, the perched
aquifer would be lost as a viable groundwater source. The
mixing of the upper aquifer water with the lower aquifer
water would not be considered as significant, because these
zones have similar water quality and, if the vertical hydraulic
gradient remained downward, the lower aquifer yuality
could be marginally improved. However, the rise in water
levels in the lower aquifer at the mine site may steepen
Flow gradients away from the mine, and could result in
more rapid transport of saline minerals from the mined out
cavities. An extensive monitoring system (Appendix C) will
help verify that the actual impacts to the groundwater system
are not as great as predicted in Wesson's report.
Water For production will be supplied from a well in
Township 1 South, Range 98 West, Section 24 for all
alternatives except the 500,000 TPY. Limited data indicates
that the Mahogany Zone has more vertical permeability
00 lease than in other portions of the basin, thus it is of
limited effectiveness as a confining or separation zone
between the upper and lower aquifers (Wright Water
Engineers and Daub & Associates 1985). [f the vertical
permeability across the Mahogany Zone is as predicted by
WRC, then drawdown at the water supply well in Section
24 would induce a small but detectable reverse flow gradient
with lower aquifer water moving upward through the
Mahogany Zone into the upper aquifer. This would be
reFlected in a quality change of the water in the supply
well. Over the life of the project, this would produce a
lowering of the water quality in the upper aquifer in the
area immediately surrounding the water supply well. This
would also hold true for additional water supply wells needed
for production water under the 500,(100 TPY Altemative.
To insure that the hydrologic system reacts similar to
the predicted assumptions and to mitigate against potentially
significant impacts, monitoring of water quality will be
required in the perched aquifer and A-groove of the upper
aquifer, as well as the B-groove and the dissolution surface
of the lower aquifer. All aquifer zones will be monitored
for baseline conditions 6 months prior to well drilling/
solution mining operations and will be continuously
monitored during mining operations. If subsidence effects
and/or increased dissolved solids are detected in the lower
aquifer, the upper aquifer zones may require more frequent
additional monitoring.
If the dedicated groundwater monitors detect increased
dissolved solids above the State of Colorado groundwater
quality standards entering the system, remedial action will
be required (Appendix C). These remedial actions will have
to be designed to remove and/or isolate the brine from
the rest of the aquifer system. A technique that may be
used is the pumping of the brines for surface disposal. The
number of dedicated groundwater monitoring wells needed
will vary by alternative, depending on the size of the well
field. (See discussion below.)
In the event mitigation failed to provide intended results,
the following signifipnt impacts could occur. There would
be increased total dissolved solids entering the lower aquifer
from natural dissolution and diffusion at the well field, and
eventually moving into Piceance and Yellow creeks, and
finally to the White River. Table 4-3A shows estimates
of these increases. The estimates assume that one-half of
groundwater Flow across the mine area goes to each stream
and the TDS concentration is expressed as a range due
to uncertainties concerning natural versus post-mining
dissolution rates (Robson and Saulnier 1981).
TABLE 43A
PREDICTED SALINITY INCREASES (UNMITIGATED)
Alternative TDS increase (mg/0 96 Increase from Average
YELLOW CREEK (2,520 mg/1)
50,000 Less than I to 70 Less than 1 to 2.7
125,000 2 to 160 Less than 1 to 6.5
500,000 6 to 600 Less than I to 25.0
PiCEANCE CREEK (1,800 mg/0
50,000 Less than 1 to 5 Less than I
125,000 Less than I to IS Less than I
500,000 Less than I to 50 Less than 1 to 3
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