2011-05-23_REVISION - C1981019 ‎(2)‎

• Permit No: C1981019
• IBM Index Class Name: REVISION
• Doc Date: 5/23/2011
• Doc Name: 2nd Adequacy Response Review
• From: Colowyo Coal Company
• To: DRMS
• Type & Sequence: PR3
• Email Name: JHB
• Media Type: D
• Archive: No

Colowyo Response - PR3 adequacy No. 2 23 May 6, 2011
estimated recharge to ground water at 0.2 to 0.35 inches per year, based on a comparison of
precipitation and base flow numbers in Spring, Taylor and Wilson Creeks. Since the majority of
the recharge area for Collom Lite pit will be disturbed land, it is anticipated that the recharge
rate may be slightly higher than for undisturbed lands. Therefore, Colowyo is doubling the
estimated recharge rate to 0.75 inches per year.
(Any higher recharge rate is not supported by the current conditions found in the Danforth
Hills area. The geologic materials of the Williams Fork in this area have a higher
concentration of mudstones and siltstones [clays and silts] versus sandstones than the lower
sequence of the Williams Fork. Also, the depositional environment of the Williams Fork is
different to the east and tends to have more sand units. For example, there is no Twenty Mile
sandstone in the Williams Fork in the Danforth Hills.)
The development of a spoil aquifer above the saturated water table/piezometric surface is
dependent on recharge rate and spoil porosity. The porosity of the spoil is estimated at 35%.
(The porosity of the spoil is determined by its bulk (moist) unit weight, its moisture content, and
the specific gravity of its mineral fraction. The spoil's unit weight was reported as 110 lb/cu ft
in Shannon & Wilson's geotechnical report, July 30, 2009, (Volume 20, Exhibit 23, Item 1).
This value is the swelled (bulked) unit weight, so the percent swell is already included in this
value. For a soil with bulk (moist) unit weight of 110 lb/cu ft, a moisture content of 4% to
15% and a mineral specific gravity of 2.6 (an average value for this material), the porosity
(Vvaids/V ,oral) is between 35% and 41 %).
This equals a rate of filling of 2.15 inches per year or nearly 6 years per foot of rise. With a
height of 125 feet to fill, this equals a total time of approximately 697 years before any
discharge to Little Collom Gulch will occur. Even increasing recharge rates to 1.5 inches per
year, the amount of time to potentially create a spoil aquifer is slightly less than 350 years.
This is assuming there is no drainage of the ground water in the spoil to the pitwall or the high
probability of the downward movement of ground water.
The above scenario has several assumptions or conditions that will affect the recharge of the
pit spoils. By discounting any pit discharge and increasing the recharge rate above that
projected from past studies, the time for pit spoil discharge to occur is actually longer than the
previously calculated amount of time. Therefore the actual time for pit spoil discharge to
occur, if ever, would be in excess of 697 years.
With respect to alluvial valley floors (AVFs), lower portions of Collom Gulch have been
studied before and since the release of the 1985 OSM Alluvial Valley Floor (AVF)
Reconnaissance map. The reconnaissance by OSM was compiled on 1:100, 000-scale maps
and was meant to represent a reconnaissance level effort to identify areas which are likely to
meet the AVF definition (from Introduction to OSM report accompanying this study). Thus,
any area identified on the OSM maps are potential AVFs. It was recognized in this study that
future studies may more conclusively prove or disprove the AVF findings in the report.
Colowyo and other companies in this area performed AVF studies to more conclusively prove
or disprove the existence ofAVFs in this potential coal mining area of the Danforth Hills. For