2018-04-02_PERMIT FILE - C1981019A ‎(7)‎

• Permit No: C1981019A
• IBM Index Class Name: Permit File
• Doc Date: 4/2/2018
• Doc Name: Permits
• Section_Exhibit Name: Volume 15 Rule 2
• Media Type: D
• Archive: Yes

RULE 2 PERMITS
from the bedrock units including and stratigraphically above the Trout Creek Sandstone in the Collom
permit expansion area is to the valley fill aquifers in the northern reaches of the valleys crossing the area.
General Groundwater Quantity — Robson and Stewart (1990) indicate that aquifers in the upper part of
the Mesaverde Group in northwestern Colorado generally produce small sustained yields, about 0-10
gallons per minute (gpm) that they would not be considered aquifers for many water -supply applications.
However, these water -yielding units are the principal source of water in the local bedrock formations. The
valley -fill aquifers also yield small quantities of groundwater to wells and springs and support base
streamflows during the drier parts of the year. The finer -grained bedrock rock units consisting of
mudstone, shale and siltstone do not yield groundwater, and the thick sequences of those rocks are
considered regional confining units. In the Danforth Hills and the Colowyo revised permit area, these
rocks typically serve as low -permeability units on which perched groundwater occurs in isolated,
discontinuous lenses or which confine groundwater in the more -permeable coal and sandstone beds and
limit the interconnection of those more -permeable units.
General Groundwater Quality — Groundwater quality in the general area was determined by previous
investigations (CDM, 1985a; Dennis, 2001; Colowyo, 1992; Water Management Consultants (WMC),
2005). Data from those investigations indicate that the principal water type in the Williams Fork
Formation is calcium- or sodium -bicarbonate, containing only minor concentrations of other major ions.
The concentration of total dissolved solids (TDS) is moderate, ranging from 440 to 1,000 milligrams per
liter (mg/L). The CDM investigation revealed that the water type of the Trout Creek Sandstone ranged
from sodium -sulfate, sodium -bicarbonate type, to mixed -cation -bicarbonate with equal percentages of
calcium, magnesium, and sodium (CDM, 1985a). TDS concentrations ranged from 600 to 710 mg/L. The
water quality of the bedrock did not exhibit significant seasonal variability. This was confirmed by the
sampling of Colowyo Trout Creek Sandstone wells in 1996 and 1997.
The water quality of the valley fill in the general area was also investigated by CDM in 1985, Dennis in
1997 and WMC in 2005. These investigations revealed two distinct trends in water quality: a temporal
uniformity in the relative percentages of major cations and anions, and general variability in the water
quality from well to well. The water types are magnesium -sulfate and magnesium- and/or calcium -
bicarbonate, with moderate to high concentrations of TDS, ranging from 420 to 3,780 mg/L (CDM,
1985a). In contrast to the water quality of the bedrock, the alluvial water quality showed more significant
seasonal variation, with TDS concentrations changing in response to seasonal recharge.
General Groundwater Use — Groundwater withdrawals in the Lower Yampa River basin counties of
Moffat, Rio Blanco and Routt totaled six million gallons per day (mgd) in 2000 (U.S. Geological Survey
(USGS), 2004). Groundwater consumption in the area is predominantly associated with irrigation use,
with about 65 percent of the groundwater withdrawals (3.9 mgd) used for irrigation. USGS data show
public water supply (12 percent, 0.7 mgd) and self -supplied domestic uses (23 percent, 1.4 mgd) as
accounting for the remainder of groundwater withdrawals in those counties. However, based on Colorado
Division of Water Resources water well data, livestock and mining use appear to account for a portion of
the groundwater withdrawals in the area.
Detailed Groundwater Information
In the Colowyo revised permit area, groundwater occurrence, flow and quality are functions of the
stratigraphy and geologic structure. Groundwater monitoring in the general area and the Collom permit
expansion area has been conducted since 1983 (CDM, 1985a; Dennis, 1997, 2001, 2004). Water
Management Consultants (WMC, 2005) performed additional hydrologic studies of the general area
around the Collom permit expansion area to establish baseline hydrogeologic conditions. The WMC
(2005) report, entitled Collom Project Pre -Feasibility Hydrology Report, is included as Exhibit 7 Item 21.
Collom — Rule 2, Page 21 Revision Date: 1/27/16
Revision No.: PR -04