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RULE 2 PERMITS <br />10 gallons per minute (gpm), that they would not be considered aquifers for many water - supply <br />applications. However, these water - yielding units are the principal source of water in the local bedrock <br />formations. The valley -fill aquifers also yield small quantities of groundwater to wells and springs and <br />support base streamflows during the drier parts of the year. The finer- grained bedrock rock units <br />consisting of clay, claystone, shale and siltstone do not yield groundwater, and the thick sequences of <br />those rocks are considered regional confining units. In the Danforth Hills and the Colowyo revised permit <br />area, these rocks typically serve as low- permeability units on which perched groundwater occurs in <br />isolated, discontinuous lenses or which confine groundwater in the more - permeable coal and sandstone <br />beds and limit the interconnection of those more - permeable units. <br />General Groundwater Quality — Groundwater quality in the general area was determined by previous <br />investigations (CDM, 1985a; Dennis, 2001; Colowyo, 1992; Water Management Consultants (WMC), <br />2005). Data from those investigations indicate that the principal water type in the Williams Fork <br />Formation is calcium- or sodium - bicarbonate, containing only minor concentrations of other major ions. <br />The concentration of total dissolved solids (TDS) is moderate, ranging from 440 to 1,000 milligrams per <br />liter (mg/L). The CDM investigation revealed that the water type of the Trout Creek Sandstone ranged <br />from sodium - sulfate, sodium - bicarbonate type, to mixed - cation - bicarbonate with equal percentages of <br />calcium, magnesium, and sodium (CDM, 1985a). TDS concentrations ranged from 600 to 710 mg/L. The <br />water quality of the bedrock did not exhibit significant seasonal variability. <br />The water quality of the valley fill in the general area was also investigated by CDM in 1985 and WMC <br />in 2005. These investigations revealed two distinct trends in water quality: a temporal uniformity in the <br />relative percentages of major cations and anions, and general variability in the water quality from well to <br />well. The water types are magnesium- sulfate and magnesium- and /or calcium- bicarbonate, with moderate <br />to high concentrations of TDS, ranging from 420 to 3,780 mg/L (CDM, 1985a). In contrast to the water <br />quality of the bedrock, the alluvial water quality showed more significant seasonal variation, with TDS <br />concentrations changing in response to seasonal recharge. <br />General Groundwater Use — Groundwater withdrawals in the Lower Yampa River basin counties of <br />Moffat, Rio Blanco and Routt totaled six million gallons per day (mgd) in 2000 (U.S. Geological Survey <br />(USGS), 2004). Groundwater consumption in the area is predominantly associated with irrigation use, <br />with about 65 percent of the groundwater withdrawals (3.9 mgd) used for irrigation. USGS data show <br />public water supply (12 percent, 0.7 mgd) and self - supplied domestic uses (23 percent, 1.4 mgd) as <br />accounting for the remainder of groundwater withdrawals in those counties. However, based on Colorado <br />Division of Water Resources water well data, livestock and mining use appear to account for a portion of <br />the groundwater withdrawals in the area. <br />Detailed Groundwater Information <br />In the Colowyo revised permit area, groundwater occurrence, flow and quality are functions of the <br />stratigraphy and geologic structure. Groundwater monitoring in the general area and the Collom permit <br />expansion area has been conducted since 1983 (CDM, 1985a; Dennis, 2001, 2004). Water Management <br />Consultants (WMC, 2005) performed additional hydrologic studies of the general area around the Collom <br />permit expansion area to establish baseline hydrogeologic conditions. The WMC (2005) report, entitled <br />Collom Project Pre - Feasibility Hydrology Report, is included as Exhibit 7 Item 21. The monitoring <br />program described in the WMC (2005) report was continued through May 2006 to complete the <br />prescribed baseline data collection. Monitoring of water levels, sampling to determine groundwater <br />quality, and aquifer testing conducted during these studies provide a basis for identifying detailed <br />hydrogeologic conditions as described in the following sub - sections. <br />Collom — Rule 2, Page 35 Revision Date: 9/28/11 <br />Revision No.: PR -03 <br />