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CHAPTERFIVE Dirait EIS Reulslons
<br />n.,.:..,, a_:n:.,...,..a .,a,..e,...o.,« ~.,.,,..r.......r...,..,:.,..:..,...,on nn c n r:..,., t:,,...~
<br />~,...,.o..«..,,:,..... ,.ra,.,.~ao rc .,. u z .,,..m ,.t,t,..:ae iuo .....m ...tom .e iron .,. zoa
<br />~~ c .,,.,m ,.ti.,,..,:...., m n~~ ...,,m ., m c .,, t z z .„..n~ .„ o m nc t~e
<br />~:..,. n z z ..,,.m .,.,a .,u i+: v~ e..,.ooaea ~,.t,...,a,. ,.. ,..a,,,.,.a. ~..,..a.,.a~
<br />Mahogany Zone
<br />The Mahogany Zone is a kerogen-rich oil shale zone that separates the Upper Aquifer from the
<br />Lower Aquifer spatially, but not entirely hydrologically. The Mahogany Zone is more resistant
<br />to fracturing than leaner shales and is, therefore, less permeable than the rock units immediately
<br />above and below it (Weeks et al. 1974). The Mahogany Zone ranges in thickness from 100 to
<br />225 feet in the Piceance Creek Basin (Donnell and Blair 1970). The Mahogany Zone is
<br />considered asemi-cotefining layer and it is approximately 180 feet thick at American Soda well
<br />20-1.
<br />The vertical hydraulic conductivity, which controls the vertical exchange of water between the
<br />Upper Aquifer and the Lower Aquifer, varies throughout the rock formation. Estimated values
<br />of vertical hydraulic conductivity of the Mahogany Zone ranged from 2.2 x l Od to 2.2 x 10"' fee
<br />per day directly west of the Piceance Site (Robson and Saulnier 1980). At that site, the hydraulic
<br />head of the Lower Aquifer is
<br />indicating there is a downwazd vertical gradient. There aze insufficient data ;available at the
<br />Piceance Site to evaluate vertical gradients. However, published data indicate that downwazd
<br />vertical gradients probably exist in the upland azeas of the project and upwazd gradients may •
<br />occur near Piceance Creek (Robson and Saulnier 1980).
<br />Based on geologic information from American Soda boreholes (Table 3.4-1), the thickness of the
<br />Lower Aquifer ranges from 342 feet bgs in we1129-2 to the south to 440 feet bgs in well 19-2 in
<br />the northwestern part of the Project Area. In the wells in section 20 at the project site, the
<br />thickness ranges from approximately 367 to 405 feet bgs. The Dissolution Surface (i.e., base of
<br />the Lower Aquifer) is located at an approximate elevation of 4,700 to 4,800 i[eet msl within the
<br />project boundary.
<br />Lower Aquifer water is generally classified as a sodium bicazbonate-chloride: type. TDS
<br />concentrations of the Lower Aquifer range from about 500 mg/1 to greater than 670,000 mg/1 and
<br />aze dependent on depth and location. Data indicate that the salinity of the Lower Aquifer...
<br />Page 3-21
<br />...increases and the water quality deteriorates as the Dissolution Surface is approached (Welder
<br />and Saulnier 1978, Daub 1996).
<br />Water quality information for the Lower Aquifer is available for American Soda wells 20-1 and
<br />20-9 (Table 3.4-2).
<br />TDS measured at the B-Crroove (we1120-9) ranged from 2,995 to 9,616 mg/l, with a mean of
<br />7,330 mg/1. Mean concentrations of chloride (1,310 mg/I), fluoride (14.4 ml;/1), nitrite (1.88
<br />mg/1), chromium (0.18 mg/l), and iron (1.79 mg/1) exceed Colorado groundwater standards.
<br />Standards aze also exceeded for sulfate, azsenic, and manganese in one of tlu~ee samples for each
<br />pazameter.
<br />5-14
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