Ground-Water Atlas of Colorado Special Publication 53

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, Colorado Geological Survey origin. The complex interfingering of sandstones and shales restricts the movement of ground water in many areas. Additionally, faults and fractures found along the many uplifts in the state, such as the Apishapa Arch, also limit large -scale ground- water flow (Robson and Banta, 1987). Robson and Banta (1987) have constructed a predevelopment potentiometric surface for the Dakota - Cheyenne aquifer in eastern Colorado based on water -level measurements from water wells and drill -stem tests from exploration holes (Figure 6.8 -3). The surface indicates ground -water movement in a general northeast direction. The potentiometric surface ranges in altitude from less than 2,000 feet in the northeast corner of Colorado to more than 5,000 feet in Boulder County, in the area between Colorado Springs and Trinidad, and along the Apishapa Arch northeast of Trinidad. The larger variations in water levels along the recharge areas, on the western margin of the aquifer, are due to the aquifer's complex structural configuration resulting from faulting and folding. Hydraulic gradients range from less than 5 feet per mile northeast of Denver to greater than 400 feet per mile in areas farther west. For the aquifer in eastern Colorado, the majority of recharge, esti- mated at 5.5 cubic feet per second, is attributed to the area between Colorado Springs and Pueblo (Robson and Banta, 1987). Discharge occurs predominantly along the Arkansas River within Pueblo County. The hydraulic conductivity of various units within the Dakota - Cheyenne aquifer ranges from 20 feet per day (ft /d) in the well - sorted sandstones to less than 0.001 ft /d in poorly sorted, clay -rich portions of the aquifer. Average hydraulic conductivity values range from 0.001 ft /d along the western margins of the Denver Basin to 2.0 ft /d in the shallow aquifer zone near the Arkansas River and Apishapa Arch (Figure 6.8 -4) (Robson and Banta, 1987). Vertical conductivity in the aquifer is negligible, ranging from 4 x 10 -7 to 9 x 10 -8 ft /d (Robson and Banta, 1987), and very little flow is thought to occur between the overlying and underlying units. The storage coefficient of the Dakota - Cheyenne aquifer is estimated to range from 1.5 x 10-4 to 4.5 x 10-4 where the sandstone thickness ranges from 100 to 200 feet near the Arkansas River. The thin, low - porosity sandstones along the deep axis of the Denver Basin have a storage coefficient of about 8 x 10 -5. Specific yields determined from pump tests in the unconfined parts of the aquifer generally produce values of 10 percent (Robson and Banta, 1987). DRAFT 105" 104° 103° 4. Rive SEDGWICK i Fo t t L O A N r4 o W E ��L D Co lins g N o a o �� Sterling �\ \` T ��` PHILLIPS E R \ire River Greeley icuth 3 f t i i MORGAN Re ;doe w R i 4 f t \ Y U M A r f i EN 'E t \ ADAMS\ WASHINGTON ¢ve o nver ` i 1 — 2500 i AR iA PA-4�C / Prikaree = Ric e rk s~T LAS E B E R T B x south K I T C A R S O N y "00 1 Z<< Colorado -a. °° Springs ° s { o Q' ` �� L I N 0 L N Y E N N E ` L A \SO K I 0� W Pueblo CROWLE r J J t�C-1 W E R S Sys V P E B L O R'ver rlcansas River mar ° B ' ° O T R O E FA 0 o � ` L A S A h 7 B A C A Trinidad 550 GROUND -WATER ATLAS OF COLORADO EXPLANATION Outcrop of Dakota Sandstone A Outcrop of pre Cretaceous rocks — 2000 -- Potentiometric contour — Shows altitude of standing - water level in wells completed in the Dakota – Cheyenne aquifer; dashed where approximately located; 0` contour interval -500 feet Limit of the Dakota – Cheyenne aquifer in subsurface Fault 39° 38° Figure 6.8 -3. Predevelopment potentiometric surface of the Dakota — Cheyenne aquifer in 37° eastern Colorado. Modified from Robson and Banta, 1987 0 100 Miles SP 53 r�