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Ralf Topper
<br />Table 1. Hydrogeologic units of the Denver Basin
<br />the deeper parts of the Basin, the aquifers become con-
<br />fined due to the intervening layers of shale and claystone.
<br />The increased withdrawals from the aquifer system, result-
<br />ing from accelerated population growth over the past two
<br />decades, is beginning to convert more of this aquifer sys-
<br />tem from confined to unconfined conditions. As water lev-
<br />els decline, pumping costs will increase and more wells
<br />will be required to extract the same volume of water.
<br />For the Dawson and Denver aquifers, water -level changes
<br />over the five -year period, 1995 -2000, as documented by the
<br />CDWR from select key wells, show both rises and declines
<br />depending upon location. With the housing development
<br />boom of the last decade, lawn irrigation recharge has
<br />become a significant recharge process for parts of these two
<br />aquifers. The picture for the dominant municipal water sup-
<br />ply aquifers, the Arapahoe and Laramie -Fox Hills, however is
<br />not so stagnant. Over the past ten years, water levels have
<br />declined throughout the Arapahoe aquifer. Extensive devel-
<br />opment in the south metropolitan area of northern Douglas
<br />and eastern Arapahoe counties has resulted in declines from
<br />100 to almost 300 ft (VanSlyke, 2000). With declines of up to
<br />30 ft /yr, the future prospects for this aquifer are of great con-
<br />cern to water managers. Likewise, over the past 10 years
<br />water levels in the Laramie -Fox Hills aquifer have also
<br />The Rocky Mountain Association of Geologists 148
<br />Modified from Robson and Banta, 1995
<br />declined, though not at the rates reported for the Arapahoe
<br />(VanSlyke, 2000). The Laramie -Fox Hills aquifer is exten-
<br />sively used for municipal water supply in the southeast Den-
<br />ver metropolitan area, resulting in a substantial area of
<br />water -level decline exceeding 125 ft.
<br />The volume of water released from storage varies signifi-
<br />cantly, depending upon whether the aquifer is under con-
<br />fined or water table conditions. Given the water -level
<br />declines over the past 20+ years, the margins of the con-
<br />fined portions of the aquifers are receding. Calculations of
<br />aquifer storage capacity are conducted under the assump-
<br />tion of no pressure head in the aquifer, i.e. water table con-
<br />ditions. The storage coefficient or specific yield under these
<br />conditions is a function of the porosity and permeability of
<br />the material. Robson (1987) estimated that about 467 mil-
<br />lion acre -ft of water are stored in the Denver Basin aquifer
<br />system. The actual amount of recoverable water is signifi-
<br />cantly less due to physical and practical limitations. In 1985,
<br />as a part of the research done to support Senate Bill 5, the
<br />Colorado Division of Water Resources estimated that
<br />approximately 295 million acre -ft of water was potentially
<br />recoverable (Table 2). The 295 million acre -ft represents
<br />about 1200 times the volume of Dillon Reservoir. This
<br />recoverable reserve represents a theoretical upper limit,
<br />Unit
<br />Saturated
<br />Era
<br />System
<br />Series
<br />Stratigraphic
<br />Thickness
<br />Physical Description
<br />Hydrogeologic
<br />Thickness
<br />Hydrologic Characteristics
<br />Unit
<br />(feet)
<br />Unit
<br />(feet)
<br />Holocene
<br />Unconsolidated gravel,
<br />Alluvial aquifer
<br />0 -100
<br />Unconfined, shallow aquifer; very permeable; yields as
<br />Quaternary
<br />Alluvium
<br />0 -125
<br />sand, silt, and clay
<br />high as 3,000 gpm
<br />Pleistocene
<br />Castle Rock
<br />Fine to coarse arkosic sand-
<br />None
<br />0
<br />Exposed in cliffs; forms cap rock on buttes; well drained,
<br />.
<br />i5
<br />Oligocene
<br />i ocene
<br />g
<br />Conglomerate
<br />0-50
<br />stone and conglomerate
<br />does not yield water
<br />0
<br />Water table aquifer In sheilow units, and confined at depth.
<br />0
<br />Terdary
<br />Eocene
<br />Dawson
<br />Transmissivity ranges from 500 -5,000 gpolft; storage
<br />U
<br />Dawson
<br />0 -1,200
<br />Sandstone and conglomeratic
<br />interbedded
<br />aquifer
<br />0 -400
<br />coefficients range from 0.002 - 0.009; specific yields vary
<br />Paleocene
<br />Formation
<br />sandstone with
<br />from 15 -25 %; domestic water source and municipal supply
<br />siltstone end shale
<br />for Castle Rock; may yield as much as 300 gpm
<br />w
<br />a)
<br />Denver
<br />Shale, silty ciaystone, and
<br />Interbedded sandstone;
<br />Dpmver
<br />0 -350
<br />Water table aquifer near outcrop area; generally confined;
<br />least permeable of Denver Basin aquifers; transmissivity
<br />800 -1,000
<br />beds of lignite and carbons -
<br />w
<br />>.
<br />agtrlfer
<br />ranges from 250 -2,000 gpd1t; storage coefficient 0.002;
<br />Formation
<br />ceous siltstone and shale
<br />ti
<br />specific yield 10 -17 %; domestic and municipal water
<br />common
<br />y
<br />source; yields up to 200 gpm
<br />w
<br />�
<br />Water table aquifer near outcrop area; generally con -
<br />Arapahoe
<br />Sandstone, conglomeratic
<br />Q
<br />"81110e
<br />0
<br />fined; most permeable of Denver Basin aquifers; trans -
<br />Fornation
<br />400-700
<br />sandstone, and interbedded
<br />a
<br />e
<br />-400
<br />missivity ranges from 500 -5,000 gpd(ft; storage coeffl-
<br />10 %; aquifer
<br />shale and siltstone
<br />cient 0.002 -0.009; specific yield -25 principal
<br />Upper
<br />y
<br />m
<br />source for municipal water; yields up to 700 gpm
<br />0
<br />Cretaceous
<br />Cretaceous
<br />N
<br />Upper part shale, silty shale,
<br />Laramie
<br />y
<br />sihstone, and interbedded fine
<br />C
<br />confining
<br />0-400
<br />Shale is impermeable
<br />1:
<br />Laramie
<br />sandstone; bituminous coal
<br />w
<br />unit
<br />Formation
<br />100 -600
<br />seams common
<br />Lower part sandstone and
<br />Water table aquifer near outcrop area; generally confined;
<br />shale
<br />PO x amils
<br />moderately permeable; transmissivity ranges from
<br />0 -250
<br />1,000 -7,000 gpolft; storage coefficient 0.001; specific
<br />Fox Hills
<br />Sandstone and siltstone
<br />aquifer
<br />yield 15 -20 %; source for domestic and municipal water;
<br />Sandstone
<br />100 -200
<br />interbedded with shale
<br />yields up to 350 gpm
<br />Pierre Shale
<br />4,500-
<br />Shale, calcareous, silty, and
<br />Confining unit
<br />0
<br />Impermeable
<br />7 000
<br />dense.
<br />the deeper parts of the Basin, the aquifers become con-
<br />fined due to the intervening layers of shale and claystone.
<br />The increased withdrawals from the aquifer system, result-
<br />ing from accelerated population growth over the past two
<br />decades, is beginning to convert more of this aquifer sys-
<br />tem from confined to unconfined conditions. As water lev-
<br />els decline, pumping costs will increase and more wells
<br />will be required to extract the same volume of water.
<br />For the Dawson and Denver aquifers, water -level changes
<br />over the five -year period, 1995 -2000, as documented by the
<br />CDWR from select key wells, show both rises and declines
<br />depending upon location. With the housing development
<br />boom of the last decade, lawn irrigation recharge has
<br />become a significant recharge process for parts of these two
<br />aquifers. The picture for the dominant municipal water sup-
<br />ply aquifers, the Arapahoe and Laramie -Fox Hills, however is
<br />not so stagnant. Over the past ten years, water levels have
<br />declined throughout the Arapahoe aquifer. Extensive devel-
<br />opment in the south metropolitan area of northern Douglas
<br />and eastern Arapahoe counties has resulted in declines from
<br />100 to almost 300 ft (VanSlyke, 2000). With declines of up to
<br />30 ft /yr, the future prospects for this aquifer are of great con-
<br />cern to water managers. Likewise, over the past 10 years
<br />water levels in the Laramie -Fox Hills aquifer have also
<br />The Rocky Mountain Association of Geologists 148
<br />Modified from Robson and Banta, 1995
<br />declined, though not at the rates reported for the Arapahoe
<br />(VanSlyke, 2000). The Laramie -Fox Hills aquifer is exten-
<br />sively used for municipal water supply in the southeast Den-
<br />ver metropolitan area, resulting in a substantial area of
<br />water -level decline exceeding 125 ft.
<br />The volume of water released from storage varies signifi-
<br />cantly, depending upon whether the aquifer is under con-
<br />fined or water table conditions. Given the water -level
<br />declines over the past 20+ years, the margins of the con-
<br />fined portions of the aquifers are receding. Calculations of
<br />aquifer storage capacity are conducted under the assump-
<br />tion of no pressure head in the aquifer, i.e. water table con-
<br />ditions. The storage coefficient or specific yield under these
<br />conditions is a function of the porosity and permeability of
<br />the material. Robson (1987) estimated that about 467 mil-
<br />lion acre -ft of water are stored in the Denver Basin aquifer
<br />system. The actual amount of recoverable water is signifi-
<br />cantly less due to physical and practical limitations. In 1985,
<br />as a part of the research done to support Senate Bill 5, the
<br />Colorado Division of Water Resources estimated that
<br />approximately 295 million acre -ft of water was potentially
<br />recoverable (Table 2). The 295 million acre -ft represents
<br />about 1200 times the volume of Dillon Reservoir. This
<br />recoverable reserve represents a theoretical upper limit,
<br />
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