The Mountain Geologist

• Year: 2004
• Title: The Mountain Geologist
• Author: Rocky Mountain Association of Geologists
• Description: October 2004, Volume 41, Number 4
• Publications - Doc Type: Other

IVER77CAL HYDRAULIC CONDuc77VITY MEASUREMENTS IN THE DENVER BASIN, COLORADO
estimated by the USGS using computer modeling is
summarized in Table 1 and includes estimates for both
horizontal discharge through the four aquifers as well as
interflow between the aquifers. For this paper, the impor-
tant values are the vertical flow estimates, which are signif-
icant volumes ranging between 1350 and 5200 acre- ft /yr.
The USGS also used estimates for vertical hydraulic con-
ductivity in the modeling efforts that resulted in the esti-
mated water budget shown in Table 1 (Robson, 1987).
These estimates of vertical hydraulic conductivity, listed in
Table 2, were arrived at through calibration of the model
wherein aquifer parameters were adjusted until model
results closely matched known conditions of the aquifer
and are reasonable values for the type of geologic materi-
als. Subsequent modeling efforts, such as those used for
the Senate Bill 74 (SB -74) investigations (CWCB, 1996) and
the South Metro Study (Black and Veatch, et al., 2004),
which stems off of the SB -74 efforts, have arrived at equiv-
alent values for vertical hydraulic conductivity through sim-
ilar groundwater modeling.
The USGS model also assumed that there was vertical
connection within the aquifers and that the aquifers would
remain confined for an extended period of time. Recent
rapid water level decline rates in excess of 30 ft /yr that
continue to decline at this rate on the west side of the
basin where the aquifers rise near the Front Range, even
after the water levels drop below the tops of the aquifers,
would suggest that the vertical connection within the
aquifers is less than originally assumed.
This conceptual model treats the Denver Basin aquifers
as a relatively simple seven -layer system based on each
designated aquifer along with the separating confining lay-
ers. In reality, the real -world aquifer system consists of
many more individual layers of varying geometry and with
varying degrees of interconnection as previously described.
It is likely that each individual water - bearing sandstone or
conglomerate layer, in effect, can be treated as an individ-
ual aquifer.
Not only is a scientifically based understanding of verti-
cally hydraulic conductivity within the sedimentary sequence
Table 1.
Model derived water budget for the four main Denver Basin aquifers. Positive numbers indicate flow into the aquifer and negative
numbers indicate flow out of the aquifer. Based on a transient -state 20 -yr groundwater model. Adapted from Robson, 1987.
Table 2.
Model- derived vertical hydraulic conductivity values. A from Robson (1987); B from Barkmann and Edington, 2001).
Vertical
Hydraulic
Source Method Layer Conductivity (cm /sec)
Robson, 1987 Model Derived Laramie confining layer 0
Arapahoe- Denver confining layer 9.17x10-9
Denver - Dawson confining layer 1.23x10 -8
Edington Model Derived Denver Basin confining layers 1.76x10 -7
175 The Rocky Mountain Association of Geologists
Precipitation
Recharge
(acre -feet per year)
Groundwater
Discharge
(acre -feet per year)
Net Inter- aquifer flow
(acre -feet per year)
Dawson
29,400
- 24,200
-5,200
Denver
4,000
-5,350
1,350
Arapahoe
2,050
-5,900
3,850
Laramie -Fox Hills
4,200
-4,200
0
Total
39,650
- 39,650
0
Table 2.
Model- derived vertical hydraulic conductivity values. A from Robson (1987); B from Barkmann and Edington, 2001).
Vertical
Hydraulic
Source Method Layer Conductivity (cm /sec)
Robson, 1987 Model Derived Laramie confining layer 0
Arapahoe- Denver confining layer 9.17x10-9
Denver - Dawson confining layer 1.23x10 -8
Edington Model Derived Denver Basin confining layers 1.76x10 -7
175 The Rocky Mountain Association of Geologists