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 CoNDucTivrry MEASUREMENTS IN THE DENVER BASIN, COLORADO
of fine - grained sediments and the overall geometry of flu-
vial channel- deposited, coarse - grained, water - bearing inter-
vals interlaced with fine - grained over -bank sediments.
Fine - grained sediments tend to dominate the sedimentary
sequence; particularly within the Denver aquifer, where
the net saturated sand thickness is at most 35% of the total
thickness of the unit interval (Robson, 1987). As pointed
out above, this shale- dominance increases to the east,
away from the sediment source area.
Not only do fine - grained sediments tend to dominate
the Basin, but also the lenticular fluvial channel sand bod-
ies are interpreted to be separated by shale, and therefore
are believed to be poorly interconnected hydraulically
(Nielsen, 2001). Figure 3 shows geophysical logs from a
pair of production wells in northern Douglas County that
include nearly all of the Denver aquifer and the entire Ara-
pahoe aquifer. Shale clearly dominates the interval covered
by these two logs. Furthermore, many of the water - bearing
sandstone layers vary considerably in thickness even
though these wells are within 100 ft of each other. The
limited lateral extent of the water - bearing sandstone layers
trapped within relatively impermeable shale that is most
evident in the Denver aquifer portion of the logs; this
geometry suggests limited hydraulic interconnection
throughout this interval.
GROUNDWATER FLOW IN THE DENVER BASIN
AQUIFER SYSTEM
Groundwater flow in the Denver Basin aquifer system
is complex, given the vastness of the aquifers and the
complexity of the sedimentary package. The USGS has
developed a concept of the flow system (Robson, 1987)
that, by necessity, is relatively simple in order to form a
foundation for describing the dynamics of the resource
and to predict how the resource will respond to develop-
ment. The conceptual model also conforms to the layered
aquifer system fixed by statute. In this conceptual model,
groundwater recharges an individual aquifer by infiltration
of precipitation and /or vertical flow from another aquifer.
Groundwater discharges from the aquifer by: 1) outflow to
connected surface water; 2) flow to an underlying aquifer;
or 3) by pumping.
Unfortunately, there are few empirical data that can be
used to describe groundwater flow conditions in the
aquifer system prior to development; however, the USGS
has used 1978 as a time starting point when development
was relatively minor given the vastness of the resource.
Groundwater flow in the four principal aquifers using the
1978 benchmark was believed to have been generally out-
ward from the southern part of the Basin in the vicinity of
the Palmer divide topographic high (Fig. 4). The majority
01MONO
r, �—
Figure 3. Resistivity logs from two production water wells less
than 100 ft apart in T6S, R68W in Douglas County show the great
lateral variability in thickness of many of the individual water -
bearing sandstone layers. The dominance of shale within the
sequence is clearly evident in the Denver aquifer. Provided by
Glenn Graham, Colorado Division of Water Resources.
of flow was directed north toward the South Platte River
(Robson, 1987). A water budget for the principal aquifers
173 The Rocky Mountain Association of Geologists