Evaluation of a bedrock aquitard for regional- and local-scale
groundwater flow
Kenneth R. Bradbury, Madeline B. Gotkowitz, and David J. Hart
Wisconsin Geological and Natural History Survey,
University of Wisconsin-Extension, Madison, WI, USA
Three-dimensional geologic mapping for groundwater applications
Workshop: Denver, Colorado
10/27/07
Three-dimensional geologic mapping for groundwater applications
Workshop: Denver, Colorado
10/27/07
Demonstrate the importance of a thin aquitard in a regional flow system
Show how regional and local characterizations differ
Purpose of this talk…
Madison
aquifer
aquiferaquitard
Units in the “sandstone” aquifer
Core of Wonewoc sandstone
Core of Eau Claire shale and Mt Simon sandstone
Eau Claire aquitard is not the same as Eau Claire Formation
Here, the aquitard makes up only 10 feet of the 235-ft thick formation.
Here, the aquitard makes up only 10 feet of the 235-ft thick formation.
Typical geologic logTypical geologic log
Eau Claire aquitard
Aquitard presence indicated by gamma signature and head drop
Natural gamma logs provide excellent stratigraphic correlation across the county
Is the aquitard continuous?
Thickness of “shale” facies ranges from 50 meters to absent in Dane County
Hydraulic conductivity
Kh= 6 x 10-5 m/day (packer/slug tests)
Kv = 2.2x10-4 m/day (regional model calibration)
•Measurement of K is difficult; field measurement of Kv is nearly impossible
•Although we generally expect Kh>Kv, we also know that regional values of K are usually greater than local values.
Eau Claire aquitard
Eau Claire is part of a 3D county-wide model
Pumping has caused significant drawdown in the deep sandstone aquifer and steep vertical hydraulic gradients across the aquitard
Drawdowns in feet
Downward flow potential: hatching shows areas where there is a downward hydraulic gradient from the upper to the lower bedrock aquifers
Regional conclusion
Eau Claire is a “good” aquitard, and is a key part of the regional groundwater system
In general, water beneath the aquitard should be quite old (100’s-1000’s of years)
But what about locally?
Profiles of head in the interior of aquitards are critical for understanding and evaluation
Hydrostratigraphy often does not match “classic” rock stratigraphy Hydraulic properties not always apparent from lithology Aquitards are rarely monolithic or uniform
G
G
G
G !
Shallow-marine (<30 feet)?
Low-energy offshore(H O depth >30 feet) 2
Nine Springs
Shorezone?
Lower shoreface (~H O depth <30 feet)
2
Faint discont. laminae of sandy clayey material
Highly bioturbated
Clayey to sandy siltst. w/ thin laminae of fine-grained sst.
Sst. w/ clayey laminae
Clean, white sst.
CementationIndex0 1 2 3 4
310
300
290
280
270
claysiltvf san
df sandm
sandc sand
Eau
Cla
ireF
m.
Mt.
Sim
onF
m.
Wo
new
ocF
m.
Clean yellow-brown sst.
Description Depositional Environment
Packer results – aquitard interval
Aquitard characterization: •The Eau Claire Formation includes intervals of varying thicknesses of shaley sandstone and shale.
•variation results from changes in the regional sedimentary depositional environment
•supports the variable thickness of the aquitard interpreted from geologic logs.
•The aquitard consists of two hydrofacies
•an upper, more conductive, and thicker unit of interbedded sandstone and shale
•a lower, less conductive and thinner shale facies.
Locally, viruses detected in deep groundwater in two city wells
Well #7
Well #24
LakeMendota
LakeMonona
5 km
Madison Municipal Wells Sampled
• Wells 7 and 24 located in dense urban area
Human enteric viruses were detected in two wells cased below the aquitard
Travel time must be less than 2 years because this is the upper time limit for virus survival in groundwater.
Possible Transport Pathways Through Aquitard
1) fractures
2) depositional or erosional stratigraphic windows
3) down cross-connecting open wells or boreholes
4) along damaged, deteriorated, poorly sealed well annulus or breaches in well casings
The exact transport routes from the surface to depth are presently unclear, but calculations show that transport via groundwater flow is possible if the aquitard is fractured
Implications
Describing the three-dimensional spatial distribution and hydrogeologic properties of aquitards at all scales is essential for proper understanding and predictive simulation of groundwater systems.
Aquitards should receive the same attention as aquifers in groundwater investigations
Regional- and local-scale properties may differ
The roles of heterogeneities, fractures, and well construction should not be ignored