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 aquitard 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. Typical 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 K h = 6 x m/day (packer/slug tests) K v = 2.2x10 -4 m/day (regional model calibration) Measurement of K is difficult; field measurement of K v is nearly impossible Although we generally expect K h >K v, 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

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 Lake Mendota Lake Monona 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