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Fracture Control of Ground Water Flow and Water - Info Ngwa ...

Fracture Control of Ground Water Flow and

Water Chemistry in a Rock Aquitard

by Timothy T. Eaton 1 , Mary P. Anderson 2 , and Kenneth R. Bradbury 3

Abstract

There are few studies on the hydrogeology of sedimentary rock aquitards although they are important controls

in regional ground water flow systems. We formulate and test a three-dimensional (3D) conceptual model of

ground water flow and hydrochemistry in a fractured sedimentary rock aquitard to show that flow dynamics

within the aquitard are more complex than previously believed. Similar conceptual models, based on regional observations

and recently emerging principles of mechanical stratigraphy in heterogeneous sedimentary rocks, have

previously been applied only to aquifers, but we show that they are potentially applicable to aquitards. The major

elements of this conceptual model, which is based on detailed information from two sites in the Maquoketa Formation

in southeastern Wisconsin, include orders of magnitude contrast between hydraulic diffusivity (K/Ss) of

fractured zones and relatively intact aquitard rock matrix, laterally extensive bedding-plane fracture zones extending

over distances of over 10 km, very low vertical hydraulic conductivity of thick shale-rich intervals of the aquitard,

and a vertical hydraulic head profile controlled by a lateral boundary at the aquitard subcrop, where

numerous surface water bodies dominate the shallow aquifer system. Results from a 3D numerical flow model

based on this conceptual model are consistent with field observations, which did not fit the typical conceptual

model of strictly vertical flow through an aquitard. The 3D flow through an aquitard has implications for predicting

ground water flow and for planning and protecting water supplies.

Introduction

Aquitards are lithologically diverse, and their hydrogeology

is poorly understood because characterization

methods are more challenging than for aquifers (van der

Kamp 2001; Cherry et al. 2006). The relative lack of

studies on aquitards, particularly indurated sedimentary

aquitards, accounts for a common misconception that

aquitards are hydrogeologically homogeneous and flow is

predominantly vertical. Classic aquifer system testing

1 C orresp onding author: Q ueens C ol l eg e— The C ity U niv ersity

of N ew Y ork , S c hool of Earth and Env ironmental S c ienc es, 6 5 - 30

Kissena Boul ev ard, F l ushing , N Y 1136 7 ; ( 7 18 ) 9 9 7 - 3327 ; f ax ( 7 18 )

9 9 7 - 329 9 ; timothy.eaton@ q c .c uny.edu

2 U niv ersity of W isc onsin— Madison, D ep artment of G eol og y

and G eop hysic s, 1215 W . D ayton S t., Madison, W I 5 37 0 6 .

3 W isc onsin G eol og ic al and N atural H istory S urv ey, 38 17

Mineral Point Road, Madison, W I 5 37 0 5 .

Rec eiv ed J ul y 20 0 6 , ac c ep ted Marc h 20 0 7 .

C op yrig ht ª 20 0 7 The Author( s)

J ournal c omp il ation ª 20 0 7 N ational G round W ater Assoc iation.

doi: 10 .1111/ j .17 4 5 - 6 5 8 4 .20 0 7 .0 0 335 .x

methods rely on the vertical propagation of drawdown

through overlying aquitards for estimating aquitard properties

(Neuman and Witherspoon 1972). However, there is

increasing evidence that flow through many aquitards is

much more complex (Remenda 2001; Cherry et al. 2006)

than can be accounted for by simple one-dimensional

(1D) vertical flow models. Indurated rock aquitards, in

particular, appear to be very different from the more commonly

studied shallow glaciolacustrine aquitards because

of the multiple sedimentary layers of differing properties

found in stratified rock sequences.

Detailed studies of rock aquitards are rare. In contrast,

unlithified surficial aquitards, which are generally

more homogeneous than interbedded indurated aquitards,

have been studied more extensively (Cherry et al. 2006).

Surficial weathering in unlithified aquitards causes the

formation of vertical fractures that can become important

preferential flowpaths for contaminant transport and

result in increased bulk hydraulic conductivity (Bradbury

et al. 1985; Keller et al. 1989; Jorgensen and Fredericia

1992; Simpkins and Bradbury 1992; McKay et al. 1993a,

Vol. 45, No. 5—GROUND WATER—September–October 2007 (pages 601–615) 601


1993b; McKay and Fredericia 1995; Jorgensen et al.

2004 ). However, this study and previous work (Cherry

et al. 2006) indicate that such a conceptual model does

not always apply to rock aquitards.

Recent work (described subsequently) has emphasized

that ground water flow through sedimentary rock is

controlled by fractures that are themselves constrained by

mechanical stratigraphic properties of sedimentary layers.

O bservational evidence for preferential flow along subhorizontal

bedding-plane fractures has been used to construct

conceptual and numerical models for wellhead

protection (Muldoon et al. 2001; Rayne et al. 2001), as

well as understanding contaminant transport and ground

water flow (P effer 1991; Michalski and Britton 1997;

Maguire 1998) and the origin of springflow (Swanson

et al. 2006). E xtensive study of the P aleozoic sedimentary

sequence in Minnesota has caused a complete revision

of the regional hydrostratigraphy (Runkel et al. 2003;

Tipping et al. 2006), resulting in delineation of new hydrostratigraphic

units based on fracturing. These studies

show that in relatively undeformed sedimentary rock,

fracturing associated with bedding planes or changes in

lithology can be correlated laterally over scales of kilometers

to tens of kilometers.

Furthermore, the following studies explain why, in

contrast to the conceptual model of vertical fractures in

unlithified materials, the lithologic heterogeneity in rock

aquifers and aquitards favors the formation of beddingplane

fractures. Fracture propagation is inhibited across

the interfaces created by sedimentary layers with contrasting

lithologies and mechanical stiffness (Helgeson

and Aydin 1991; Rijken and Cooke 2001; U nderwood

et al. 2003; Cooke et al. 2006; G raham Wall 2006). V ertical

fractures in indurated sedimentary rock tend to be

isolated within individual layers and their frequency

dependent on layer thicknesses (P ollard and Aydin 1988;

Narr and Suppe 1991; G ross et al. 1995; Renshaw 1997).

Therefore, in thick rock aquitards that contain numerous

shale and siltstone beds, few if any vertical fractures will

propagate across all the layer interfaces. In the absence

of continuous vertical flowpaths, these aquitards can be

very effective confining units (E aton and Bradbury

2003).

In addition to our study of the Maquoketa aquitard in

Wisconsin (E aton 2002), we are aware of only one other

major rock aquitard in the U nited States, the P ierre Shale,

that has been characterized extensively (Neuzil 1980, 1986,

1993; Bredehoeft et al. 1983), although hydrogeologic

properties of low-conductivity fractured chalk have been

characterized for geotechnical purposes (Dutton et al.

1994 ; Wang and Myer 1994 ). In Switzerland, the O palinus

Clay, a fractured, diffusion-dominated shale, is being

studied for possible nuclear waste disposal sites (Mazurek

et al. 1998; G autschi 2001; Croise et al. 2004 ; Wersin et al.

2004 ). P ore water chemistry and transport processes also

have been studied in the Tournemire tunnel argillites in

France (Boisson et al. 2001; P atriarche et al. 2004 ).

New conceptual models are needed to explain complex

flow in stratified rock aquitards, particularly in

regionally important aquitards such as the Maquoketa

Formation. In this paper, we use detailed data collected

at two field sites 10 km apart (Figure 1) to describe a

conceptual model for the O rdovician-age Maquoketa

shale aquitard in southeastern Wisconsin, emphasizing

the importance of bedding-plane fractures in lithologically

complex rock aquitards. The work presented here extends

and amplifies the findings of E aton and Bradbury (2003),

who showed that the vertical distribution of hydraulic

head in the Maquoketa aquitard is currently not at equilibrium

in response to drawdown in the underlying aquifer

system. An early, widely cited study of the Maquoketa

aquitard (Walton 1960) used a steady-state analysis of

flow between the overlying and underlying aquifer systems

to estimate rates of leakage through the aquitard.

Similarly, the selection of parameters assigned to the

Maquoketa aquitard for recent regional simulation studies

relies on an equilibrium assumption (Y oung 1976, 1992;

Feinstein et al. 2004 ). Although Hart et al. (2006)

recently used one of these regional models to investigate

cross-connecting well and fracture hypotheses about

Figure 1. Generalized bedrock geology near field sites (inset) at DOT and Minooka Park, showing proximity to western

Maquoketa subcrop (shown in black shading). The Paleozoic bedrock sequence dips gently eastward into the Michigan

Basin. Diamond shape around DOT site is the boundary of the flow model in Figure 8.

602 T.T. Eaton et al. GROUND WATER 45, n o. 5: 601–615


egional properties of the Maquoketa aquitard, our work

suggests that significant deviations from steady-state conditions

occur within the aquitard at the tens of kilometers

scale of our study.

In order to understand hydraulic properties and flow

through regional aquitards, data are needed at multiple

scales. L arge-scale simulations can be used to estimate

regional hydraulic properties (Bredehoeft et al. 1983;

Hart et al. 2006) but may not have the resolution to incorporate

features that control flow. Data from field studies

can also provide insight into hydrologic conditions such

as whether hydraulic heads have equilibrated to pumping

stresses (Husain et al. 1998; E aton and Bradbury 2003). If

these hydrologic conditions are different from previous

assumptions, alternative conceptual models are needed

and can lead to new interpretations of aquitard properties.

The more local scale of field observations avoids the

uncertainty that anthropogenic factors such as multiaquifer

wells can introduce in parameterization of large

regional simulations.

We evaluate a three-dimensional (3D) conceptual

model (Figure 2) developed for flow through the Maquoketa

aquitard to show how detailed hydrogeologic and hydrogeochemical

data point to a complex internal flow

system at the site scale. While simple 1D vertical flow

models do not explain these data, the conceptual model

we propose provides a basis for numerical modeling and

understanding of vertical hydraulic head distribution

within the aquitard. As more field data are collected on

other sedimentary rock aquitards, this model can be further

tested and refined in other similar settings.

H ydrog eolog ic S etting

The Cambrian-O rdovician Aquifer underlying the

Maquoketa Formation is heavily pumped for municipal

supply in the rapidly growing inland suburban area near

Milwaukee, Wisconsin (Figure 1). As a result, hydraulic

head in the aquifer system has been drawn down considerably

below land surface since the early 1900s (Fetter

1981), forming a regional cone of depression extending

south into Illinois (Feinstein et al. 2004 ). O ver the past

50 years, the rate of regional head decline averages more

Figure 2. C onceptual geological model of conditions within

10 km of the subcrop of a rock aquitard based on the

Maquoketa Formation. I n this hydrostratigraphic setting,

subcrops of different units at the bedrock surface are ov erlain

by a shallow unlithified aquifer. The aquitard is shaded,

with laterally extensiv e bedding- plane fracture zones shown

by lines. V ertical scale is greatly exaggerated.

than 2 m/year, and total head loss exceeded 130 m over

the last century. The Maquoketa Formation constituting

the aquitard is 50 to 60 m thick and dips gently eastward

into the Michigan Basin. It overlies the 600-m-thick sandstone

and dolomite aquifer system and isolates it from

a shallower Silurian dolostone aquifer and unlithified

P leistocene sediments near the land surface. The Maquoketa

Formation in southeastern Wisconsin, described

from rock core (E aton 2002), consists of numerous lithofacies

ranging from a fissile greenish-gray dolomitic

shale at the base to multiple interbeds of mudstone and

more resistant crystalline dolomitic packstone and grainstone

in the upper part of the formation (Figure 3). As

with all the bedrock formations, it is traversed by

a regional fault zone, the Waukesha Fault (Figure 1),

whose hydraulic properties are unknown.

S tudy Ap p roach and M ethods

The conceptual model proposed in this paper is

based on a large and detailed data set from two field sites

10 km apart (Figure 1) within the regional cone of

depression produced from municipal pumping in the

underlying aquifer. It consists of, to our knowledge, the

first hydraulic head profiles with depth within the aquitard,

data from single- and multiple-well hydraulic testing

and laboratory core tests, as well as hydrogeochemical

data from water samples. More details of the two field

sites and the methods of downhole geophysical logging

and single-well hydraulic testing of multiple boreholes

are given by E aton (2002) and E aton and Bradbury

(2003).

From the analysis of these data, we develop the

hypothesis of a 3D conceptual model for the hydrogeology

of rock aquitards and describe the construction of

a site-scale numerical flow model to test this hypothesis.

Finally, we compare simulated hydraulic heads to measured

hydraulic heads with depth in and below the

Maquoketa aquitard and conclude with some implications

for regional hydrogeology.

H ydrog eolog ic D ata Collection

In this study, due to the depth (60 m) of the Maquoketa

aquitard below land surface, hydrogeologic characterization

was limited to borehole studies. At both study

sites (Figure 1), rock-core (Figure 3) and downhole geophysical

logs (Figure 4 ) were collected from boreholes

through the aquitard. L aboratory hydraulic testing of

Maquoketa rock core (E aton et al. 2000) is described

in detail by Hart et al. (2006). P neumatic packers and

multilevel monitoring systems to measure hydraulic head

with depth were then installed in two boreholes, one at

each site, which were also used for ground water sampling.

Horizontal hydraulic conductivity (Figure 4 ) was

estimated from single-well hydraulic testing at both locations.

The multilevel monitoring systems were constructed,

to the extent possible, to isolate fracture zones in

different vertical intervals. After initial stabilization over

several months, hydraulic head measurements in these

and another type of multilevel monitoring system (E aton

T.T. Eaton et al. GROUND WATER 45, n o. 5: 601–615 603


Figure 3 . C orrelation of six lithofacies recognized in the Maquoketa Formation based on rock- core analysis from (A )

Minooka Park and (B) DOT field sites. The S innipee Group dolostone ov erlies the most productiv e sandstone formations in the

C ambrian- Ordov ician A quifer system.

Figure 4 . S ingle- well hydrogeological data at (A ) Minooka Park and (B) DOT field sites (note the formation elev ation differences):

(at left) representativ e downhole geophysical logs from selected wells with dashed lines showing maj or bedding- plane

fracture zones and triangles indicating no measured flow; (at right) results from slug testing and straddle- packer testing in the

same wells. Bar thickness indicates interv al length tested, gray bars indicate the use of a submersible pump, and triangles indicate

v alues below testing limit (E aton and Bradbury 20 0 3 ).

604 T.T. Eaton et al. GROUND WATER 45, n o. 5: 601–615


2002; E aton and Bradbury 2003) responded to various

pumping tests, then equilibrated to similar long-term values

within the aquitard (Figure 5). Head values in the bottommost

monitoring intervals below the Maquoketa Formation

continued to exhibit a slow decline over time, and the

unusual vertical profile has remained essentially the same

up to the present (K.R. Bradbury, personal communication).

At the Department of Transportation (DO T) field site

(Figure 1), two additional wells of differing construction

(bentonite sealed with transducers embedded in sandpacks—

see E aton and Bradbury [ 2003] for details) were installed

to verify the observed hydraulic head profile with depth,

and a third deep well open to the Maquoketa Formation

was then constructed for hydrogeologic testing. The addition

of this well did not change the stable hydraulic head

distribution with depth, attributed to the small crosssectional

well area (0.018 m 2 ) and the anastomosing

nature of fracture pathways (Tsang and Neretnieks 1998).

A downhole video log indicated that either single large

fractures up to 0.001-m aperture or clusters of hairline

fractures correspond to each of the three zones identified

at the two sites (E aton 2002). Subsequent digital borehole

imaging also showed multiple bedding-parallel fractures.

Bedding fracture zone transmissivity and storage coefficient

were estimated from two pumping tests in this well

that caused rapid head changes in fractured intervals of

multilevel observation wells at distances of 15 to 25 m,

indicating interconnected transmissive flowpaths. Head

drawdown and recovery were analyzed using standard

time– drawdown methods (Cooper and Jacob 194 6; Jacob

1963), as described in E aton (2002). A constant aperture

value, b, of 0.001 m was used to obtain estimates of interwell

fracture hydraulic conductivity from the pumping

test results (Figure 6). More sophisticated methods of analyzing

effective fracture apertures (Halihan et al. 1999)

indicate that the selected value is a conservative estimate

of fracture aperture based on the range of assumed hydraulic

conductivity.

V alues from field and laboratory hydrogeologic testing

(Figure 6) were used to assign parameters for the

numerical model. With the enormous contrast in hydraulic

properties between fractures and rock matrix in the

shale, it is assumed that field transmissivity and storage

coefficient values estimated from interwell testing are

primarily representative of the fracture network rather

than the rock matrix. Complex flowpaths (Tsang and

Neretnieks 1998) are likely through such fracture systems,

similar to those that have been studied in detail in

carbonate outcrop settings elsewhere (Muldoon et al.

2001; Cooke et al. 2006; G raham Wall 2006; Tipping

et al. 2006).

H ydrog eochemical D ata Collection

Anticipating primarily slow vertical fluxes and very

low hydraulic conductivity within the Maquoketa aquitard,

we designed monitoring wells and sampling for

analysis of vertical hydrogeochemical trends. However,

the significant heterogeneity observed during field hydrogeologic

testing complicates interpretation of hydrogeochemical

data.

Figure 5. H ydraulic head in multilev el wells at (A ) Minooka Park site and (B) DOT site. Outermost plots in both (A ) and (B)

prov ide detail of the same data at an expanded horizontal scale. Open circles represent data from longer open interv als in

W aterloo TM systems, while solid circles represent data from embedded pressure transducers in different sealed wells. S mall

arrows at the locations of packers (gray) or bentonite seals (black) show flow directions interpreted from head measurements.

T.T. Eaton et al. GROUND WATER 45, n o. 5: 601–615 605


Figure 6 . R ange in hydraulic conductiv ity (K) estimated

from rock- core testing, slug and straddle- packer testing, and

interwell pumping. A fracture aperture b ¼ 0 .0 0 1 m was

used to calculate K from transmissiv ity for interwell pumping.

N ote that many single- well field results were less than

10 25 m/ d, below the assumed equipment testing limit.

Samples were collected from each of the intervals in

the multilevel packer systems within the Maquoketa aquitard

at the two field sites (Figure 7). Due to low sampling

flow rates and depths of the sampled intervals, only limited

purging was possible. Measurements of electrical

conductivity, pH, dissolved oxygen, redox potential (E h),

and temperature were taken in the field prior to filtering

through 0.4 5-lm membrane filters. Samples were acidified

in the field and collected in polyethylene containers

and preserved on ice for delivery to the laboratory (Wisconsin

State L aboratory of Hygiene and a commercial

lab), where they were analyzed for major ions using

inductively coupled plasma mass spectroscopy and other

standard methods. Saturation indexes of mineral phases

and pCO 2 for major ion analyses were calculated using

the geochemical speciation program P HRE E Q C (P arkhurst

1995) using the thermodynamic data provided. Charge

balance errors in all samples were less than 10% .

N umerical M odeling

A 3D, transient numerical ground water flow model

was constructed to test the new conceptual model (Figure 2)

and specifically to evaluate the hydrogeologic role of

laterally extensive bedding-plane fracture zones in the

aquitard. The MO DFL O W96 (McDonald and Harbaugh

1996) code was used to represent the flow system. E quivalent

porous medium models have been used successfully

in sedimentary fractured rock settings (Y ager 1997;

Rovey 1998; Rayne et al. 2001; Swanson and Bahr 2004 ;

Swanson et al. 2006) and even in crystalline fractured

rock settings (Shapiro and Hsieh 1998; Tiedeman and

Hsieh 2001) by previous researchers. As in other modeling

applications, important differences exist between

606 T.T. Eaton et al. GROUND WATER 45, n o. 5: 601–615

centimeter- or meter-scale model fracture zone layers and

the sets of discrete submillimeter-scale features they represent.

Therefore, parameter values often need to be

adjusted from field values during the calibration process.

The model was designed to represent a vertical parallelepiped

through the aquitard centered on the DO T field

site (Figure 8) within the regional cone of depression in

the deep aquifer system. The principal output of interest

is the vertical distribution of head within and below

the Maquoketa aquitard. Therefore, 30 layers were used

allowing delineation of head at a high vertical resolution

(Figure 8). Mapped regional hydrostratigraphic data

(E aton et al. 1999) were interpolated to layer elevations

across the model domain. Specific layers of uniform

thickness (0.33 m) at certain depths were used to represent

major fractured zones in the Silurian Aquifer and

Maquoketa aquitard (Figure 4 ), but other layer thicknesses

varied across the model domain. At the subcrop at

the western corner of the model, all layer thicknesses

within the Silurian Aquifer and upper Maquoketa aquitard

were reduced to 0.33 m where the bedrock is erosionally

truncated. A variable column and row spacing was

used, from 900 m at the periphery to 1 m at the central

area of interest. The major elements of surface hydrography

(Figure 8) are represented by constant-head and

head-dependent nodes in MO DFL O W’ s River P ackage.

Recharge is estimated to range between 0 and 100 mm/

year over the model domain based on precipitation-runoff

modeling of stream low-flow discharge data in southeastern

Wisconsin (Cherkauer 2004 ). A uniform recharge rate

of 8 3 10 25 m/d (29 mm/year) best reproduced observed

head and flux values in the shallow aquifer system.

L ateral boundary conditions were selected to represent

the hydrogeologic conditions at the western Maquoketa

subcrop and correspond to the regional drawdown

on the other sides. L ateral boundary conditions in layers

2 to 8 were set at constant heads of generally higher

values than those of layer 1, consistent with the local

recharge-discharge conditions in the water table aquifer

in the unlithified sediments. At the western corner of the

model domain, where the Maquoketa subcrop is in contact

with the shallow aquifer system (Figures 2 and 8),

constant-head boundaries were specified for layers 9 to

24 , at 260 m above mean sea level (msl), the elevation

of P ewaukee L ake (Figure 8). Head-dependent (G eneral

Head Boundary [ G HB] ) boundaries were used for all

other edges and the bottom of the model, to simulate transient

decreasing hydraulic heads caused by regional drawdown

in the Cambrian-O rdovician Aquifer system.

Hydraulic conductivities were based on field and laboratory

data for the Maquoketa aquitard (Figure 6) and

unpublished and published data for the other formations

(E aton 2002). Hydraulic conductivity zones were delineated

in layer 1 representing the lateral and vertical variation

in unlithified P leistocene materials (Clayton 2001),

but hydraulic conductivity for layers 2 to 30 was specified

as constant and isotropic across the model domain. Two

classes of hydraulic conductivity and storage values

(Table 1), reflecting the vastly different properties of

fractured zones and relatively unfractured rock matrix

(Figures 4 and 6), were specified for both the Silurian


Figure 7 . Field parameters and maj or ion concentrations at different lev els through the Maquoketa Formation at

(A ) the Minooka Park site and (B) the DOT site. C ircle data symbols correspond to lower axis, while triangle symbols correspond

to upper axis. S haded area represents thickness of the Maquoketa Formation. The central column is a schematic

diagram of packers (black) and sampled open interv als (white). The approximate elev ation of bedding- plane fractures is

shown with asterisks.

Aquifer and the Maquoketa aquitard. During initial model

runs, hydraulic conductivity for fracture zone layers was

reduced from 100 to 1 m/d to avoid rapid lateral drainage

and dry cells. Hydraulic conductivity of relatively unfractured

Maquoketa model layers was reduced from 1 3

10 28 to 1 3 10 29 m/d, and hydraulic conductivity for the

Sinnipee G roup model layers was increased to 1.5 3

10 27 m/d in order to simulate the observed head profile.

V alues of specific storage, Ss, calculated from laboratory

rock-core testing (E aton et al. 2000; Hart et al. 2006)

ranged between 1 3 10 28 and 7 3 10 26 /m, so the initial

value of storage coefficient, S, used for unfractured model

layers of thickness 4 to 5 m was 1 3 10 26 , later adjusted

during calibration to 1 3 10 25 . In contrast to dual

overlapping continuum or double-porosity fracture models,

the thin fracture layers here do not represent entire

fractured rock systems in which storage is relatively low

because it depends on the extremely small ratio of fissure

volume to bulk rock volume (Moench 1984 ). In this

study, model layers representing fractured rock zones

were assigned a storage coefficient of 1 3 10 23 , consistent

with field hydraulic testing values as reported later,

and higher than values in layers representing relatively

unfractured rock (Table 1).

The model was used to simulate steady-state predevelopment

conditions (circa 1900) when hydraulic head

was considerably higher (Weidman and Schultz 1915) in

the underlying Cambrian-O rdovician Aquifer system,

T.T. Eaton et al. GROUND WATER 45, n o. 5: 601–615 607


Figure 8. The 3 D flow model showing (A ) a plan v iew of layer 1 with local hydrographic features forming boundary conditions

around DOT field site. C ontours represent mapped water table elev ations (S E W R PC / W GN H S 20 0 2), and arrows indicate

locations of cross sections; (B) cross sections showing relativ e elev ations and thicknesses of layers representing different

formations. The bottom and lateral boundary conditions in lower layers are head dependent except for the western corner of

the model where there are constant heads in the upper 24 layers as described in the text. Fracture zone layers are not apparent

at this scale.

inducing an assumed upward gradient across the Maquoketa

Formation. For all steady-state and subsequent transient

simulations, the P reconditioned Conjugate G radient

(P CG 2) solver (Hill 1990) was used. For model calibration,

we used eight head targets in the water table aquifer

in the Silurian and glacial aquifers and three flux targets

from observed streamflow, in addition to head measured

in and below the Maquoketa aquitard. These were used to

identify standard deviation and 90% confidence intervals

following Hill (1998).

All but one of the simulated water table head values

and all the flux values fell within the target confidence

intervals for both transient and steady-state runs and were

used in combination with the measured vertical head

distribution in the field to assess model calibration.

Monitoring well open intervals correspond to multiple

model layers, so transmissivity-weighted arithmetic mean

head calculations (Sokol 1963; Bennett et al. 1982) were

needed for calibration purposes. These mean simulated

hydraulic head values were compared to composite field

hydraulic head values measured in open intervals

608 T.T. Eaton et al. GROUND WATER 45, n o. 5: 601–615

spanning more than one model layer thickness. The root

mean square (RMS) error criterion was used for both flux

and head calibration. Head RMS error (5.791 m) was less

than 10% of the simulated total head gradient and flux

RMS error (176.1 m 3 /d) was less than 5% of the maximum

simulated flux. Model results are presented here as

head profiles at the location of the DO T field site, for

comparison to measured hydraulic head data (Figure 5).

More detailed model analyses, including comparison to

models without fracture zone layers and two-dimensional

models with no-flow lateral boundaries, showed that the

fully 3D model presented here best matched the field data

(E aton 2002).

Results and D iscussion

Field D ata

Transmissivity values of fractures within the Maquoketa

aquitard calculated from five analyses of drawdown

or recovery in observation wells during the two interwell


Table 1

Parameter A ssignments for N umerical Model S howing S ignificant C ontrast in Transmissiv ity (Bold)

between Fractures and Matrix

L ayer Type Thickness b (m) S torage 1 Kx ¼ Ky (m/ d) Kz (m/ d) T ¼ Kxb (m 2 / d)

U nlithified V arious 0.3 0.2– 30 2E -6– 0.03 V arious

Silurian matrix 9– 13 1 E -5 1E -3 1E -3 2 1E - 2

Silurian fracture 0.33 1E -3 1 1 0 .3 3

Transition 3 0.33 1E -3 0.1333 0.1333 0.0399

Maquoketa matrix 4 – 5 1E -5 1E -9 1E -9 2 4 E - 9

Maquoketa fracture 0.33 1E -3 1 1 0 .3 3

Sinnipee V arious 1E -5 1.5E -7 1.5E -7 V arious

St. P eter V arious 1E -6 1 1 V arious

1 Specific yield in the unconfined, unlithified layer 1, storage coefficient in all other layers.

2 E xcept where these formations are represented by thin layers at the subcrop, and vertical hydraulic conductivity is 1 m/d.

3 A transition layer with intermediate values of hydraulic conductivity was required for numerical convergence.

pumping tests were within a range of 0.7 to 5 m 2 /d, with

storage coefficient values between 1 3 10 25 and 6 3

10 23 (E aton 2002). The overall range of estimated

hydraulic conductivity exceeds 11 orders of magnitude,

from less than 1 3 10 28 m/d to greater than 1 3 10 13 m/d,

assuming a fracture aperture of 0.001 m (Figure 6).

Hydraulic conductivity of rock core is generally limited

to between 1 3 10 29 and 1 3 10 27 m/d. Horizontal

hydraulic conductivity of the Maquoketa aquitard from

single-well hydrogeologic testing ranged over 5 orders of

magnitude, from 3 3 10 25 to 3 m/d (Figure 6). Many

hydraulic conductivity testing results were below the limit

of the field equipment (estimated at 2 3 10 25 m/d) and

are not included on Figure 6.

Based on detailed lithology and downhole geophysical

logging data (E aton 2002; E aton and Bradbury 2003),

three major bedding-plane fracture zones were correlated

in the heterogeneous upper part of the Maquoketa aquitard

(Figure 4 ). They account for the range of hydraulic

conductivity (Figure 6) above that of the unfractured and

lithologically homogeneous basal shale (less than 1 3

10 28 m/d). The hundred or more meters of drawdown in

the underlying aquifer system have affected head only in

the bottommost monitoring intervals in the Sinnipee

G roup dolostone, where heads are 60 to 80 m lower than

in the Maquoketa aquitard (Figure 5).

G round water in the Maquoketa aquitard is primarily

a calcium-magnesium– bicarbonate type, with abundant

sodium and sulfate. Saturation indexes, log(IAP /Ksp),

indicated that ground water is oversaturated with CO 2(g),

and generally in equilibrium with calcite and dolomite,

but undersaturated with respect to gypsum (E aton 2002).

V ariation in field-measured parameters and concentrations

of major ions with depth is shown (Figure 7) in comparison

to data from the overlying Silurian Aquifer (interval

7) and the underlying Sinnipee G roup dolomite (interval

1). The variation in values with depth at both sites is particularly

notable because samples presumably represent

mixing over long open intervals between packers. The

trend in hydrochemical parameters with depth provides

clues to the principal directions (horizontal or vertical) of

high-conductivity flowpaths within the Maquoketa Formation.

Relative variations in ground water chemistry are

especially pronounced between intervals containing

major horizontal fracture zones and adjacent unfractured

intervals. While the vertical boreholes of necessity sample

primarily horizontal fractures, variations in hydrochemical

parameters at different levels in the aquitard

strongly suggest that there is little mixing of water from

top to bottom. Hydrogeochemical parameter values vary

between the sites, which is not unexpected for ground

water in complex fractured systems. However, relative

variations in hydrochemistry at different levels, observed

at both sites, are difficult to explain by simple 1D vertical

flow, and suggest that hydrochemical differences

result from sampling water from different sources.

Water chemistry distribution in the Maquoketa Formation

is inferred to represent complex local mixing of

sodium bicarbonate (Na-HCO 3)– rich water from the

unfractured rock matrix and calcium-magnesium bicarbonate

(Ca-Mg-HCO 3)– rich water from major beddingplane

fracture zones (E aton 2002). The high-sodium,

low-chloride concentrations in the Maquoketa water

probably originate from cation exchange processes in the

clay-rich shale (Na/Cl molar ratios 3 to 19). High-sulfate

water are consistent with possible localized occurrences

of gypsum in the shale (Weaver and Bahr 1991; E aton

2002). The calcium-magnesium bicarbonate (Ca-Mg-

HCO 3) water, thought to be representative of fracture

zones, is similar to water from shallow aquifer systems in

the region (Saad and Thorstenson 1998).

The differences in hydrochemistry at different levels

within the Maquoketa aquitard (Figure 7), as well as the

head differences (Figure 5), suggest few to no vertical interconnections

between bedding-plane fractures or flow

due to vertical fractures but are consistent with lateral

advective transport along bedding-plane fractures (E aton

and Bradbury 2003). This analysis is also in agreement

with ground water tritium and oxygen isotope data (E aton

2002) that indicate water in the Maquoketa aquitard is

probably of P leistocene age compared to recent water in

the overlying Silurian dolomite aquifer.

T.T. Eaton et al. GROUND WATER 45, n o. 5: 601–615 609


Concep tual M odels for Aquitards

Neither hydrogeochemical (Figure 7) nor hydraulic

head data (Figure 5) in the Maquoketa aquitard are

consistent with the conventional conceptual model that

flow is predominantly downward in aquitards overlying

heavily pumped aquifers (Remenda et al. 1996; E aton and

Bradbury 2003). In the traditional conceptual model, vertical

flow under steady-state conditions is associated with

a linear head decrease with depth, and under transient

conditions, a monotonic head decrease with depth, both

characteristic of uniformly downward gradients. Instead,

we find that flow between monitoring intervals is bidirectional,

and there is an abrupt decline in head across the

base of the formation at both field sites (Figure 5), indicating

nonequilibrium and a very low effective vertical

hydraulic conductivity (E aton and Bradbury 2003).

High-transmissivity channels (Tsang and Neretnieks

1998) within fracture planes greatly increase the effective

hydraulic conductivity in a porous medium depending on

fracture orientation, extent, and especially connectivity

(Robinson 1984 ; L ong and Witherspoon 1985). In the

P ierre Shale, through-going vertical fractures spaced at

kilometer intervals may explain the discrepancy between

effective vertical hydraulic conductivity values estimated

from small-scale measurements on rock core and relatively

higher values based on results of regional flow

modeling (Bredehoeft et al. 1983; Neuzil 1986, 1994 ). A

similar discrepancy in estimates from field data (this

paper) and regional model values (Hart et al. 2006) exists

for the Maquoketa aquitard. However, in contrast to the

P ierre Shale, the Maquoketa aquitard in southeastern

Wisconsin is penetrated by numerous, old multiaquifer

wells (SE WRP C/WG NHS 2002; Hart et al. 2006). The

Maquoketa Formation is usually cased off in these wells,

but many wells are uncased above and below the shale,

allowing flow through the borehole from the surficial

aquifer to the deeper aquifer. In regional flow modeling

under transient pumping conditions, borehole flux

through such wells can be accommodated using an apparent

higher vertical hydraulic conductivity of the aquitard.

A major structural discontinuity whose hydraulic

properties are poorly known (the Waukesha Fault) occurs

directly between the field sites (Figure 1). Y et, a conceptual

model of through-going vertical fractures is inconsistent

with available field evidence at the tens of kilometers

scale encompassed by our two sites. Flowpaths through

such vertical fractures, if they existed, would intersect

the numerous bedding-plane fractures that have been

observed in the Maquoketa aquitard (E aton 2002; E aton

and Bradbury 2003) and are known from similar hydrogeologic

settings. In such a hypothetical high-diffusivity

interconnected fracture network, hydraulic head must

respond rapidly to more than 100 m of drawdown caused

by pumping in the underlying Cambrian-O rdovician Aquifer

system. The hydraulic response time to equilibrium

(Alley et al. 2002; E aton and Bradbury 2003) in such

a fracture network would be on the order of days. Furthermore,

flow within a through-going fracture network would

cause vertical head profile equilibration to a monotonic

pattern and homogeneous hydrogeochemistry with depth

in the formation, contrary to observations (Figure 7).

610 T.T. Eaton et al. GROUND WATER 45, n o. 5: 601–615

Measurements over a period of 3 to 4 years indicated

that there is no drawdown trend in head change or systematic

head fluctuation within the Maquoketa Formation

at our sites (Figure 5). Several large municipal supply

wells within a few miles of the field sites were concurrently

being pumped on various schedules, and the underlying

aquifer system has been pumped for over a century.

Together, the hydraulic head data (Figure 5), the stratified

hydrogeochemistry (Figure 7), and limitations on vertical

fracture propagation due to heterogeneous mechanical

stratigraphy provide important evidence against continuous

large vertical fractures in the Maquoketa aquitard, at

least at the scale of our two field sites.

A new geological conceptual model for the aquitard

(Figure 2) accounts for all the available hydrogeological

observations in the Maquoketa aquitard at our study sites

and is consistent with recent understanding of the mechanisms

and patterns of fracture formation and porosity

development in similar, relatively undeformed, heterogeneous

sedimentary rocks. The major elements of the new

conceptual model are as follows:

1. O rders of magnitude contrast between hydraulic diffusivity

(K/S s) of horizontal bedding-plane fractured zones and

relatively intact aquitard rock matrix. In the case of the

Maquoketa aquitard, this accounts for the rapid responses

in observation wells to the interwell pumping tests and

horizontal fracture flow, resulting in the vertically stratified

ground water chemistry (Figure 7).

2. L aterally extensive bedding-plane fracture zones extending

over distances of at least 10 km and probably more. Development

of bedding-plane fracture porosity in carbonate

rocks is similar to evolution of preferential flowpaths in

karst through progressive dissolution (Kaufmann and Braun

1999; G abrovsek and Dreybrodt 2001; Bloomfield et al.

2005). These studies have shown that fracture porosity is

highly dependent on boundary conditions. For example,

given a carbonate rock subcrop in an aquifer system,

increased fracture porosity and transmissivity develop progressively

over time with distance away from the subcrop. If

numerous interbeds of different lithology impose a mechanical

stratigraphic control on vertical fractures but allow the

development of bedding-plane fractures, the result would be

laterally extensive bedding-plane fracture zones extending

over distances of up to tens of kilometers in carbonate rocks.

3. V ery low vertical hydraulic conductivity in thick unfractured

shale intervals. In this case, effective hydraulic

conductivity of the apparently unfractured shale near the

base of the Maquoketa aquitard appears to be similar

to values from laboratory rock-core testing (E aton and

Bradbury 2003), a type of scale independence suggested

for some aquitards by Neuzil (1994 ).

4 . Hydraulic influence of the aquitard subcrop, where

hydraulic head in the shallow aquifer system is controlled

by numerous surface water bodies. This last element of

the conceptual model does not excessively limit its general

applicability because unrecognized edges and proximity

to areas where the aquitard has been eroded through

to the aquifer are common uncertainties about aquitards

(Cherry et al. 2006).


N umerical M odeling Results and S ensitiv ity

Simulation results are shown using vertical profiles

of head (Figure 9A) for comparison to hydraulic head

data collected at the field sites (Figure 5). V ertical head

distribution for the Maquoketa aquitard and below is

illustrated from predevelopment steady-state conditions

(1900) to present-day transient conditions (2000), with

head profiles at two intermediate time periods: 1960

and 1980. Simulated head values in different layers

(curves) are most different from measured head values

(points) at the base of the formation, where head

changes rapidly over a short vertical distance. The discrepancy

is likely due to inadequate vertical resolution

of the model grid.

The simulated 2000 head profile with depth generally

matches the observed abrupt loss in head across the

bottom contact of the Maquoketa Formation. The match

between simulated transient heads at present (year 2000)

and the observed head distribution with depth (Figure 5)

suggests that the flow field in the Maquoketa aquitard is

not at steady state with respect to pumping in the underlying

aquifer system, as previously inferred (E aton and

Bradbury 2003). Since long-term changes in head values

were not observed within but only below the Maquoketa

Formation, model parameters represent an upper bound

on aquitard vertical diffusivity, and hence indicate a long

time constant to equilibrium.

The model was also used to investigate the transient

nature of the ground water flow system. The evolution of

head is dependent on inherently unpredictable patterns of

pumping for municipal supply. However, if the current

head in the deep aquifer system is assumed to remain at

the present value of 14 0 m above msl, the evolution of

head in the Maquoketa aquitard can be projected. Results

of predictive simulations (shown in Figure 9B for years

2200 and 2500, and steady state), using initial heads from

the present-day (year 2000) simulation, show that the current

hydraulic head profile is far from steady state. The

steady-state head profile shows a distinct change in vertical

gradient below the upper fractured portion of the

Maquoketa aquitard, where the hydraulic head gradient

with depth becomes constant and linear. In all simulations,

head in the transmissive bedding-plane fracture

layers tends to maintain the vertical head distribution

close to the head values at the lateral model boundary

(260 m above msl). The difference between present-day

(year 2000) and future profiles suggests that a steady-state

flow system within the Maquoketa aquitard might not

occur for many hundreds of years.

In order to test the effect of both the constant-head

boundary condition at the aquifer subcrop (represented

by P ewaukee L ake) and the bedding-plane fracture zones

on the head profile in the aquitard, we arbitrarily reduced

the value of the western constant-head boundary condition

by 80 m and reran the steady-state and transient

simulations. Bedding-plane fracture zones that are laterally

extensive over scales of kilometers can provide highdiffusivity

flowpaths, causing distant lateral boundary

conditions to become more important than much closer

lower boundary conditions for rock aquitards. The resulting

effect on the head profile (Figure 9C) indicates that

hydraulic head at the lateral boundary can play a more

important role in controlling the Maquoketa vertical head

distribution than the lower boundary condition in the

immediate underlying heavily pumped aquifer. Although

it is well known that boundary conditions affect steadystate

hydraulic heads, the dominant influence of lateral

boundary conditions here, under transient conditions, is

Figure 9 . Field- measured v ertical head distribution (points) within the Maquoketa Formation (shaded) and modeling results

(curv es) at the DOT field site (A ) after different stress periods (dates) to present; (B) from present (20 0 0 ) to years 220 0 and

250 0 and steady- state future head distribution; (C ) with constant- head boundary condition v alues reduced by 80 m to demonstrate

role of head at western subcrop. Dashed lines indicate approximate elev ation of layers representing bedding- plane fracture

zones. Open and solid circles as in Figure 5. The simulated decrease in head across the bottom of the Maquoketa

Formation is discretized by the number of layers used— it would be more abrupt with ev en thinner layers.

T.T. Eaton et al. GROUND WATER 45, n o. 5: 601–615 611


a significant contrast to the dominance of the top or bottom

boundaries inherent to the traditional vertical flow

conceptual model.

A sensitivity analysis was conducted using the

inverse code U CO DE (P oeter and Hill 1998). Composite

scaled parameter sensitivities, which are an aggregate of

all the dimensionless scaled sensitivities, were calculated

for each rock parameter value in Table 1. Results indicated

that sensitivity to all horizontal hydraulic conductivity

parameters was negligible. The highest sensitivities

were to the vertical hydraulic conductivity of the Sinnipee

G roup dolostone, followed by the storage coefficient of

model layers representing unfractured Maquoketa rock.

Next most sensitive was the vertical hydraulic conductivity

of layers representing unfractured Maquoketa rock,

followed by recharge. Sensitivity to the storage coefficient

of layers representing bedding-plane fracture zones

was very low, indicating that results would not change

with lower storage values than used in the calibrated

model. High sensitivity to storage coefficient of layers

representing Sinnipee G roup and unfractured Maquoketa

rock is consistent with the simulated reduction in hydraulic

head caused by pumping over the 20th century

(Figure 9) and with the interpretation that the flow field

in the Maquoketa Formation has not yet equilibrated to

the aquifer drawdown.

D iscussion

Results from this study indicate the vertical hydraulic

conductivity of the Maquoketa aquitard is on the order of

1 3 10 29 m/d based on field data and numerical modeling

at the tens of kilometers scale of our field sites. This

very low conductivity is due largely to the bottom shale

layer in the formation, which the hydraulic head profile

suggests is unfractured at this scale. A complex flow system

(Figure 10B) surrounding the bedding-plane fracture

zones is inferred to exist within the aquitard. A remnant

higher head zone may be present within the very low conductivity

base of the aquitard, accounting for the observed

head distribution measured within the multilevel monitoring

systems. Slow drainage from the low-conductivity

rock matrix into high-diffusivity bedding fractures causes

the variation in water chemistry signatures and the

converging vertical flow directions observed between

fractured and unfractured intervals. Additional field

observations are needed to determine the farthest lateral

extent of the fractured zones away from the aquitard subcrop.

There are significant differences in the inferred flow

field in the aquitard under predevelopment steady-state

(Figure 10A) and present transient conditions (Figure 10B)

caused by municipal pumping.

Mass balance calculations were made to estimate

leakage fluxes for the calibrated flow model in the area

encompassed by the field sites, assuming constant aquifer

drawdown. They show that transient leakage fluxes from

the aquitard to the underlying aquifer system peak at

about 0.017 m 3 /d/km 2 , declining to about 0.006 m 3 /d/km 2

at steady state. Intrinsic hydraulic conductivity and leakage

fluxes presented here for the Maquoketa aquitard are

almost 3 orders of magnitude lower than values previously

reported by others, who based estimates on steadystate

flow assumptions (Walton 1960; Y oung 1992).

Results presented here suggest that recent higher estimates

based on regional numerical modeling reported by Hart

et al. (2006) are attributable to numerous cross-connecting

multiaquifer wells rather than through-going vertical

fractures.

Conclusions and Imp lications

We developed a 3D conceptual model for flow in

a rock aquitard that emphasizes the importance of laterally

extensive, transmissive fracture zones along bedding

planes. The conceptual model allows explanation of the

unusual but observed vertical profiles of head and hydrochemical

data at two sites 10 km apart in the Maquoketa

aquitard, southeastern Wisconsin. Results from a numerical

model show that lateral head boundaries at the rock

Figure 10 . S chematic diagram showing complex flow system within the Maquoketa aquitard (shaded) within 10 km of subcrop.

(A ) C orresponds to steady- state predev elopment conditions and (B) corresponds to transient present- day conditions.

H eav y lines represent transmissiv e bedding- plane fracture zones extending east from the subcrop and dashed contours represent

hydraulic head. N ote that field sites are within the area of fracture zones (Figure 2), but the extent of these away from the

subcrop is uncertain. I n (A ), hydraulic head is highest in the underlying aquifer system. I n (B), closed contour in lower

unfractured Maquoketa Formation represents highest head. Deep well at right represents municipal pumping that has drawn

down head in underlying aquifer system.

612 T.T. Eaton et al. GROUND WATER 45, n o. 5: 601–615


aquitard subcrop can be a dominant control on the vertical

distribution of hydraulic head and hydrochemistry in

the aquitard because of the presence of transmissive

bedding-plane fractures.

Major conclusions from this study are as follows:

1. The traditional conceptual model of 1D vertical flow

through an aquitard was inadequate to explain the field

data in an indurated rock aquitard while a 3D conceptual

model of flow and hydrochemical mixing satisfactorily

explained the pattern of both the heads and water chemistry

observed in the field. The Maquoketa aquitard, while

restricting downward ground water flow, has a complex

internal flow system (Figure 10) due to near-horizontal

bedding-plane fractures. Accordingly, conceptual models

of similar aquitards should not necessarily assume strictly

vertical flow of water and/or solutes/contaminants.

2. Transmissive bedding-plane fractures play an important

role in the hydrogeology and hydrogeochemistry of rock

aquitards such as the Maquoketa Formation. These fractures

promote a range of water chemistry with depth as

pore water from the rock matrix mixes with very different

ground water from the more fractured horizons.

3. Simulation results show that the effects of lateral boundary

conditions are propagated over multikilometer-scale

distances through transmissive, high-diffusivity, beddingplane

fractures controlling the vertical distribution of

head and delaying equilibration to pumping from underlying

aquifers. In the Maquoketa aquitard, the flow field

has not equilibrated to the effects of pumping in the

underlying aquifer system (E aton and Bradbury 2003),

partly owing to the presence of the highly transmissive

fractures in the upper part of the aquitard and partly

owing to the extremely low hydraulic conductivity (10 29

m/d) of the apparently unfractured base of the aquitard.

4 . Numerical models that incorporate the most important

elements of the conceptual model presented in this study,

in combination with stresses (for example, pumping in an

underlying aquifer) that allow the development of a significant

vertical hydraulic gradient, are likely to be useful in

interpreting hydrogeologic data in other rock aquitards.

O ur conceptual model for control of hydraulic head

and hydrogeochemistry in rock aquitards potentially is

applicable to a broad area within at least 10 km of the

subcrop of the Maquoketa Formation, which extends over

parts of the states of Iowa, Illinois, Wisconsin, Indiana,

O hio, and Michigan in the northern Midwest U nited

States. It also provides a framework for evaluating hydrogeologic

and hydrogeochemical data from similar settings

in other rock aquitards, where multilevel hydraulic head

data are available from within and below the aquitard.

The results of this study have implications for assessment

of effective properties of rock aquitards on regional

scales. E stimates of intrinsic vertical hydraulic conductivity

of the Maquoketa aquitard reported here are up to 3

orders of magnitude lower than previously reported for

regional studies of this aquitard, as are estimates of flux

to the underlying Cambrian-O rdovician Aquifer system.

Since the results of this study are based on site-specific

field data from within the aquitard, the estimates of fluxes

to the underlying aquifer system do not include the effects

of the numerous deep wells that are cased through

the Maquoketa aquitard but open to the aquifers above

and below (SE WRP C/WG NHS 2002; Hart et al. 2006).

Flow through these multiaquifer wells could explain the

discrepancy between our estimates of aquitard properties

and the larger values reported in regional studies.

O n a regional scale, the proliferation of multiaquifer

wells over the 20th century in southeastern Wisconsin

(SE WRP C/WG NHS 2002) has increased the effective

vertical hydraulic conductivity of the aquitard. Therefore,

a higher vertical hydraulic conductivity for the aquitard

than used in this study is appropriate for assessments of

regional leakage to the underlying aquifer system. Such

assessments are needed for projections of regional municipal

water supply, and the change in effective properties

presents challenges for regional numerical modeling studies.

Regional models often do not explicitly represent

multiaquifer wells or have the resolution to account for

hydrogeologic field data needed to evaluate intrinsic

properties of aquitards. However, the results presented

here provide a baseline for evaluation of predevelopment

conditions or intrinsic properties of the Maquoketa aquitard

and therefore may be useful for isolating the effects

of the multiaquifer wells.

Acknowledg ments

Funding for this work was provided by the Wisconsin

Department of Natural Resources and the U niversity of

Wisconsin Water Resources Institute. The authors gratefully

acknowledge the comments and thorough reviews

provided by Chris Neuzil, who acted as editor-in-chief for

this paper, two anonymous reviewers, and Tamie Weaver.

Their input has greatly improved the paper.

E ditor’ s N ote: The use of brand names in peer-reviewed

papers is for identification purposes only and does not

constitute endorsement by the authors, their employers, or

the National G round Water Association.

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