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

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

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).

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