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C O N T A M I N A N T T R A N S P O R T
T H R O U G H
A B A N D O N E D B O R E H O L E S
I N
F R A C T U R E D R O C K
by
S E A N
R A Y M O N D P A T R I C K B U R N S
B . A S c , University
of
B r i t ishColumbia,
1997
A T H E S I S S U B M I T T E D
IN
P A R T I A L F U L F I L L M E N T
OF
T H E R E Q U I R E M E N T S F O R T H E D E G R E E
OF
M A S T E R
OF
A P P L I E D S C I E N C E
in
T H E F A C U L T YOFG R A D U A T E S T U D I E S
Department ofEarthand Ocean Science
W e accept this thesis as conforming
to the required standard
T H E U N I V E R S I T Y OFB R I T IS H C O L U M B I A
June2000
Sean
Raymond
PatrickBurns,2000
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In present ing th i s t h e s i s i n p ar t i a l ful f i lme nt of the requirements
for an advanced
degree
at the Uni ve rs it y of B r i t i sh Co lumbia , I
agree that the L i b r a r y sha l l make i t fre el y av ai la bl e for refe rence
and stud y. I fu rt he r agree tha t per mis si on for e x te n s i v e copying of
t h i s t h e s i s f or s c h o l a r l y pur pos es may be gr an te d by the he ad of my
department or by hi s or her re pr es en ta ti ve s. It i s underst ood t hat
copying
or pub l i ca t io n o f th i s t h e s i s for f in an ci al gain sh al l not
be all owed without my wri tt en p er mis si on.
Department
The
Univ ers i t y o f
Br i t i sh Columbia
Vancouver,
Canada
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Abstract
Abandoned explorationboreholes arecommonlyfound around mine sites in a fractured
crystallinerock environment.
I f
the abandoned boreholes have not been properly
decommissionedthey have the potential tocreateconnections through the rock fractures
and
influence ground water flow and contaminant transport. Afullythree-dimensional
discrete fracture model is used to investigate the impact of abandoned boreholes on
contaminant transport froma waste-rockpile overlyinga fractured rock mass. Dissolved
contaminants travel through the fractured rockmassunder the influence of a sub-
horizontalregionalhydraulicgradient towards a downstream compliance boundary. A
number of different fracture geometries are investigated to gain an understanding of the
fieldsituations inwhichabandoned boreholes can be expected to have an impact. The
effect of fracture density,transmissivitycontrasts, and borehole diameter and
location
are
studied.
Thesimulationresults show
that
verticalabandoned boreholes are most l ikelyto
have an impact when large,
sub-horizontal, high-transmissivity
featuresarepresentin the
network. L o w fracture density,aperturevariability,
relatively
highhorizontal
transmissivity,
and the presence of major
features
in the fracture network all lead to
abandoned boreholes having agreateroverallinfluence. If an abandoned borehole is
transversely offset fromthe centralflow linepassing through the source zone the
contaminant plume can migrate towards the borehole in adirectionnot predicted by the
average regionalhydraulicgradient. In field-scale fracture networks smaller borehole
diameters leads to shorter breakthrough times and higher contaminant concentrations at
the downstream boundary due to the interplay between the fracture network and borehole
void
space. The presence of abandoned boreholes can be expected to have important
implicationsin the design ofmonitoringnetworks todetectground water contamination
whenthese
fracture network and abandoned borehole properties exist.
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Table
of Contents
Abstract ii
Table
of Contents iii
List
ofTables v
Listof
Figures
vi
Acknowledgements ix
1.0 Introduction 1
2.0 Review 6
3.0 Methods
13
3.1FractureNetworkModeling 13
3.2Overview
of the
FRACMAN/MAFIC suite 13
3.2.1 FRACWORKS 14
3.2.2 MESHMONSTER 17
3.2.3
EDMESH
18
3.2.4
MAFIC
19
3.3SoluteTransport 20
3.4Implementation
of
Abandoned Boreholes 22
3.4.1 Borehole
as a
Discrete
Fracture
23
3.4.2
Borehole-Fracture
Intersections 27
3.4.3 Test
of
FlowthroughBorehole 28
3.4.4 Test
of
Particle
Tracking
throughBorehole 29
3.4.5 Effect
of
BoreholeDiameter 31
3.5
Conceptual Model Description
33
4.0
Results
48
4.1
Base
CaseFractureNetwork 48
4.1.1 Fracture
Generation Region
50
4.1.2 Base CaseBorehole 50
4.1.3 Grid
Discretization
52
4.1.4 Case1 -
Base
Case
Results
54
4.2Influence
of Background FractureNetwork
60
4.2.1 Case
2 - Higher
HorizontalTransmissivity 60
4.2.2 Case
3
-Higher Vertical
Transmissivity
64
4.3
Influence
of Major
Features
66
4.3.1 Case4 - Major
Feature Transmissivity
=
lxl0
3
m
2
/s 67
4.3.2 Case
5 -
Threshold Effect 69
4.3.3 Case
6 -
LowerFeatureTerminates
atx = 30 m
70
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4.4
Influence of
Borehole Location and Diameter
72
4.4.1
Case -Boreholeatx = 0 m 72
4.4.2 Case
8 -
Borehole
atx = -35 m
73
4.4.3 Case
9 -
Borehole
aty = 15 m 75
4.4.4 Case
10-
Influence
of
Borehole Diameter 76
4.5
Case 11
-
Variable Network
with No
Major Features
77
5.0Discussion 112
5.1Results Summary 112
5.2
Fracture Network Structure
116
5.3
Implications
for
Monitoring Network Design
119
5.4
Proper Borehole Decommissioning
124
6.0 Conclusions 126
References 128
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List o fTables
Table 3-1:Parametersfor analytical problem 32
Table 4-1: Generation parametersfor
base
casefracturenetwork 48
Table 4-2:
F l o w
andtransport output
parameters
for al l simulations 55
Table 4-3: Generation
parameters
for Case 11
fracture
network 78
Table 4-4: Borehole
parameters
for Case 11 80
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Listof
Figures
Figure
1-1: Contaminants leaching from a
waste
rock pile into a fractured rock
aquifer 5
Figure
2-1:
Aquifer
cross-contamination due to abandoned
w e l l
in
multiaquifer
system 12
Figure3-1:
Definition
of orientation convention used inF R A C M A N 36
Figure3-2: Orthogonalview o fthe
base
casefracture network 37
Figure3-3:Parametersfor incorporation of a circular borehole as a
rectangular fracture 38
Figure3-4:
Discretization
algorithm around the intersection of a borehole
with
afracture 39
Figure3-5: Test of
flow
through a 10 cm diameter borehole 40
Figure3-6: Fracture network designed totestparticle-tracking through the
borehole 41
Figure3-7: Particle breakthrough at downstream boundary fortestfracture
network 42
Figure3-8: Pathway of a single particle traveling through thetestfracture
network 43
Figure
3-9:
A n a l y t i c a l
model used to
test
the effect of borehole size on
f low
rate
and residence times 44
Figure3-10:
F l o w
behaviour of a large
void
space
within
a thin conduit 45
Figure3-11: Hypothetical mine site used to develop the conceptual model 46
Figure3-12: Cross-sectionviewthrough the conceptual model showing
boundaries and dimensions 47
Figure4-1:Cross-section of
base
casefracture network showing fracture
traces
for background fractures and major
features
83
Figure4-2: Orthogonal
view
ofmodelingdomain showing
flow
andtransport
boundaries 84
Figure
4-3: Diagram of
modeling
domain and fracture generation region 85
Figure4-4: Finite element
grid
discretization 86
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Figure4-5: Case - Particlebreakthrough at downstream exit boundary 87
Figure4-6: Case
-
Lateral
breakthrough distribution of particlesacrossexit
boundary 88
Figure4-7: Pathways of selected particles
after
exitingthe borehole 89
Figure4-8:
Snapshots
of particles traveling through the fracture network when
no borehole ispresent 90
Figure4-9: Case 2 - Particle breakthrough at downstream boundary 91
Figure
4-10: Case 2 -
Lateral
breakthrough distribution of particles 92
Figure4-11: Case 3 - Particle breakthrough at downstream boundary 93
Figure4-12: Case 3 -
Lateral
breakthrough distribution o fparticles 94
Figure4-13: Case 4 - Particle breakthrough at downstream boundary 95
Figure4-14: Case 4 -
Lateral
breakthrough distribution o fparticles 96
Figure4-15: Case 5 - Particle breakthrough at downstream boundary 97
Figure4-16: Case 6 - Particlebreakthrough at downstream boundary 98
Figure4-17: Case 6 - Lateral breakthrough distribution of particles 99
Figure
4-18: Case 7 - Particle breakthrough at downstream boundary 100
Figure4-19: Case 7 - Lateralbreakthrough distribution of particles 101
Figure
4-20: Case 8 -
Particle
breakthrough at downstream boundary 102
Figure 4-21:Case 8 -
Lateral
breakthrough distribution of particles 103
Figure4-22: Case 9 - Particlebreakthrough at downstream boundary 104
Figure4-23: Case 9 - Lateralbreakthrough distribution of particles 105
Figure4-24: Case 10 - Influence of borehole diameter on downstream
breakthrough curves 106
Figure4-25: Case 10 - Influence of borehole diameter on total system
f low
and median particle residence times 107
Figure4-26: Case 11 - Particlebreakthrough at downstream boundary for a
random fracture network
with
no majorfeatures 108
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Figure
4-27: Isometric views of borehole locations for Case 14 109
Figure4-28: Case 11 - Particle breakthrough at downstream boundary for an
entirelyrandom fracture network 110
Figure4-29: Particle breakthrough at downstream boundary for aconstant
aperturefracture network 111
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A c k n o w l e d g e m e n t s
I
would
like
to thank my supervisor,
Leslie
Smith,for his support, advice, guidance and
patience during the preparation of this thesis. I am grateful for the support and advice of
Petros Gaganis, Bob Parney, and the other graduate studentsin the U B Chydrogeology
group who have
given
me advice over thepasttwo years.
GolderAssociates
L t d .
developed the discrete fracture modeling code used in this study,
and
special thanks go to the F R A C M A N group in Seattle,includingTomDoe,
Bi l l
Dershowitz,
Paul
L aPointe and
G l o r i
Lee, who made the source code
available,
without
whichIcouldnot have made the necessary
modifications.
I
would l iketo give a very special thank you to my parents, G a i land RayBurns,who
have beenincrediblysupportive over myuniversitycareer, and who have always
encouraged me to
follow
my dreams.
Claire
Bradfordalso deserves special mention for
her emotional support and editing prowess.
Funding
from theNationalScience andEngineeringResearch
C o u n c i l
providedfinancial
support for this research.
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1.0
Introduction
Protecting
and maintaining the quality of natural water supplies is necessary for the
wel l
being
of
a ll life
on earth,
including
humanity. As the population
of
the
world
expands the
demand for material goods
w i l l
increase, and new sources of raw minerals
w i l l
need to be
uncovered and developed. In the
past,
poor and
ill-advised
mining
practices have
resulted in catastrophic
pollution
to water supplies and long-term environmental damage.
Whilemining
practices and accountability have improved significantly in recent years,
environmental damage continues to occur at active sites and
there
is stillroom for
improvement.
Groundwater is the most vulnerable fresh water source to long-termpollutionfrom
miningwaste, as residence times in the subsurface can be on the order oftensto hundreds
ofyears.
Once a ground water resource has been contaminated it is technically
difficult
and very expensive to remediate, i f it can be done at
all.
It is more cost-effective to
engage
in practices
that
prevent ground water contamination from occurring in the first
place,
than to
attempt
to clean up the problem afterwards. If proper care is taken a
balance can be met between meeting the resource requirements of society while
sustaining the quality of the ground water for the long term.
M i n i n g
typically
requires the extraction of a small amount
of
valuable ore minerals from
large volumes of sub-economicalganguematerial. Open-pitminingcan result in waste
rockpiles and tailings dams
that
need to be managed fordecadesor even centuries.
Waste rock is the material removed to gain access to the ore body, and usually contains
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metallicminerals such as pyrite,whichhave no economical value. Figure 1-1 depicts a
situation
inwhicha waste rock pile is situated in an elevated area
overlying
a fractured
rockmass. Over long time periods exposure to
rainfall
and atmospheric oxygen can
result in acidic water leaching out
of
the waste rock piles and
mobilizing
toxic heavy
metals. This process is commonly referred to as
acid
rock drainage
( A R D ) .
If no
impervious
barrier is
present
to impede migration the metals
w i l l
travel
vertically
through
the unsaturated zone and reach the water table, contaminating the underlying ground
water.
Underthe Environmental Assessment Act in
B r i t ishColumbia
a potential mine site must
be rigorously studied to
assess
any potential for ground water contamination ( R S B C
1996). A compliance boundary for a site is established to delineate the maximum extent
atwhichsome degradation to the
local
ground water quality is deemed acceptable by the
regulatory agency. A network of compliance monitoring wells is installed at the
compliance
boundary and ground water is regularly sampled. If contamination above a
threshold value is detected at the compliance boundary then the site owner
w i l l
face
financial
penalties on top of the cost of
containing
and remediating the contamination.
The processes governing
A R D
are
still
an active area of research, and
there
is a
great
deal
of
uncertainty when predicting contaminant fluxes to the subsurface. Once contaminants
have reached the water table
there
is the potential for off-site migration in the direction of
ground water
f low.
Numericalmodels are useful to help predict contaminant fluxes from
potential sources at a site and
assess
the risk of exceeding the threshold concentration at
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the compliance boundary. Ground water modeling for a fractured rock terrain is more
complicatedthan in porous media due to theheterogeneous
nature
of a fractured rock
mass.
F l o w
is often concentrated in asmallsubsetof the total number of fractures and
contaminant transport occurs along complicated, tortuous pathways. Gathering
hydrogeologicalcharacterization data in crystalline rock is very expensive, so the amount
ofinformation available to the modeler is usually quite
limited.
The higher uncertainty in
characterizing
fractured rock leads to additional engineering effort and expense.
One source of uncertainty at a mine site
overlying
fractured rock is the presence of any
abandoned exploration boreholes around the ore body. Before a mine can be developed
the ore body must be delineated, so a developed mine site may havetensto hundreds of
explorationboreholes.
I f
previous
phases
ofexplorationat a site
failed
todetect
economic
mineralizationthen all
dri l l ing
records for a sitecouldbe lost. If the boreholes
were not properly grouted before being abandoned they can act as conduits between
stratigraphic layers, or
form
preferential pathways in the fracture network. The potential
for
cross-contamination of layered aquifers
that
are perforated by boreholes is
wel l
understood. It is hypothesized
that
in a fractured rock the boreholes
could
connect
fractures creating a preferential pathway and
allowing
contamination to travel faster to
the compliance boundary, or contaminating a previously protected zone. Connections
can
be formed
that
cannot be anticipated based solely on knowledge o fthe statistical
fracture geometry and the contaminant plume may move in a direction not anticipated
based on conventional porous medium approximations.
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Thisthesis investigates the potential impacts
that these
abandoned boreholescouldhave
at a mine site underlain by a fractured rock mass. The conceptual model assumes
that
contaminated leachate
from
a waste rock
pile
has reached the water table. A three-
dimensionaldiscrete fracture code is used to model the flowand contaminant transport
through the fracture network and to
assess
the impacts the boreholes have on the system.
Theobjectiveo fthis thesis is to gain an understanding of the types of subsurface
conditionswhere abandonedexplorationboreholes have the potential to exacerbate a
contaminationproblem,or reduce the effectiveness of amonitoringnetwork design.
The
first two chapters present themotivationfor the work and a review of related work
on
fracture networkmodelingand the effects of abandoned
wells
and boreholes. In the
third
chapter themodelingmethodology is presented along
with
themodifications
that
were necessary for the purposes of
this
study. The fourth chapterpresentsthe results of
the set ofsimulationsused to demonstrate the effects of boreholes in different fracture
networks.
Thef inaltwo chapters explaintheimplicationsof thesimulationresults and
summarize theconclusions
that
can be drawnfromthis study.
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2.0Review
Numericalmodels for simulating flow and transport in porous media have been available
forseveraldecades(e.g. Freeze, 1971;McDonaldand
Ffarbaugh,
1988; Zheng,1990).
Recently, the potential for siting high-level radioactivewasteindeepcrystalline rock has
prompted
interest in modeling flow and transport through fractures (e.g. Dverstorp et al.,
1992; Chanet al., 1993; Bodvarsson et al.,1997).
Fracturemodeling has been applied on a variety of lengthscalesfrom a single fracture to
field-scale networks. The simplest model for fracture flow is that of two parallelplatesof
constant aperture (e.g. Snow, 1965; Witherspoon et al.,1980). Several
studies
have been
conducted on flow and transport through a single fracture with various complicating
factors including spatial variability in the aperture (Brown, 1987; Raven et al., 1988;
Morenoet al.,1988),diffusion into the porous rock matrix (e.g. Sudicky andFrind, 1982;
Neretnieks, 1980) and surface sorption (e.g.Burkholder,1976; Freeze andCherry, 1979;
Wels and Smith,1994). Laboratoryexperiments on flow through a single fracture have
suggestedthat the cubic law is not valid for most naturallyoccurringrough-walled
fractures (e.g. Raven et al., 1988) but most discrete fracture models make this assumption
due to limitations on the complexity that can be modeled over large domains.
Fieldscalemodels for flow and transport through fracture networksfallunder one of
three broad categories: (National ResearchCouncil,1996) (1) equivalent continuum
models which model the fracture network using average properties (e.g. Neuman, 1987;
Carreraet al.,1990),(2) discrete network simulation models which account for each
fracture individually (e.g. Smith et al., 1985; Dershowitz et al,
1995),
and (3)
hybrid
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techniqueswhichapply somecombinationof the previous two approaches (e.g. Schwartz
and
Smith,
1988;
L a
Pointe et al. ,1995).
Equivalent
continuum models do notattemptto model each fracture pathway
individually,but use an average representation
o f
the
hydraulic conductivity
field. The
main
advantage o f
these
models is
that
they can be less computationally intensive than
discrete fracture models so a larger domain can be modeled.
This
approach is most
appropriate whenmodelinga rock
with
many fracture connections or significant matrix
permeability.
When
the network is heterogeneous
with
large contrasts in hydraulic
conductivity,equivalent continuum models require a fine
grid
discretizationand can
become as computationally intensive as discrete fracture methods
( N a f f
et
al. ,
1998a,b).
In
sparsefracture networks the
individual
fracture connections and tortuous pathways
may
need to be incorporated torealisticallymodel solute transport.
Discretefracture network modelsattemptto include every important fracture in the rock
mass as an
individual
feature in themodel. The models can be eithertwo-dimensional
where the fractures are represented by interconnected
line
elements, or three-dimensional
where the fractures are planar features. There are three-dimensional models available,
based onfinitedifference (e.g. Therrien andSudicky, 1996) orfiniteelement (e.g.
Dershowitzet
al. ,
1995)discretizationschemes,that
allow
for secondary
f low
through
the porousmatrix. Nordqvistet
al .
(1992) presented a three-dimensional model
that
couldsimulate aperture variability
within individual
fractures. The more complex the
model,the smaller the sizeo fthe domain
that
can be modeled, due to computational
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limitations.
The mainadvantageof discrete network models is
that
they include the
physical
structure of the fracture network and can account for fracture connections and
preferential
flowpaths that
may be of
primary
importance, especially in
sparse
networks.
The disadvantage to
these
models is
that
they are more computationally intensive, and
require extensive site characterization data to be any more meaningful than an equivalent
continuum
approximation.
H y b r i d
models have evolved from anattemptto combine the benefits of both equivalent
continuum
and discrete fracture models. There are a variety of approaches used to
combine the physical accuracy of discrete networks
with
the computationaladvantagesof
a
statistical continuum. One approach has been to analyze the fracture network and only
include
the major conducting conduits (e.g. La Pointe et a l , 1995). This approach is
valuable only if it can be assumed
that
the smaller fractures are not important to the
f low
and transport. Another method
uses
small-scale discrete fracture networks to
generate
statistics for
flow
and transport
that
are subsequently applied to a field-scale continuum
model(Schwartz and Smith, 1988; Parney and Smith, 1995). This modeling approach is
asubject ofongoingresearch to try and determine appropriate
links
between small and
large-scale simulations.
One common purpose for modeling subsurface
flow
from a potential contamination
source is to aid in the design ofmonitoringnetworks. A significant amount of research
has been done onoptimizingthe design ofmonitoringnetworks in porous media.
Massmannand Freeze (1987)presenta framework for the design ofmonitoringnetworks
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whichincorporates uncertainty in the selection of a
best
network alternative.
Meyer
et al.
(1994) provide a comprehensive method for
optimizing
monitoring network design by
minimizing
network cost,maximizingthe probability of detection, and
minimizing
the
sizeof the plume at the time of detection. Storck et al.(1997) expanded this work to a
fully three-dimensional analysis for porous media. Jardine et al.(1996) expand upon the
method ofMassmannand Freeze (1987) to provide a decision-analysis framework for the
design of
monitoring
networks in fractured rock, using a two-dimensional discrete
fracture model to evaluate the
best
alternative from a number of
alternate
monitoring
strategies. A comprehensive presentation of
monitoring
network design in a
three-
dimensional
fractured rock
mass
has yet to be provided.
The potential environmental hazards from abandoned
drinking
water wells arewidely
known. Most
o f
these
wells contain a steel casing but through time the casing can
corrode exposing the contacting sediments to the
we l l
bore water. If the
w e l l
crosses a
low
permeability unit, such as a clay aquitard,
there
is the potential for cross
contamination into previously protected aquifers. Figure 2-1 shows a
typical
situation
where the potential for cross contamination exists. Pumping from the lower confined
aquifer cancausecontaminants to migrate downwards from the upper unconfined aquifer
through the abandoned w e l l . The ground water resource in the lower aquifer is polluted
and the potential exists for contaminants to reach the water supplyw e l l .
Many
jurisdictionshave
legislation
requiring landowners to properly grout and decommission
abandoned wells on their property.
I f
the landowner
does
not comply they can be held
liable
for any contamination problems broughtaboutfrom their
wells.
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A l l
of the above studies dealwiththe environmental impacts of abandoned boreholes in a
porous medium or an equivalent
continuum.
No
attempt
has yet been made to
assess
the
impact of abandonedexplorationboreholes in a fractured rock environment using a
discrete fracturemodel. The heterogeneous
nature
of contaminant transport through
fractured rock means
that
equivalent continuum models are not sufficient to properly
address the influence ofleakyboreholes. The present study
uses
a discrete fracture
modelto investigate the impact of abandonedexplorationboreholes on contaminant
transport froma surface waste rockpilethrough a fractured rock mass.
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3.0Methods
3.1
FractureNetworkModeling
In
this study a three-dimensional discrete fracture model was used to model
flow
and
transport through the fracture network. It was necessary to take account of the fracture
scale heterogeneity to properly model the influence of an abandoned borehole in a
fractured rock mass. The influence of
individual
fractures and connected pathways
through the rock matrix are an important
physical
process
that
needed to be included in
the chosenmodel. For this reason, using an averaging method such as the statistical
continuum
methodwouldnot be appropriate. The
F R A C M A N / M A F I C
suite of programs
developed by
Golder
Associates (Dershowitz et
al .
1995) was chosen as it provides
nearly a ll of the required tools,fromfracture network generation to
flow
and contaminant
transport modeling.The mainmodificationrequired was the addition of a routine to
incorporate abandoned boreholes.
F R A C M A N / M A F I C
provides the advantageof
allowing
complex fracture geometry to be included in the analysis. This could
allow
for
a
wide variety of fracture networks to be investigated byfollowingthe method described
in
this thesis.
3.2
Overview
of the
FRACMAN/MAFIC suite
Four
applicationsfromthe
F R A C M A N
suite were used to model the discrete fracture
network and solve the
f low
and transport
problem: F R A C W O R K S , M E S H M O N S T E R ,
E D M E S H ,
and
M A F I C . F R A C W O R K S
is used to
generate
the three-dimensional
fracture networks,
M E S H M O N S T E R
is used to
create
a basic finite-element mesh,
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E D M E S H is used to make appropriate refinements to the mesh, and M A F I C is used to
solve flow
and contaminant transport through the fracture network.
3.2.1 FRACWORKS
F R A C W O R K S allows a wide variety of discrete
features
to be generated from
deterministic or stochastic descriptions. The
base
casefracture geometry for the
simulations
presented in the
following
chapter was generated using a combination of
deterministic fractures to model two majorfeaturesand two stochastic fracture setsfor
the background network. The first
stage
of generating a three-dimensional fracture set is
to choose a generation
model.
The random networks used in this study were generated
using
the Poisson Rectangle model. This is a
simplified
version
o f
the Enhanced Baecher
model
(Dershowitz et
al.,
1989),
with
fracture dimensions specified using length and
width
instead of an effective radius.
The Poisson model
assumes that
the fracture
centers
are randomly distributed in space.
Once
the fracture center has been chosen, the fracture geometry is determined by
specifying
the dimensions and orientation ofthe fracture. Fracture mechanicssuggest
that
the general
shape
of a fracture in homogeneous rock
w i l l
be
e l l ipt ical
(Baecher et al.
1977). The
base case
geometry assumed
square
planar fractures for
simplicity,
although
polygonal
approximations to
e l l ipt ical
fractures are incorporated in the
f inal
simulations
with
minimal
additional computational effort.
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The fracture density is determined by specifying the number of fractures togenerate,and
the sizeo fthe generation region. In the
base
casethe background network consisted of
two
sets
of 750 fractures, generated in the
following
region:
-62.5 m
st
a3
T3
O
co
o
+
LU
o
Csi
CO
O
+
LU
O
o
O
CM
O
CO
O
CD
d
CM CO
O
CO
o
o
CN
O
O
O
+
LU
O
S
bxi
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3 - 9 : Schematicofanalyticalmodel usedto test theeffect of borehole sizeonflowrate andresidence time
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s / c i u )
a j e y w v o y
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4.0 Results
4.1 Base Case Fracture Network
Thebase
case fracture network consists oftwo orthogonal fracture setsand two large
horizontalfractures. The large fractures represent major features ofhighconductivityin
the horizontal plane, such as sedimentary bedding planes or large fissures caused by
erosional unloading. The smaller fractures are randomly sampledfrom twosetsof
statisticalparameters, and represent the background fracture network. Figure 4-1 shows
the fracture
traces
on a cross-section through the center of the modeling domain. The
major features are connected through the background fracture network, but no single
background
fracture is large enough to directly connect the two major features.
Therefore, all
flow
and transport must take place along some indirect pathway through
the network. A single, transmissive fracture
that
connected the two major features would
createa preferential pathway through the network, and producesimilarresults to
includinga borehole in the network. The generation parameters for a llof the fractures in
thebasecase network are provided in Table 4-1.
Parameter
Fracture
set one
Fracture
set two
Generation
Region
125 m x 65 m x 40 m 125 m x 65 m x 40 m
Number
of Fractures 750
750
Fracture
M o d e l Poisson
Rectangle
Poisson
Rectangle
GenerationMode Centers
Centers
TruncationMode Of f Of f
Number
of Sides 4 4
Pole(trace, plunge)
0 ,90
0 ,
0
Poledistribution Constant Constant
Fracture dimension lOmx 10m
lOmx
10m
Transmissivity
lxl0
6
m
2
/s lxl0
6
m
2
/s
Aperture
l x l 0
4
m l x l 0
4
m
Table4-1:Generation parameters forbasecase fracture network
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The
statistics for both
sets
of
background
fractures are
identical,
except for orientation.
The base
case network is meant to represent the simplest possible situation, therefore all
fractures in a given set have the same orientation, aperture, and size. The center of each
fracture is randomly positioned
within
the fracture generationregion. It is possible to
sample any
ofthese
parameters from probabilitydistributions, but constant values are
used in the
base
casesimulationfor
simplicity
and toeasecomparisons
with
later
simulations.
In
the
base
case scenario, both of the large fractures have atransmissivityof
l x l
0
4
m
2
/s
and extend laterally across the entire modeling domain. The transmissivity
of
the major
features is two orders of magnitude higher than
that
of the background fractures
which,
combined
with
their large areal extent, causes them to dominate the
flow
system. The
uppermost feature located at z = 10m is bounded onal lsides by impermeable
boundaries,whilethe lower feature located at z =
-8
m intersects a constant head
boundary on the right-hand side. The contaminant source zone is located at the top
of
the
domainon the left-hand side as shown in Figure4
-2 .
For contaminants to reach the exit
boundary on the right-hand side they must travel at least partly through the background
fracture network to reach the major features.
Most
of the particles representing the
contaminant release
w i l l
travel partly along the major features and partly along the
backgroundfractures to reach the exit boundary, although asmallpercentage w i l l bypass
the major features and travelonlyin the background network.
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4.1.1
Fracture
Generation Region
The generation region for the background fractures is larger than the modeling domain to
avoid
'boundary effects'
within
themodeling
domain.
The generation algorithm
randomly
positions a fracture center
within
the generationregion. Because fracture
centers
w i l l
not be positioned outside the boundaries, a certain amount of fractures
that
overlap the boundaries
w i l l
be
missing,
resulting in a sparser fracture network near the
generation region boundaries.
I f
the generation region is larger than themodelingby the
widthof the largest fracture generated this error w i l lbeeliminated. For this reason the
fractures were generated in a 125 x 65 x 40 meter region for the 100 x 50 x 25 meter
modelingdomain as shown in Figure 4-3.
4.1.2Base
Case
Borehole
For
al l
of
the different fracture networks investigated in this study, the
f low
and transport
modelis first
solved
without any boreholes present, and then the same model is
solved
witha borehole in aspecified location. The resultsfromthe two simulations are then
compared
to determine the effects of the borehole on the fracture networkf lowand
contaminant transport. Theverticalborehole for thebasecase fracture network is located
at x = -25 m and y = 0 m.
A s
can be seen inFigure4-1, the borehole intersects the two major conductive zones at z
=
8 m and -10 m respectively and any background fractures in between.
F l o w
canenter
or
leave the borehole at any intersection, depending on the
l oc a l
hydraulichead
distribution.
In thebasecase
simulation
the flowdown the borehole variesonlyby a
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small
percentagebetween the major features, representing the
limited
hydraulic
interactionof the boreholewiththe background network. I fthe f lowthrough the
boreholedecreasesby 1 after a fracture intersection thenthereis a1 chancethat
particles w i l lleave the borehole at
that
intersection andenterthe background fracture
network. There is a9 9 chance they w i l l continue traveling down the borehole.
The borehole diameter for thissimulationwas 10cm. The mathematical algorithm
allows
for boreholes of any size and orientation to beincludedin the finite-element
model,
as
long
as the
finite
element
grid
is designed to
ensure
an appropriate refinement
around
any borehole-fracture intersections.
A s
explained in Chapter 3,
i f
a borehole's diameter is large it may not act as a fast-
pathway, butrathercan increase contaminant residence times. The large radius borehole
w il l
contain a significant volume of water compared to the
rest of
the fracture network,
and will act likea reservoir or holding-tank in the system. This w i l lbe thecase i fthe
volumeofvoidspace in the borehole is large in comparison to the volume ofvoid space
in therestof the fracture network, and the borehole isonlyconnected to the constant
head boundaries through thethinbackground fractures. Whilea larger borehole radius
w il l
raise theoverallhydraulic conductivityof the fracture network and therefore the total
flow
through the fracture network, the volume of water
w i l l
be much larger and the
residence times
w i l l
increase
significantly. S m a l l
diameter boreholes may
form
previouslyunavailable pathways,
while
not
significantly
increasing the volume of
void
space in the system, thereby decreasing contaminant residence times and increasing
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maximumconcentrations at the downstream boundary. The effect ofusingdifferent
borehole diameters in the
base
case fracture network is investigated later in this chapter.
Figure4-1 shows
that therew i l l
be numerous possible
paths thatw i l l
lead
from
the
source to the exit boundary. However,no
individual
fracture islongenough to connect
the major features, and particles must travel along both
of
the background fracture
sets.
It should be noted
that
atwo-dimensionaltrace plot such as thisw i l lalways appear less
connected than the corresponding three-dimensional network asthereis another
dimension
for connections to be made
in .
This
base
case fracture network is
we l l
connected
with
many possible
paths
for particles to travel along to reach the exit
boundary.
4.1.3
Grid
Discretization
Figure
4-4 shows the finite-element
grid
discretization
of
the lower large
high-
conductivityfracture. Note the further gridrefinement around the borehole location and
the fracture intersections. Thisrefinement was done to reduce model errors where the
hydraulicgradients were l ikelyto be large. The
grid
refinement shown was chosen as the
point
atwhichfurther
grid
refinement did not lead to significant changes in the
flow
or
particle
transport results.
Thislevel
ofdiscretizationresulted in approximately 20000
nodes and 35000 elements for the
base-case
scenario. Some subsequent simulations
with
more complicated fracture networks required close to 100000 elements.
Available
computer memory allowed simulations of up to 300000 elements, but this
level
of
discretizationwas not found to be necessary for the fracture networks considered here.
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A nincrease in the fracture densities overthoseusedherewouldbe approaching the limits
ofa Pentium II, 450
M H z
machine
with
196
M B
ofR A M . A single
base-case
simulation
including
fracture generation,
discretizing,
editing and
solving
the finite element model
takesapproximately 15 minutes on the above machine. Some of the more complex
simulations
took several hours to complete.
It was found
that
approximately 1000 particles were needed to provide a reliable
breakthrough curve at the exit boundary for a single realization of the fracture geometry.
Certain
simulations required significantly more than 1000 particles to be injected at the
source due to some particles becoming stuck in the system.
Using
a constant
concentration at the source zone for all simulationswouldresult in a higher number of
particles entering the system for fracture networks
with
high
f lowrates.
For the purposes
o fthis study the amount of
mass
entering the system for different fracture networks is not
as important as the behaviour of the
mass
once it has entered the network. For this reason
the number
o f
particles traveling through the system was kept constant
rather
than the
source concentration, so concentration values shown on breakthrough curves should be
interpreted in a relativesense. The longitudinal and transverse dispersivities were set at
1.0 m and 0.707 m respectively. Test simulations found
that
most o fthe plume spreading
was due to tortuous pathways in the fracture network and not due to the dispersivity.
Setting
the dispersivities several orders of magnitude lower yielded
similar
results.
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4.1.4 Case1 - Base CaseResults
Table4-2 shows a summary of the results ofa llsimulations described in this chapter,
includingthe
base
case scenario.
Q
s y s
is the total system
f low,
representing the
f low
entering through the upstream constant head boundary and
leaving
through the
downstream boundary.
V b
0 r
is the downward ground water
velocity
in the borehole, and
Qbor
is the
volumetric flowrate
through the borehole. The parameter
% Qb
0 r
is equal to
Qbor /
Qsys
x 100% andrepresentsthe percentage
o f
the total system
f low
thattravels
through the borehole. The T parameters are as defined in Chapter 3.
It can be seen
from
Table 4-2
that
the total
flow
through the fracture network increases by
afactor of3.3from 3.07x10
5
m
3
/s to 1.02xl0
4
m
3
/s when the borehole isincluded, while
the mean contaminant breakthrough timedecreasesby a factor of 7.8. When the borehole
is
present in the
base
case scenario it controls the
flow
system, channeling 87% of the
network
flow
and 98.5%o fthe particles released at the source.
Figure4-5 shows the breakthrough of particles at the exit boundary for the
base
case
simulation
with
and without a borehole.
Both
plots are normalized bydividingthe
number of particles exiting during a specific time period by the total number of particles
travelling
through the network. For instance, the spike in the breakthrough curve for the
case
with
a borehole means
that
9% of the particles exited during
that
particular time
period. Eachtime period is 2000 slong. The curve is smoothed using a
moving
average
o fthe
three
closest time periods.
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T
ii
u
o
a
a
Q
CS
S
3
Vi
u
u
CN
CN
0
S
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The
effects of
adding
a borehole in this scenario can be quite clearly seen on this plot.
The
simulation
with
the borehole has a much higher spike of particles exiting in a short
time period,which
represents
a higher concentration of contaminant than the
case
without
the borehole. Furthermore, contaminants reach the downstream boundary earlier
in
time for the
case
with
the borehole. In fact, most of the particles have already exited
byt = l x l 0
5
s in thecase
with
the borehole, whereas in thecasewithout the borehole the
first
few particles are just beginning to reach the downstream boundary at this time. The
parametersused to quantify theshapeof the breakthrough curve are T io ,T
5
oand
T
9
o. Tio
is
the time when 10% of the particles have exited at the downstream end of the modeling
domain,and
represents
the nose o fthe breakthrough curve.
T 5 0
is the time when
half
o f
the particles have exited and is
identical
to the median of the particle breakthrough times.
T9 0is the time when 90%o fthe particles have exited and
represents
the
tail
o fthe
breakthrough curve. The difference between
T9 0
andT ioquantifies the spread of the
breakthrough curve. A lower spreadwouldcorrespond to a more tightly contained
plume,and higher contaminant concentrations at the downstream boundary, as is seen in
the
base
casescenario
with
the borehole. Theparameters T25 ,T
7 5
, and
T 9 5
are also
included
to give a complete description of the breakthrough curve. Table 4-2 gives the
values of the
parameters
described above for all simulations,
including
the
base
case.
Figure4-6 is a plot of the exit
location
of each particle against its exit time. The vast
majorityof the particles reach the downstream boundarywhile
travelling
along the lower
major conduit, and exit at thesame
longitudinal
and
vertical
location (x = 50m, z = -10
m). For this reason the plot shows the transverse, or y-component o fthe exit
location
vs.
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time.
The time axis islogarithmic.
This
plot shows the temporal and transverse spread
ofthe particles at breakthrough, and where along the y-axis particles show up first.
The
mostapparentdifference between the two scenarios is the order-of-magnitude longer
time for particles to travel through the fracture network when no borehole is
present.
Anotherinterestingfeature
thatcould
not be seen on the breakthrough plots is the
structure of the contaminant plume when a borehole is
present.
It can be seen
from
this
plotthatthe borehole controls the
flow
system in the large fracture, andthatthe
f low
lines
in the fracture are
similar
to
thosethat
occur in a confined aquifer
with
an injection
w e l l . Particles close to the center of the domain tend to exit first as they
fol low
the most
direct pathwayfrom the borehole to the exit boundary. Particles
that
exit further from the
center of the domain take longer to reach the boundary, as they have to travel along a
longer arc, even backwards forpartof the journey. The particles spread laterally almost
across the entire domain. The impermeable boundaries at either side of the domain
interfere
with
the
f low
system and
l imit
the spread o f
particles.
These boundaries
representundisturbed
flow
lines in the regional
flow
system, andwhileitwouldbe
desirable to have the boundaries as far away as possible, computational requirements
l imi tthe sizeo fthe domain.
One
interesting
feature
to
note
for the
case
with
the borehole is
that
the temporal spread
o fthe plume is even less than depicted on the breakthrough curve. The breakthrough
curve for any given lateral
location w i l l
encompass a smaller time spread than when the
breakthrough is averaged across the entire
y-axis. Thismeans that
the breakthrough
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curve is underestimating the maximum concentrations through averaging along the lateral
direction,and concentrations for thecase
with
the borehole
w i l l
be even higher than
already shown.
Figure4-7 shows the pathway a single particletakesfromthe source zone to the exit
boundary.
After
the particle exits the borehole it travels backwards along the x-axis to
the upstream impermeable boundary. As it approaches the boundary the ground water
velocitycarries it outwards in the negative ydirection. Near the lateral boundaries the
velocity
is in the positive x
direction
which
carries the particle towards the exit boundary
again.
The particle exists at y = -24.1 m. Particles
exiting
towards the outskirts
of
the
domain
must travel along a much longer path resulting in a later exit-time for both the
first
and last particles to reach the boundary. Particles
that
leave the borehole on the
downstream side travel
directly
to the downstream boundary under a strong gradient, and
exitthe system at the earliest time. When a particle leaves an element at any intersection,
theprobabilityof it entering any other intersecting element is based on the proportion of
flow
to
that
element. Stream-tube routing is not incorporated in the
M A F I C flow
and
transport code.
Some clustering can be observed in the lateral
exit-location
of particles for the
case
without
the borehole.
This
is due to heterogeneity in the discrete fracture network caused
by the random distribution of fractures in the domain. Althoughall fractures have the
sameequivalent radius andtransmissivity,areas
with
greaterthan average fracture
densities
w i l l
result in higher
l oc a l
permeability and preferential
f low
paths.
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The
following
set of simulations investigates the effect of changing the fracture network
transmissivities,while
keeping the geometrical properties the same. To facilitate
comparison
with
the
base
casescenario thesamerandom-number seed was used to
generate
the fracture network, resulting in the exactsamelocations for the fractures.
4.2 Influenceof Background Fracture Network
4.2.1Case2
Higherhorizontal
transmissivity
The next simulation investigates the effect of anisotropy in the background fracture
network. In thiscasethe transmissivityo fthe horizontal fracture set was raised by two
orders of magnitude to l x l0
4
m
2
/s (corresponding to anapertureof
5 x l 0
4
m),whilethe
vertical
set was left at 1x10 m /s. The transmissivity of the two large fractures was kept
at
l x l 0
4
m
2
/s.
This
alteration to the
base case
scenario
represents
an introduction of
anisotropy into the system. A
typical field
scenario
that
corresponds to this fracture
network is horizontal fractures along consolidated sedimentary bedding planes, connected
by
tighter vertical fractures. Other than the above modifications the
parameters
for this
simulationare thesameas provided in Table 4-1.
This
alteration to the background fracture network raised the
overall
permeability of the
geologicunit andcausesthe
flow
through the system to increase by 64% over the
base
case, to 5.04x10
5
m
3
/s when no borehole is
present.
When the borehole is
present
at x =
-25 m the
flow
through the system is
L l O x l O
4
m
3
/s. The total
f low
through the system is
higher both
with
the borehole and without compared to the
base
case. However, the
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relative
impact of
adding
the borehole in this scenario is reduced. In the
base
casethe
borehole increased the total system
flow
by a factor of
3.3,
whereas in this scenario the
borehole increased the system
flow
by a factor of2.2. The proportion of the total system
flowthat
channeled through the borehole was also less in thiscaseat 77%.
It is expected
that
a
vertical
boreholewouldhave a more significant effect where the
horizontalfracture transmissivities are higher than the
vertical.
A
highly
conductive
vertical
conduit should be more effective in this situation
becausewith
low
vertical
fracture transmissivities the competing preferential
flow
paths
through the background
network are at a disadvantage. A n yconnected flow-path through the background
network must include someverticallow-transmissivityfractures. The reason
that
the
borehole has less of a relative effect on the system
flow
than the
base
caseis not the
anisotropy but the overallincrease in background networkconductivity. The significance
ofthe borehole relies upon the fact
that
it connects two relatively hightransmissivity
fractures and
creates
a preferential pathway
that
did not exist before. The
greater
the
contrast between the background transmissivity and the major
features
the
greater
an
impact the borehole can be expected to have.
This
relationship is investigated later in this
chapter.
Figure
4-9 shows the breakthrough of particles at the downstream boundary for Case 2.
The
breakthrough time
parameters
are provided in Table 4-2.
Generally,
the behaviour of
these
curves closely resembles the breakthrough curves for the
base
case. The borehole
has the effect of
allowing
contaminants to reach the downstream boundary sooner and at
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muchhigher concentrations. The breakthrough curve for thesimulationwithout the
borehole shows
that
the particle breakthrough is dispersed over a much longer time
frame, but
with
lower concentrations.
Morespecifically,
although the borehole was
shown
to have less of an effect on the system
flow
in thissimulationvs. the
base
case, the
borehole has agreatereffect on the contaminant breakthrough as measured by the median
residence time, T 5 0 T 5 0 decreasesby a factor of 8.5 in Case 2
with
the inclusionof the
borehole, butonlyby a factor of 7.8 in the
base
case. Thisresult is more significant
whentaken in conjunction
with
the borehole's decreased effect on the
f low
in Case 2.
The
reason for this increased borehole effect on transport in Case 2 is the longer
residence times for Case 2 without the borehole. It can be seenfromTable 4-2
that
Q
s y s
and the breakthrough times aresimilarfor Case 2 and thebase casewhen the borehole is
present. Thisis expected,becausethe higher transmissivity horizontal fractures
w i l l
not
have a large effect when the borehole ispresent. The majority of the system
f low wil l
take place through the borehole, the major features, and through the
vertical
background
fractures
from
the source zone to the upper major feature. 97.5%
o f
the particles released
from
the source zone travel along this path through the borehole. However, except for
the
ini t ial
breakthrough, the breakthrough times are longer for Case 2 when no borehole
ispresent. Thisis due to theoverallvolume increase caused by the largeraperture
horizontalfractures being introduced. Increasing the fracture transmissivity by two
orders of magnitude results in an increase in fracture volume of 4.64 according to the
cubiclaw (Tocb ). The total systemconductivityincreased by a factor of 1.6 as
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4.2.2Case3 -Higher
Vertical
Transmissivity
In
this
case
the transmissivity
o f
the vertical fracture set was raised by two orders of
magnitude over the
base
caseto
l x l 0
4
m
2
/s, while the horizontal set was left at
l x l O
6
2
* 4 2
m
/s. The transmissivity
o f
the two large fractures was maintained at 1x10 m /s.
Increasing the transmissivity of the vertical fracture set had a larger effect on the
f low
system than increasing the horizontal transmissivities in Case 2. Without the borehole,
the total
f low
through the system increased by a factor of 4.8 to 1.47x10
4
m
3
/s. The
reason for this is
that
in the
base case
the vertical background fracture set
represents
the
longest constriction in the major flow-path.
Most
of
the horizontal distance from the
source zone to the downstream constant-head boundary can be traversed along one of the
major conductive features. To reach the lowerfeatureand the exit boundary, however,
water must travel along the vertical background fractures from the source zone to the
upperfeatureand again between the upper and lower features. These vertical fractures
act as the bottleneck in the
base
case, and increasing their transmissivities by two orders
ofmagnitude opens this bottleneck significantly.
When
the vertical borehole was included in the system the total
f low
was 2.24x10
4
m
3
/s,
or
2.2 times the
base
casescenario
with
the borehole. The overall effect of the borehole
on the
f low
rate
in Case 3 was therefore an increase by a factor of 1.5 (relative to Case 3
with
no borehole),which
means
the borehole had a less significant effect
here
than in
Case 2 or the
base
case. The
percentage
of the
flow
through the borehole was also less in
this
case
at 55%. The reason for this is the overall increase in the hydraulic conductivity
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ofthe background fracture network,whichmakes the preferential pathway created by the
borehole less significant.
It can be seen by Figure 4-11 though,
that
the borehole is stillcausing a large spike in the
particlebreakthrough curve early in time. In fact, in Table 4-2 it can be seen
that
the
earlybreakthrough
with
the borehole is even faster than
that
of Case 1. However, the
effect of the borehole on contaminant transport, measured by the change in the median
particle
residence time, is
only
a factor of
3.8. This
is due to faster particle transport for
Case 3 without the borehole compared to the
base
case without the borehole. The
proportion
of particles
travelling
through the borehole is reduced to 89%, because in this
situationthere
are several conductive alternate pathways.
This
effects the
tai l
of the
breakthrough curve for the case
with
the borehole, as particles traveling through the
background
network pathways stilltake longer than those traveling through the borehole.
Figure4-12 shows the lateral breakthrough distribution for Case 3. On this plot it is
evident
that
the borehole is having a less significant impact on contaminant transport than
inthe
base
case. For the case
with
the borehole the lateral distribution is less structured
due to the larger percentage of particles
that
bypass the borehole and travel through the
background
fracture network. There is an overlap of
particle
exit times for the
simulationswith
and without the borehole
that
was not seen in the
base
case simulations.
Even
the particles
that
do travel through the borehole exhibit a greater spread in their
residence times in Case 3.
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Fromthe previous two simulations a number of observations can be made. It is obvious
that
a borehole w i l lhave a much more significant effect on the
f low
system whenthereis
less choice of alternative fast-pathways for the
flow
andtransport. For a vertical
borehole, thegreatestimpact w i l loccur when the background network has a lower
verticalhydraulic conductivity, as the borehole w i l lcreatea fast vertical pathway where
none
existed
previously.
As was observed in Case 2, increasing the transmissivity of a
fracture set
does
not necessarily result in faster contaminant
transport,
although it wil l
result in a
greater
hydraulic conductivity and
hence
a
greater
f lowrate. The important
consideration is whether any restrictions along the preferential pathways are increased.
When
a borehole waspresent,increasing the vertical transmissivities resulted in earlier
breakthrough and higher concentrations. The vertical fractures were the bottleneck in the
preferential pathway along the section between the sourceareaand the upper major
feature. Increasing the horizontal transmissivities did not have much of an effect when
the borehole waspresentbecausethe main pathway through the systemdoesnot
extensively sample the horizontal background set.
4.3
Influence
of MajorFeatures
The next set of simulations investigates the effect of changing the properties ofthe two
main
featureslocated at z = 10 m and z = -8 m for thesamebackground network
considered in the
base
case. The first twocaseslook at the effect of increasing and
decreasing the transmissivity of the major fractures, to see how the influence of the
borehole
depends
onthesepathways. The thirdcaseinvestigates the effect of a high
contrast between the major featuresand the background by raising the transmissivity of
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The
contaminant breakthrough curves for Case 4 are
given
in Figure 4-13. The
breakthrough curve for the case
with
the borehole reaches a higher peak and occurs
earlier
in time than in the
base
case. The time difference can be seen more
clearly
looking
at the T ioand T
5
o values in Table 4-2. The breakthrough curve without the
borehole occurs later i n time than in the
base
case, leading to anoverall greater contrast
between the borehole and no-borehole simulations for Case 4. For the no-borehole case
contaminant residence times are longer because the increased hydraulic conductivity
fromthe more transmissive major features does not overcome the larger volume of the
void
space. The background fractures connecting the major features constrict the
flow.
When
a borehole is present theconductivityis much greater due to the new pathway it
forms between the major features.
This
causes contaminant travel-times to be shorter
than in the
base
case. The borehole controls both the
flow
and transport through the
system when it is present, channeling 99%o fthe system
f low
and 98.6% of the particles.
Thelateral breakthrough distribution for Case 4givenin Figure 4-14 shows the extent
that
the borehole controls theshapeo fthe plume at breakthrough. The breakthrough
pattern is very structured and reflects the flow-paths coming out of the borehole in the
lowermajor fracture. Because the borehole contains nearly a ll of the
f low
in the system,
there
is
limited
interference in the pattern due to
localized flow
frombackground
fractures into the major fracture.
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4.3.2Case5 - Thresholdeffect
In
this
case
the transmissivity of the major
features
was lowered
from
the
base case
to the
threshold value where they just begin to have an effect on the system.
When
the major
fracture transmissivity was set at the
same
value as the background fractures
there
was no
flow
through the borehole and all particles traveled down one of the
alternate
pathways to
the exit boundary. The transmissivity was subsequently increased
until
the borehole
began to conduct
f low.
The threshold value for
thus
fracture geometry was T = 1.86xl0
6
m
2
/s.
A tthis point the total
flow
through the system was found to be 8.38x10 mIswithout the
borehole, and 8.63x10
6
m
3
/s
with
the borehole; a difference of3%. The
flow
through the
borehole accounted for 13% of the total system
flow.
It can be seen
from
Figure 4-15
that there
is no appreciable difference in the
breakthrough curve when the borehole is
included
in the system. No particles travel
through the borehole
because
it
does
not
create
a more preferable pathway to
those
already existing in the background fracture network.
Even
though the major
features that
the borehole connects extend across the entire domain and have a transmissivity
that
is
86%higher than theresto fthe fractures, the network is already well-connected and the
pathway created by the borehole is not transmissive enough to channel the particles.
It is interesting to
note that,
when no borehole is
present,
the contaminant residence times
were longer than the
base case
both when the major-features' transmissivity was raised
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T he flow
and transport
parametersfrom
thissimulationare given in Table 4-2. The
f low
through the system is less than the
base case
both
with
and without the borehole. The
relative
effect
of
the borehole on the system
flow
is also less, increasing the
flow
by 13%
compared
with
330% for thebasecase. The borehole stillcontrols 77%o fthe
f low
in
this system. The borehole makes less of an impact in Case 6 than in thebasecase, but it
stillcontrols the fracture
flow
when it ispresent.
T he
breakthrough of contaminants occurs much later in time when compared to the
base
case.
T 5 0
for Case 6 is 9.22x10
5
s compared
with
2.48x10
5
s for the
base
case. As can be
seen in Figure 4-16, the borehole stillhas the effect of shortening the particle residence
times in the fracture network, but the breakthrough is more dispersed in time and the
concentrations are lower. The borehole had theoveralleffect of accelerating the median
breakthrough by a factor of 1.7 in Case 6, compared
with
a factor of 7.8 in the
base
case.
This
caseis representative of any situation inwhichtwo or more relatively largefeatures
are connected by a borehole
within
the fracture network. When the
features
do not
form
part
of any highconductivitypathway to the source or sink the borehole has some effect
on
contaminant transport, but it is substantially
diminished.
The plot of the lateral breakthroughdistributionfor Case 6 is given in Figure 4-17. The
particles have to
f ind
their way to the exit boundary through the background network
along
discrete fractures, making the lateral distribution of
particle
exit locations more
clustered. In the
base case
the locations were spread across the entire width of the
domain,
making the plume easier to
detect.
In this
case
clustering leaves
gaps
in the
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plume,
which could
allow
contaminants to travel
past
a discrete monitoring location
undetected. The particles exit insimilarlocations for the case
with
the borehole and
without,
but the borehole causes the particles to exit earlier in time.
4.4
Influence
of
Borehole
DiameterandLocation
Thenext group ofcasesconsiders the effect ofchangingthe borehole properties for the
base
case scenario. The effect of both the borehole's location and its diameter on the
flow
and transport through the fracture network are investigated. Because the fracture
network used in this section is
identical
to the
base
case, the
f low
and transport properties
without
the borehole are the same as the
base
case. For this reason
only
the results
with
the borehole are compared to the
base
case scenario when the borehole was located at x =
-
25 m, y = 0 m
with
a diameter of 10 cm.
4.4.1Case -Boreholeatx = 0m
Inthebasecase scenario the borehole was located at x = -25 mwhichisonly5meters
downstreamfrom the upper constant-head boundary and 15metersdownstream of the
contaminant source zone. In this case the borehole was moved 25metersfurther
downstreamfromthe
flow
and contaminant source zones to x = 0 m. The results of the
simulationare summarized in Table 4-2.
Positioning
the borehole further downstream had verylittleeffect on the groundwater
flow
through the fracture network. The total
flow
through the system and the
f low
through the borehole are essentially the same as the
base
case.
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Figure4-18 shows the breakthrough of contaminants at the downstream boundary for
Case 7 plotted against the
base
caseresults. It can be seenfromthis plot
that there
is no
significantdifference in the temporal breakthrough across the downstream boundary
whenthe borehole is located farther awayfromthe source zone. The geometry o fthe
systemmeans
that
thereis no change in the length orconductivity o fthe main pathway
when
the borehole is moved downstream along the centerline. There is a
slightly
lower
peak concentration
with
the borehole at x = 0 m and a longer
tail
to the breakthrough
curve. Even
though the proportion of
flow
through the borehole is the
same
as the
base
case, a higherpercentageof particles bypass the borehole and travel through the
background
network when it is located further away. In thiscase93.9% of the particles
traveled through the borehole compared
with
98.5% in the
base
case.
Figure4-19 shows the lateral breakthrough distribution for Case 7. The main difference
between this plot and the
base case
is
that
the particles are spread out more through time
when
the borehole is
present.
The front of the plume
reaches
the downstream boundary
at the
same
time, but the
tail
is more spread out along the entire widtho fthe domain.
4.4.2Case8-Boreholeatx = -35 m
Thiscaseinvestigates the effect ofmovingthe borehole 10m closer to the contaminant
source zone to x = -35 m. The constant head boundary along the top of the domain
extends
from
x = -50 to -30 m, so in thiscasethe top o fthe borehole is actually
intersecting the constant-head boundary. Based on the results of
moving
the borehole
further away in the previous case,
simply moving
the borehole closer
would only
be
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expected to raise peak concentrations somewhat, and not effect breakthrough times.
However,becausethe
high
conductivityconduit now intersects a constant head
boundary, the
flow
system and contaminant transport are
significantly
altered.
In
thissimulation92% of the groundwater
flowenters
the domain through the borehole.
The
borehole connects the upper constant head boundary to the lower major feature,
whichin turn is connected to the downstream constant head boundary. This
creates
a
highconductivitypathway through the system,significantlyincreasing the total system
flow
in this case. The system
flow
with
the borehole is
2.26xl0
4
m
3
/s for this scenario
compared to 1.02xl0
4
m
3
/s for thebasecase.
In
previous examples the borehole acted as a connection between two fractures, so
that
flow
entered the borehole at one fracture intersection and traveled under a constant
gradient to another fracture intersection. In the
present
scenario, the borehole is
connected to a constant-head boundary, andactsas a source of groundwater to allofthe
surrounding
intersecting fractures. The hydraulic head in the borehole is higher than
that
inthe surrounding fracture elements, therefore
flow
is out of the borehole at all fracture
intersections and no particles canenterthe borehole.
I f
the borehole intersected the
contaminant source zone then
there
wouldbe a large
influx
of fast
moving
contaminants
travelingalong the major
flow
path, and reaching the downstream boundary at
high
concentrations and at an early time.
This wouldrepresent
a worse
case
scenario but is
unlikely
to occur in practice.
Most
contaminant source
areas,
such as a
tailings
pond at a
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mine
site,wouldbe
lined with
someformof an impermeable barrier, and any known
boreholes in this area
would
be properly sealed
from
groundwater
flow.
T he contaminant breakthrough curve for Case 8 is provided in Figure 4-20. The borehole
in
this
case
actually slows down contaminant transport, as contaminants are pushed away
from
the fast pathway due to the higher
hydraulic
head in the borehole. Particles must
take a longer and more tortuous route through the background network after they are
forcedbackwards and to the lateral
edges
of the domain. The lateral breakthrough
distributionin Figure 4-21 shows evidence
ofthis.
When the borehole ispresentthe
majority ofparticles are stilllocated close to the domain edge, even though the
downstream boundary is 85metersfromthe borehole location.
4.4.3Case9 -
Borehole
aty = 15 m
In thiscasethe effect oflocatingthe borehole offo fthe x-axis is investigated. The
parametersfor thissimulationare thesameas the
base
case, except for the borehole
location which
is now at x = -25 m y = 15 m. The borehole
sti l l
extends
vertically
across
the entire modeling domain.
Figure4-22 shows the breakthrough ofparticles in this scenario averaged across the
entire outflow boundary.
Comparing
this curve to the breakthrough
with
the borehole at
y= 0 m shows
that
the temporal breakthrough issimilarfor both cases. The main
difference caused by the off-center borehole location is shown in the lateral breakthrough
distributionplot, Figure 4-23. It can be seen in this plot
that
the borehole's
position
has
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caused the plume to deflect laterally towards the positive ydirection. The earliest
breakthrough no longer occurs at x = 0 m but approximately at x = 15 m, the lateral
position
o fthe borehole. The impermeable boundary located at y = 25 m is interfering
with
the
flow
system and preventing particlesfrom movingfurther awayfromthex-axis,
and preventing thetrueeffect of the boreholelocationbeing demonstrated. It is clear
from
this plot
that
the borehole controls the contaminant plume exit
location,
and in this
scenario an effective monitoring network design
would
need to
take
into account the
influence
of the borehole. 96.4%o fthe particles travel through the borehole,whichis
less than the
base case
scenario, but
still
enough to influence the downstreamstructureof
the plume. The behaviour described above has implicationsfor the design of detection
monitoringnetworks,whichare discussed in Chapter 5.
4.4.4Case10 -
Influence
ofBoreholeDiameter
Thenext set ofsimulationstestthe influence of the borehole diameter on the contaminant
transportthrough the
base
casenetwork. The borehole is located at x = -25 m, as in the
base
case. The diameter was set at 2 cm, 5 cm, 10 cm
(base
case)and 20 cm, and the
flow
andtransportmodeled. A summary of the results o fthe
individual
simulations is
given
in Table 4-2.
Thecontaminant breakthrough curves for the different borehole diameters are given in
Figure4-24. It can be seenfromthis plot
that
the larger the borehole, the longer
contaminantstaketo reach the downstream boundary. The peak concentration varies
between the different runs, but is l ikelyrelated more to the random variations between
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runs due to the random dispersion process, than to any significant
physical
phenomenon.
The
peak concentration and the
tail
of the breakthrough curve are related to the
proportion ofparticles
that
travel through the borehole, whereas the nose of the
breakthrough curvedependsmore on the borehole properties.
Figure4-25 shows the total system
flowrate
and the mean contaminant breakthrough
time plotted against the borehole diameter. The
f lowrate
varieslinearly
with
the
borehole diameter over this range,whichmeans thereis a linear increase in the total
hydraulic conductivity of
the system. The interesting behaviour shown on this plot is
that
the median contaminant residence time also increases
with
borehole diameter. Intuitively
one
would
expect
that
a more conductive system
would
result in shorter residence times.
However,
while
the residence time is proportional to the
flowrate,
it is inversely
proportional
to the conductingvolume. For the geometry considered here, the increased
volumeof a larger borehole outweighs the increasedconductivity,resulting in longer
residence times. In effect, when
there
are constrictions in the main
f low
path a larger
borehole
w i l l
slowdown contaminant transport and act more
like
a
holding
tank than a
fast conduit.
4.5Case
11 - Variable
Network
with
No Major
Features
In
previous studies of the effect of abandoned boreholes in porous media the borehole
connected two aquifers across a
relatively
impermeable aquitard (Lacombe et
al.,
1995;
A v c i ,
1994). Ahydraulicgradient was established across the aquitard due to either a
pumping
or injection
w e l l
or natural hydraulic head differences between the two aquifers.
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