DRAFT
REVISED DRAFT FINAL REPORT
THEORETICAL EVALUATION OF
SITES LOCATED IN THE ZONE OF SATURATION
Contract No. 68-01-6438
Task No. 012
Prepared for:
Mr. James Bland
Project-Officer (5 AHWM)
U.S. Environmental Protection Agency
Region V
Chicago, Illinois 60604
Prepared by:
Versar Inc.
6850 Versar Center
P.O. Box 1549
Springfield, Virginia 22151
and
JRB Associates
8400 Westpark Drive
McLean, Virginia 22102
Date Prepared:
April 6, 1983
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TABLE OF CONTENTS
1.0 INTRODUCTION 1
2.0 DRAIN THEORY 4
2.1 Drain Spacing and Head Levels 4
2.1.1 Drains On Impervious Barriers 5
2.1.2 Drains Above Impervious Barriers 7
2.1.3 Drains On Sloping Impervious Barriers 11
2.2 Drain Pipe Sizing 13
2.2.1 Hydraulic Gradient (i) and Roughness Coefficient 14
2.2.2 Discharge (Q) 17
2.2.3 Pipe Size 21
2.3 Filters and Envelopes 23
2.3.1 Function of Filters and Envelopes 23
2.3.2 Design of Sand and Gravel Filters 24
2.3.3 Design of Sand and Gravel Envelopes 26
2.3.4 Synthetic Filters 26
3.0 DESIGN AND CONSTRUCTION 28
3.1 Hypothetical Site 28
3.2 Sensitivity Analysis 31
3.2.1 Drains On Impermeable Barriers 31
3.2.2 Drains On Sloping Impervious Barriers 34
3.2.3 Drains Above An Impervious Barrier 40
3.3 Application 40
3.3.1 Parameter Estimation 43
3.3.2 Landfill Size 44
3.3.3 Example Problem 44
3.4 Application 47
3.4.1 Construction Inspection 47
3.4.2 Drain System Maintenance 48
3.4.3 Future Operating Conditions 49
11
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TABLE OF CONTENTS
(Continued)
4.0 FLOW NET ANALYSIS 50
4.1 Introduction 50
4.2 Initial Conditions 52
4.3 Parameter Analysis 55
5.0 MODELS 60
5.1 Release Rate Models 60
5.1.1 Fundamentals 60
5.1.1.1 Leachate Generation 60
5.1.1.2 Leachate Constituent Concentrations. . 66
5.1.1.3 Leachate Release 66
5.1.2 Selected Release Rate Models 69
5.2 Solute Transport Models 77
5.2.1 Fundamentals 77
5.2.1.1 Analytical Models 80
5.2.1.2 Numerical Models 81
5.2.2 Selected Solute Transport Models 82
5.2.2.1 Analytical Models 86
5.2.2.2 Numerical Models 90
5.3 Model Limitations 94
6.0 RECOMMENDATIONS 95
7.0 BIBLIOGRAPHY 97
iii
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LIST OF TABLES
2-1 Values for the Equivalent Depth d of Hooghoudt 10
2-2 Minimum Hydraulic Gradients for Closed Pipes 15
2-3 Drain Grades for Selected Critical Velocities 16
3-1 Range of Site Variables 30
3-2 Drain Length Spacing (m) for Drains on an Impermeable
Barrier 32
3-3 Values of h/L for Various C=q/k^ and Angles a 36
3-4 Drain Spacing (m) for Head Maintenance Levels of 2 Meters 38
3-5 Drain Spacing (m) for Drains Above an Impermeable Barrier 41
3-6 Example Data Set 45
5-1 Major Factors Affecting Leachate Generation 62
5-2 Factors Affecting Leachate Constituent Concentrations . 67
5-3 Factors Affecting Leachate Release 68
5-4 Release Rate Models 70
5-5 Solute Transport Models 83
IV
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LIST OF FIGURES
1-1 Typical Stratigraphic Column of Zone of Saturation
Sites in Wisconsin 2
2-1 Drains Resting on an Impervious Barrier 6
2-2 Flow to Drains Above an Impervious Barrier 8
2-3 Drains on Sloping Impermeable Barrier 12
2-4 Flow Components to a Landfill 18
2-5 Division of a Symmetrical Drawdown Drain Problem Into
Two Equivalent Fragments 20
2-6 Capacity Chart n = 0.013 22
3-1 Hypothetical Zone of Saturation Landfill 29
3-2 Drain Length versus Flow Rates for Head Levels Equal to
1 Meter 33
3-3 Plot of h/L versus h/I = tan o for Drain and Sloping
Impervious Layers 37
3-A Drain Length (L) versus c - q/k^ 39
3-5 Drain Spacing (L) versus Inflow rates (q) 42
4-1 Cross Section of Typical Zone of Saturation Landfill . 51
4-2 Inflow With Typical Landfill Cell 54
4-3 Flow Nets With Different Leachate Levels 56
4-4 Flow Nets With Different Vertical Gradients 57
4-5 Flow Nets With Different Numbers of Drains 58
5-1 Major Components of Groundwater Transport Equations . . 79
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ACKNOWLEDGMENTS
This report was prepared by Dr. Edward Repa, Ms. Kathi Wagner, and
Mr. Rodger Wetzel of JRB Associates, and Mr. Michael Christopher of
Versar Inc., under the direction of Dr. G. Thomas Farmer, Task Manager.
This report was prepared for the Wisconsin Department of Natural
Resources under the U.S. EPA's Technical Assistance Project sponsored by
U.S. EPA, Region V.
VI
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1.0 INTRODUCTION
The State of Wisconsin and much of EPA Region V are located in a
portion of the United States characterized by thick surficial deposits of
saturated glacial till underlain by fractured sedimentary or crystalline
rock. A typical stratigraphic column for Wisconsin is shown in Figure
1-1. Landfill site design in this part of the country is complicated by
the hydrology of the zone of saturation. At these sites, the glacial
clay deposits into which the landfills are built act as limited
unconfined aquifers. Hydraulic conductivities of the clays range from
10~ cm/sec, where the clays are fractured, to 10 cm/sec, where
the clays are unfractured (Newport, 1962). Underlying the clays are
thick units of dolomite and sandstone which act as confined aquifers.
Confining pressures in these units can bring groundwater to the land
surface in drilled wells. Recharge of these underlying units occurs at
or near outcrops.
Siting landfills in this region, especially those which will accept
hazardous wastes, is a problem because the base grade of the facility is
typically below the water table (i.e., in the zone of saturation). As a
result of having the base grade below the groundwater table, the
potential for accelerated leachate generation and contaminate release is
greatly enhanced. To alleviate this problem, landfill operators are
required to manage groundwater and leachate in the landfills so that
inward hydraulic gradients are constantly maintained, thereby limiting
the possibility of contaminant escape. The method utilized to maintain
inward hydraulic gradients is a drainage collection system installed on
the base of the landfill. The drainage system allows for the maintenance
of landfill head levels which are lower than the natural groundwater
table (i.e., inward hydraulic gradients).
-1-
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Geologic
Log
\ \ \ \. \ \
\ v \ \
-\ \ \
\ \ \ \ \
\ \ \ \ V
\ \ \ \ V
\\x\\\
\ \ \ \ \
-'- V
Rock Unit
Glacial deposits
Red Clav
Galena Dolomite and
Platteville Formation
St. Peter Sandstone
Trempealeau Formatior
Franconia Sandstone
Galesville Sandstone
Aauifer
Limited Unconfined
Aquifer
Confined Aquifer,
solution cavities
Confined Aquifer
Vertical Scale 1"=200'
Figure 1-1. Typical Stratigraphic Column of Zone of Saturation
Sites in Wisconsin (City of Fond du Lac) (Newport, 1962)
-2-
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The purpose of this study was to perform a theoretical evaluation of
the validity of the presently used landfill management schemes for
groundwater and leachate at sites located in the zone of saturation.
This evaluation included flow net and parameter sensitivity analyses.
The key parameters that were evaluated include:
o Drain spacing.
o Hydraulic conductivities of the landfill and natural soils
surrounding the site.
o Inflow rates resulting from groundwater infringement and leachate
generation.
o Head maintenance levels within the landfill.
o Pipe sizing.
o Drainage blanket use.
Other parameters addressed include landfill dimensions, construction
inspections, and future operating conditions. Drainage theory and
selected models for predicting release rates and solute transport are
also described.
The results of this study should assist permit writers in determining
engineering design modifications and site monitoring requirements, as
well as aid in establishing a basis for future design protocols for zone
of saturation landfills.
-3-
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2.0 DRAIN THEORY
There are three major elements to consider in the design of a
subsurface drainage system suitable for zone of saturation landfills:
o The drain spacing required to achieve the desired head maintenance
levels.
o The hydraulic design of the conduit, including the pipe diameter
and gradient.
o The properties of the drain filter and envelope.
This section briefly describes the principles involved in determining
a desirable drain slope and spacing, and in selecting appropriate drain
materials.
2.1 Drain Spacing and Head Levels
There are numerous analytical solutions and models that have been
developed for estimating the drain spacing required to maintain head
levels at a predetermined height in saturated media. This section
presents the analytical solutions for determining drain spacing based on
maintenance head levels, permeabilities, and flow rates for:
o Drains resting on an impermeable barrier
o Drains installed above an impermeable barrier
o Drains resting on an impermeable barrier that slopes symmetrically
at an angle to the drains.
The equations presented here assume that steady state conditions
exist, that recharge distribution and leachate generation over the area
between the drains is uniform, and that the soil is homogeneous. Most
real world situations do not fully meet these criteria; therefore, the
results obtained should be considered approximate. In using the
equations for designing a landfill drain system, a conservative approach
-4-
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should be taken to ensure that head maintenance levels are at or below
the desired height.
2.1.1 Drains on Impervious Barriers
Groundwater flow to drains resting on a horizontal (flat) impervious
barrier can be represented (Van Schlifgaarde, 1974; Wesseling, 1973) by
the equation:
L = [(8KDh + 4Kh2)/ql°'5
where:
L = drain spacing (m)
K = hydraulic conductivity of the drained material (m/day)
D = distance between the water level in the drain line and the
impermeable barrier (m)
h = water table height above the drain levels at the midpoint between
two drains (m)
q = leachate generation rate (m/day) [equal to total inflow
3 2
(m /day) divided by landfill area (m )J
Figure 2-1 illustrates the relationship between these terms. When two
parallel drain lines are installed properly, each line causes the
establishment of a drawdown curve that, in theory, will intersect midway
between the two drain lines. In solutions to gravity flow problems, the
distance from the drain to a point where the drawdown can be considered
insignificant (Ah = 0) is equal to half of the drain spacing (L/2).
This distance, L, is commonly referred to as the "zone of influence" of
the drain.
For a pipe drain resting on an impermeable barrier, the parameter D
approximately equals the radius of the pipe and hence can be very small
in comparison to h (the water table height above the drain). Then, since
the term 8KDh becomes very small (=0), equation 1 can be simplified to:
2 05
L = KAKh )/q]
Equation 2 represents horizontal flow to the drains above the drain level.
-5-
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Figure 2-1: Drains Resting On An Impervious Barrier
-6-
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Drain spacing (L) and hydraulic head level (h) in the equations are
interdependent design variables which are a function of the leachate
generation rate (q) and hydraulic conductivity (K) of the drained
material. Assuming a constant leachate generation rate and hydraulic
conductivity, the closer two drains are spaced the more their drawdown
curves will overlap and the lower the hydraulic head levels between the
drains will be. Therefore, in order to space the drains at the required
distance to achieve the desired head maintenance levels, the hydraulic
conductivity of the landfill material and the quantity of leachate
generated must be determined to a reasonable degree of accuracy.
2.1.2 Drains Above Impervious Barriers
Equations 1 and 2 are suitable for estimating drain spacing and head
levels if the drains are located on an impervious barrier, as is the case
with most landfill operations. In using drainage system design
equations, a layer is generally considered impervious if it has a
permeability at least 10 times less than the overlaying layer (i.e.,
K /K >10) . The clay base of a landfill may not act as an
above below
impermeable layer in the design equations if:
o Clays are not adequately compacted to produce the desired
permeability
o Clays are fractured (naturally or during placement)
o Clays are not uniform (e.g., contain sandy zones)
o Landfill material has a permeability comparable to the clay liner.
Where drains are not installed on impermeable barriers, flow to the
drains is radial (as illustrated in Figure 2-2). In this case the drains
are considered to be installed at the interface of a two-layered soil
with hydraulic conductivities of K and K (as shown in Figure 2-2).
Substituting the hydraulic conductivity of the material below the drain
(K ) into the first term of the right hand side of equation 1
2
compensates for radial flow to the drain system, and gives:
2 0.5
L
.
= t(8X2Dh = AiLjh )/q] <3>
-7-
-------
m
Figure 2-2 Radial Flow to Drains Above An Im-
pervious Barrier
-8-
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However, the drain spacing calculated by equation 3 does not take into
account the fact that flow velocity in the vicinity of the drain is
higher than elsewhere in the flow region. If the flow velocity was
uniform, the reduction in hydraulic head caused by the drain would be
distributed evenly around the drain. But, because of the non-uniform
velocity, a proportionally higher loss of head occurs close to the drain,
and the actual water table elevation (h) will stand higher than expected
with drains spaced according to equation 3.
To account for the increase in h caused by radial flow, Hooghoudt
(1940) introduced a reduction of the parameter D to a smaller equivalent
depth, d. The equation that was developed to take into account radial
flow can be rewritten as:
2 05
L = [(8K2dh + A^h )/q] (4)
where the new term d is the equivalent depth (m)
Equation 4 shows the drain spacing L is dependent on the equivalent
depth d. But the value of d is calculated from a specified value for L,
so equation 4 cannot be solved explicitly in terms of L. The use of this
equation as a drain spacing formula involves either a trial and error
procedure of selecting d and L until both sides of the equation are equal
or the use of nomographs which have been developed specifically for
equivalent depth and drain spacing. Table 2-1 gives values of the
equivalent depth (d) as a function of drain spacing (L) and saturated
thickness below the drains (D). This tables show values of d for a drain
pipe with a radius (r ) of 0.1 meter. Similar tables have been
prepared for other values of r . For saturated thicknesses (D) greater
than 10 meters, the equivalent depth can be calculated from drain spacing
using the following equation:
d = 0.057 (L) + 0.845 (5)
This equation was developed by linear regression from the values given in
Table 2-1.
-9-
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7.5
10
15
20
30
50
O
D
0 * ii 0 4 7 048 049 049 049 0 50 050
0.75 0.60 0.65 0.69 0.71 0.73 0.74 0.75 0.75 0.75 0.76 0.76
1.00 0.67 0.75 0.80 0.86 0.89 0.91 0.93 0.94 0.96 0.96 0.96
1.25 0.70 0.82 0.89 1.00 1.05
1.50
1.75
2.00
2.25
1.50
.1.75
3.00
3.25
3.50
3.75
4.00
4.50
5.00
5.50
6.00
7.00
6.00
i.OO
10.00
0.88 0.97 1.11 1.19
0.91 1.02 1.20 1.30
1.08 1.28 1.41
1.13 1.34 1.50
1.38 1.57
1.42 1.63
1.45 1.67
1.48 1.71
1.50 1.75
1.52 1.78
1.81
1.09 1.12 1.13 1.14 1.14 1.15
1.25 1.28 1.31 1.34 1.35 1.36
.39 1.45 1.49 1.52 1.55 1.57
1.5 1.57 1.62 1.66 1.70 1.72
.69 1.69 1.76 1.B1 1.84 1.86
1.69 1.79 1.87 1.94 1.99 2.02
1.76 1.88 1.98 2.05 2.12 2.18
1.83 1.97 2.08 2.16 2.23 2.29
1.88 2.04 2.16 2.26 2.35 2.42
1.93 2.11 2.24 2.35 2.45 2.54
1.97 2.17 2.31 2.44 2.54 2.64
2.02 2.22 2.37 2.51 2.62 2.71
1.85 2.08 2.31 2.50 2.63 2.76 2.87
1.88 2.15 2.38 2.58 2.75 2.89 3.02
0.71 0.93 1.14 1.53 1.89
2.20 2.43 2.65 2.84 3.00 3.15
2.48 2.70 2.92 3.09 3.26
2.54 2.81 3.03 3.24 3.43
2.57 2.85 3.13 3.35 3.56
2.89 3.18 3.43 3.66
1 3.23 3.48 3.74
2.24 2.58 2.91 3.24 3.56 3.88
L — > 50 75
D
0.5 0. 50
1 0.96 0.
2 1.72 1.
3 2.29 2.
4 2.71 3.
5 3.02 3.
6 3.23 3.
7 3.43 4.
8 3.56 4.
9 3.66 4.
10 3.74 4.
12.5
15
17.5
20
25
30
35
40
45
50
60
80
97 0.
80 1.
49 2.
85 90
97 0.
82 1.
52 2.
04 3.08 3.
49 3.
55 3.
85 3.93 4.
14 4.
23 4.
38 4.49 4.
57 4.
70 4.
74 4.89 5.
5.02 5.20 5.
5.
5.
- 3.88 5.
20 5.40 5.
30 5.53 5.
5.62 5.
5.74 5.
100 150 200 250 H
to
cr
97 0.98 0.
82 1.83 1.
54 2.56 2.
12 3.
16 3.
61 3.67 3.
00 4.08 4.
33 4.42 4.
61 4.72 4.
82 4.95 5.
98 0.
0 I-1
P O
O fS)
C 1
99 0.99 0.99 H1 H
85 1.00 1.92 1.94 3 rf
60 2.
&• (D
72 2.70 2.83 a <
W u
24 3.46 3.58 3.66 jtT ft> M
78 4.
rf O e
12 4.31 4.43 C n (0
n u
23 4.70 4.97 5.15 p O
i-f >-i« l-h
62 5.22 5.57 5.81 O r-t> 6'
a. i-t» M
95 5.68 6.13 6.43 ~~ fl>' "
t-t M rrl
23 6.09 6.63 7.00 gr (0 (H"
M* !"1 C^
04 5.18 5.47 6.45 7.09 7.53 O rr H-
38 5.56 5.
60 5.80 6.
92 7.20 8.06 8.68 J3*
M *- ••!
76 5.99 6.44 8.20 9.47 10.4 ™ JJ £
87 6.12 6.60 8.54 9.97 11. 1
11.813.8 2- 5' x^
12.0 13.8 3 „ JJ
in CO O
12.1 14.3 ^*g ^
1 14.6 O 0 n
1 -^ t—O
38 5.76 6.00 6.26 6.82 9.55 12.2 14.7 O
09 II
Source: Wessellng, 1973.
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2.1.3 Drains on Sloping Impervious Barriers
Typically, landfill cells are designed so that the compacted base of
the cell slopes symmetrically at an angle towards the drains. A
cross-section of such a design is shown in Figure 2-3. By designing the
drain system on sloping barriers, the flow of water towards the
collection system is accelerated, thus decreasing the steady-state head
maintenance levels. This allows the drains to be spaced further apart or
the heads to be lowered if the other parameters in the equation are held
constant.
Drain spacing can be calculated for landfills designed with an
impervious layer sloping towards the drains at an angle by the following
equation (Moore, 1980):
L = (2h /c°'5)/[tan2a/c) +1 - (tan a/c) (tan*a + c)°' ] (6)
max
where:
c = q/R (dimensionless)
a = slope angle (degrees)
hmax = maximum allowable head level above impervious layer (m)
Note that because the "peak" formed by the two slopes between the drains
intrudes on the saturated mound between the drains, h is not found
max
directly above this peak (midway between the drains) but at some distance
to either side of this point.
For example, consider a landfill to be constructed in glacial till
with bottom drains constructed of a material with a hydraulic
-4 -6
conductivity (K) of 2 x 10 cm/sec (2 x 10 m/sec). The fill will
have slopes toward the drains of 2% (1.1°), has an estimated leachate
-4
generation rate of 6 inches per year (4.2 x 10 m/day) based on water
calculations, and must maintain the leachate level at a maximum elevation
of 2 m above the drains. The drain spacing is calculated using
equation 6:
-11-
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Infiltration (q)
Figure 2-3' Drains on Sloping Impervious Barriers
-12-
-------
L = (2h /c°'5)/[tan2a/c) +1 - (tan ot/c) (tan2a + c)°'5] (6)
max
where C = q/K = - *.2 * 10 - = 2.43 x
2 x 10 m/sec x 86,400 sec/day
T (2 x 2 m) /TO. 69 x 10~4) , (1.92 x 10~2
L _ - / - — + 1 - - -
(2.43 x 10 ) I [(2.43 x 10 ) (2.43 x 10 )
+ (3.69 x 10~4 + 2.43 x 10~3)°'5
A «tt T O
= - 2-S — — /[(1. 52 x 10 ) + 1 - (7.90) (5.29 x 10~ )]
4.93 x 10
-1
=81.1 m/7.34 x 10
= 110.6 m
These drainage system design equations assume that the drain pipe
will accept the drainage water when it arrives at the drainline and that
the drain pipe will carry away the water without a buildup in pressure.
To meet the second assumption, the pipe size and drain slope must be
adequate to carry away the water after it enters the drain pipe. The
following sections describe the methods utilized to ensure that these
assumptions are valid.
2.2 Drain Pipe Sizing
The design diameter of a drain pipe is dependent on the flow rate,
the hydraulic gradient, and the roughness coefficient of the pipe. The
roughness coefficient, in turn, is a function of the hydraulic resistance
of the drain pipe. The formula for the hydraulic design of a drain pipe
is based on the Manning formula for pipes which is:
QT = (i°-5> A/n (7)
-13-
-------
where:
QT = design discharge (nr/sec)
R = hydraulic radius of the pipe (m) , which is equal to the wetted
cross-sectional area divided by the wetted perimeter, or is
equal to 1/4 of the diameter of a full flowing pipe
i = hydraulic gradient (dimensionless)
A = cross-sectional area of flow (m?)
n = roughness coefficient (dimensionless).
Each of the above factors is described in further detail in the following
sections.
2.2.1 Hydraulic Gradient (i) and Roughness Coefficient (n)
Subsurface drains are generally installed on a gradient (i) that is
sufficient to result in a water velocity within the pipe that will
prevent silting, but is less than the velocity which will cause turbulent
flow. Past experience has shown that non-silting velocities occur above
1.4 feet per second (Soil Conservation Service, 1973). In situations
where silting may be a problem and velocities are less than 1.4 ft/sec,
filters and traps can be utilized to prevent the drains from clogging.
The minimum hydraulic gradients required to prevent siltation in three
sizes of closed pipe are listed in Table 2-2. However, steeper gradients
are generally desirable provided they are less than the gradients which
would result in turbulent flow.
To prevent turbulent flow, the hydraulic gradients should result in
velocities that are less than critical velocities. Table 2-3 gives
critical velocities for various drain sizes, gradients, and roughness
coefficients. For smooth perforated concrete or plastic pipes, roughness
coefficients can be assumed to be equal to 0.013 (Soil Conservation
Service, 1973). Knowing the velocity which results in siltation and that
which results in turbulent flow, the design engineer can select a
gradient which results in a velocity somewhere between the two extremes.
-14-
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TABLE 2-2. MINIMUM HYDRAULIC GRADIENTS FOR CLOSED PIPES
Pipe Diameter Grade
Inches Cm %
4 10.2 0.10
5 12.7 0.07
6 15.2 0.05
Source: Soil Conservation Service, 1973
-15-
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TABLE 2-3. DRAIN GRADES FOR SELECTED CRITICAL VELOCITIES
Drain Size
laches
,. , VELOCITY
1.4 ips^1' 3.5 fps 5.0 fps 6.0 fps 7.0 fps 9.0 fps
Grade - feet per 100 feet
For drains with
Clay Tile, Concrete Tile,
4
5
6
8
10
12
Clay Tile,
4
5
6
3
10
12
.28
.21
.17
.11
.08
.07
1.8
1.3
1.0
0.7
0.5
0.4
For drains
Concrete Tile, and
.41
.31
.24
.17
.12
.09
2.5
1.9
1.5
1.0
.8
.6
For drains
and Concrete Pipe (with good alignment)
3.6
2.7
2.1
1.4
1.1
0.8
with "n"
Concrete
5.2
3.9
3.1
2.1
1.6
1.2
with "n"
Corrugated Plastic
4
5
6
8
10
12
.53
.40
.32
.21
.16
.13
3.3
2.5
2.0
1.3
1.0
.8
6.3
5.1
4.0
2.7
2.0
1.6
5.1
3.9
3.1
2.1
1.5
1:2
- 0.013
Pipe (with
7.5
5.6
4.4
3.0
2.2
1.8
- 0.015
Pipe
9.8
7.3
5.8
3.9
2.9
2.3
7.0
5.3
4.1
2.8
2.1
1.6
11.5
8.7
6.9
4.6
3.5
2.7
fair alignment)
10.2
7.7
6.0
4.1
3.0
2.4
13.3
9.9
7.9
5.3
4.0
3.1
16.8
12.7
10.0
6.8
5.0
3.9
21.9
16.6
13.2
8.8
6.6
5.1
(1)—Feet per second
(2)—"n" is the roughness coefficient
Source: Soil Conservation Service, 1973.
-16-
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2.2.2 Discharge (0)
The design discharge of a pipe, Q is equal to the sum of the
individual discharges which impinge upon the drain. Figure 2-4 shows the
various flow components that could contribute to a drain discharge within
a landfill. These flow components can be broken into two major
categories—flow from within the site and flow from the surrounding
aquifer.
In most instances the flow rates are estimated for design purposes so
that the drain spacing can be determined using the previously presented
equations. Estimates of discharge can be obtained using two simplified
methods—the water balance method and the method of fragments. The water
balance method is used to calculate the amount of the percolation that
can recharge the water table between the lines of drains. This flow must
be removed to maintain steady state conditions. A simple water balance
equation is as follows:
qp = P-RO-ST-ET
where:
qp = percolation rate: amount of water that must be removed by
drainage system (m/day)
P = precipitation (m/day)
RO = surface water runoff (m/day)
ST = change in soil (refuse) moisture storage (m/day)
ET = evapotranspiration rate (m/day)
Once the percolation rate has been calculated, discharge can be obtained
by multiplying the percolation rate by the drainage area (i.e., Q
o *)
(m/day) = q (m/day) Area (m )).
When using the water balance method to calculate flow rates or
discharges for landfills, the following points should be considered:
-17-
-------
I
Lateral flow
in waste
Lateral
flow in
rounding materi
Figure 2-4s Flow Components to a Zone of Saturation Landfill
-------
o Precipitation values for those time periods with high intensity
rainfalls should be used to ensure percolation values are
maximized and drainage design is adequate to handle these
discharges.
o Soil (refuse) moisture storage changes can be significant as new
refuse is placed into the landfill; once field capacity of the
materials is attained the SI term can be considered zero.
o Variations in cover depths and the absence of vegetation can have
significant effects on percolation rates; these effects will
probably be greatest during the active operational phase when
shallow covers are presented and drainage is inadequate.
The method of fragments is used to calculate the flow rates that are
derived from the aquifer that borders the outermost drains. This flow
component is not considered in the drain spacing equations but could
significantly affect the pipe sizing of the border drain. Discharge of
the border drains can be derived exclusively from horizontal flow or
through a combination of horizontal and radial flow. Figure 2-5 shows
the division of the border drain flow into two fragments that can be
calculated separately and summed.
The quantity of flow into fragment 1 (Figure 2-5) can be estimated
(Moulton, 1979) from the equation:
Q_, = K(h2)x/2(R.-b) (9)
Gl u
where:
discharge (m3/day)
K = hydraulic conductivity (m/day)
h = height of the water table above the drain (m)
x = length of the drain (m)
Rjj= distance of the drain's influence (m)
b = half the width of the drain and the trench (m).
-19-
-------
i
i
i
Impervious Boundary
Fragment No. i
- — I
Impervious
Fragment No. 2
H
D i q2
Impervious
Figure 2-5* Division of Symmetrical Drawdown Drain Problem into Two
Equivalent Fragments (Moulton, \379)
_Rd_/2_
^-2b
-------
In order to solve the equation, the value of R. must be known or
a
estimated. Typically, the value of R, is estimated using an equation
d
such as the Sichardt (1940) equation:
R. = C(h)(K)°'5 (10)
a
where h and K are as specified as above, and C is a constant that
Sichardt set equal to 3; however, most drainage engineering books now
recommend this constant be set at 1.5-2 for "realistic" results.
The quantity of flow into fragment 2 can be estimated (Moulton, 1979)
from the equation:
<> = (K(H-D)x)/[R./D)-(l/ff)(log 0.5(sinh w/D)] (11)
G2 d
where the new terms are:
H = height of water table above impervious barrier (m)
D = height of the drain above impervious barrier (m)
Once the individual discharges for the segments are calculated, the total
discharge is the sum of the individual discharges. For sites on
impervious barriers, the total discharge is equal to Q ; for sites
with drains above an impermeable barrier the total discharge is the sum
of QG1 plus QG2.
2.2.3 Pipe Size
Once the total discharge (Q ) has been determined, an appropriate
grade selected, and the appropriate roughness coefficient determined, the
minimum drain diameter can be determined. Nomographs such as the one
shown in Figure 2-6 are typically utilized to obtain pipe diameters.
Because the nomographs are based on the Manning formula (Equation 7),
this formula can be used directly to obtain pipe size. Rearranging
Equation 7, pipe diameter (d) can be found from:
0.5 1.5
d = 4(Q n/i )
-21-
-------
i;
' •
(I
I
I >
I
,•„
'•!
n
c I
I'
•
M
o
i
H-
II'J
s
M
n
i i
i
V.
' .
n
H-
• i
i i
I
H
II
•
• '
l '
i .,
-------
A margin of safety is usually incorporated in the selected drain
diameter which will account for the reduction in drain capacity caused by
siltation over time and for any discharge in excess of the design
capacity. Because the nomographs and Manning formula estimate drain
diameter without accounting for a margin of safety, the drain diameter is
typically chosen as the next larger size.
2.3 Filters and Envelopes
Performance of a drainage system is based on the assumption that the
pipe will accept all inflow without a pressure build-up. Filters and
envelopes are used to ensure that this requirement is met.
2.3.1 Function of Filters and Envelopes
The primary function of a filter is to prevent soil particles from
entering and clogging the drain. The function of an envelope is to
improve water flow into the drains by providing a material that has a
higher permeability than the surrounding soil. Envelopes may also be
used to provide suitable bedding for a drain and to stabilize the soil
material on which the drain is being placed. The filter's function and
the envelope's function are somewhat contradictory because filtering is
best accomplished by fine materials, and coarse materials are more
appropriate for envelopes.
As percolating water approaches a subsurface drain, the flow velocity
increases as a result of convergence towards the perforations or joints
in the pipe. This increase in velocity is accompanied by an increased
change in hydraulic gradient. As a result, the potential for soil
particles to move towards the drain is increased. By using a highly
permeable envelope material around the pipe, the number of pore
connections at the boundary between the soil and the envelope will
increase, thereby decreasing the hydraulic gradient.
A filter should prevent the entry of soil particles, which could
result in sedimentation and clogging of the drains, blocking of
-23-
-------
perforation or tile joints, or blocking of the envelope. The filter
materials should not, however, be so fine that they prevent all soil
particles from passing through. If silts and clays are not permitted to
pass through, they may clog the envelope resulting in increased entrance
resistance which can cause the water level to rise above the drain.
Although filters and envelopes have different distinct functions, it
is possible to meet the requirements of both a filter and an envelope by
using well-graded sands and gravels. The specifications for granular
filters, however, are more rigid that those for envelopes. It is usually
necessary for filter materials to be screened and graded to develop the
desired gradation curves. Envelope materials, on the other hand, may
have a wide range of allowable sizes and gradings (Soil Conservation
Service, 1973).
2.3.2 Design of Sand and Gravel Filters
Detailed design procedures are available for both gravel and sand
envelopes. The Soil Conservation Service (SCS) (1973) has distinct
design criteria for filters and envelopes, whereas the Bureau of
Reclamation (1979) has developed one set of standards for a well-graded
envelope which meets the requirements of both a filter and an envelope.
The separate SCS design criteria will be considered below for the
following reasons:
o Site specific conditions may warrant the use of only a filter or
an envelope, but not both.
o Where both a filter and an envelope are needed, the SCS design
criteria for a filter can generally be used.
o It may be desirable to use a fabric filter with a gravel envelope.
The approach recommended by SCS is to determine whether the drainage
system needs a filter and then determine the need for an envelope.
Generally, this sequence is performed because a well-graded filter can
also function as an envelope.
-24-
-------
The general procedure for designing a gravel filter is to (1) make a
mechanical analysis of both the soil and the proposed filter material;
(2) compare the two particle distribution curves; and (3) decide by some
set of criteria whether the envelope is satisfactory. The Corps of
Engineers and the Soil Conservation Service (1973) have adopted similar
criteria which set size limits for a filter material based on the size of
the base material. These limits are:
50 percent grain size of the filter
_ ~ ,;:,. .. r = 12 to 58
50 percent grain size of the base
15 percent grain size of the filter
— ~ = 12 to «*0
15 percent grain size of the base
Multiplying the 50 percent grain size of the base material by 12 and 58
gives the limits within which the 50 percent grain size of the filter
should fall. Multiplying the 15 percent grain size of the base material
by 12 and 40 gives the limits within which the 15 percent grain size of
the filter should fall.
All of the filter material should pass the 1.5 inch sieve, 90 percent
of the material should pass the 0.76-inch sieve, and not more than
10 percent of the material should pass the No. 60 sieve. The maximum
size limitation aids in preventing damage to drains during placement, and
the minimum size limitation aids in preventing an excess of fines in the
filter which can clog the drain. When the filter and base materials are
more or less uniformly graded, a generally safe filter stability ratio of
less than 5 is recommended.
15 percent filter grain size _ 12 fcQ 5g
85 percent filter grain size
Consideration must also be given to the relationship between the
grain size of the filter and the diameter of the perforations in the
pipe. In general, the 85 percent grain size of the filter should be no
smaller than one-half the diameter of the perforations. SCS recommends a
minimum filter thickness of 8 cm (3 inches) or more for sand and gravel
envelopes (Soil Conservation Service, 1973).
-25-
-------
2.3.3 Design of Sand and Gravel Envelopes
The first requirements of sand and gravel envelopes is that the
envelope have a permeability higher than that of the base material. SCS
(1973) generally recommends that all of the envelope material should pass
the 1.5-inch sieve, 90 percent should pass the 0.75-inch sieve, and not
more than 10 percent should pass the No. 60 sieve (0.25 millimeter).
This minimum limitation is the same for filter materials; however, the
gradation of the envelope is not important since it is not designed to
act as a filter.
The optimum thickness of envelope materials has been a subject of
considerable debate. Theoretically, by increasing the effective diameter
of a pipe, the amount of inflow is increased. If the permeable envelope
is considered to be an extension of the pipe, then the larger the
envelope's thickness the better. There are, however, practical
limitations to increasing envelope thickness. The perimeter of the
envelope through which flow occurs increases as the first power of the
diameter of the envelope, while the amount of the envelope material
required increases as the square of the diameter. Doubling the diameter
of the envelope (and consequently decreasing the inflow velocity at the
soil-envelope interface by half) would require four times the volume of
envelope material with an attendant increase in costs. Recommendations
for drain envelope thickness have been made by various agencies. The
Bureau of Reclamation (1978) recommends a minimum thickness of
10 centimeters (4-inch) around the pipe. SCS (1973) recommends an
8 centimeter (3-inch) minimum thickness.
2.3.4 Synthetic Filters
For synthetic materials, the suitability of a filter can be
determined from the ratio of the particle size distribution to the pore
size of the fabric. The accepted design criterion for geotextile filters
is:
-26-
-------
P85 (85% pore size of the filter fabric)
D85 (85% grain size of the subgrade material) ~
or P85 < D85
Using this equation, the P85 of the filter fabric can be determined from
the D85 of the subgrade soil. Manufacturers of geotextile fabrics can
then be consulted to select the proper filter type (DuPont Co., 1981).
-27-
-------
3.0 DESIGN AND CONSTRUCTION
This chapter presents an analysis of a hypothetical zone of
saturation landfill site that is based on data provided by the State of
Wisconsin. A sensitivity analysis was performed for the site (based on
the equations presented in Chapter 2) using the following parameters:
o Groundwater/leachate generation rates, q
o Hydraulic conductivity of waste materials, K^ and native soils,
K2
o Head maintenance levels within the site, h
o Drain spacing, L
Also included in this chapter are design and construction considerations
which may be incorporated into a zone of saturation landfill.
3.1 Hypothetical Site
A hypothetical zone of saturation landfill site was developed using
available data from similar sites located in Wisconsin. A cross-section
of this site is presented in Figure 3-1. Table 3-1 presents some of the
typical ranges of values that may be encountered at similar sites. The
hypothetical site and the accompanying site data are used in the next
section as the basis for the sensitivity analysis. Assumptions made to
simplify the site conditions include:
o Landfill materials and soils are homogeneous and isotropic.
o Water tables within and outside the site are drawn down to drain
level.
o Groundwater system is at equilibrium (steady state) conditions.
o Groundwater pressures in the underlying (dolomite) aquifer do not
affect groundwater movement into the bottom of the pit.
-28-
-------
lo
-r>
i
*-
Final Clay Cover
Landfill Material (K,=IO"3to id"5)
Base grades = 1 %
4^
^IV^pvc^pjpe^1^
„
Original-^
# water table
f
in pea gravel
-25ft
•Recompacted Clay
Clay (K?=ICr5tolO~7)
125 250ft
Dolomite
X X
X X X X X XXX X X
X X X X X X XX XXXXXXXXXXXX X XXX X
Figure 3-1= Cross Section of Hypothetical Zone of Saturation Landfill
-------
TABLE 3-1. RANGE OF SITE VARIABLES
Parameter
Range of Values
Cell Dimensions
o Depth of cell
o Thickness of compacted clay
beneath drains
o Thickness of compacted clay
sidewalls
o Thickness of clay till below
base grade
o Drain spacing
Hydraulic Data
o Depth to water table
o Permeability of:
Clay till
Refuse
Compacted clay
o Drain diameters
o Slope of base
20 to 30 feet (average) up to
60 feet
3 to 5 feet
5 feet (average)
20 to 30 feet (minimum) usually
greater than 50 feet
200 to 400 feet
10 feet (average)
10~5 to 10~7 cm/sec
10~2 to 10~5 cm/sec
< 10~7 cm/sec
to 6 inches
(average)
-30-
-------
If these assumptions are invalid, the drain equations will predict drain
spacings that are too large. The degree to which these equations
over-predict will be directly related to the degree the assumptions are
invalid.
3.2 Sensitivity Analyses
Sensitivity analyses were performed on the drain equations for:
o Drains on impervious barriers
o Drains on sloping impervious barriers
o Drains above impervious barriers.
The variables in the sensitivity analyses include head maintenance
levels, hydraulic conductivities, flow rates, drain spacing and barrier
slope.
3.2.1 Drains on Impervious Barriers
The equation for drains placed on an impervious barrier is
(Equation 2, Chapter 2):
L = (AK^/q)0'5 (2)
When using this equation, a barrier is usually considered impervious if
the barrier material has a permeability (K ) at least 10 times less
than the overlying material (K ). This situation typically occurs in
landfills that have recompacted clay bases.
Table 3-2 presents the results obtained when equation 2 is solved for
drain spacing (L) using various values of flow rates (q), head
maintenance levels (h), and hydraulic conductivities (K ) of the
landfill material. A log-log plot of drain length versus flow rate for
four hydraulic conductivities (holding the head level constant at one
meter) is shown in Figure 3-2. Table 3-2 and Figure 3-2 show how the
proper drain spacing (L) is directly related to head maintenance levels
(h), inversely related to the square root of the leachate generation rate
(q), and directly related to the square root of the landfill permeability
-31-
-------
TABLE 3-2. DRAIN LENGTH SPACING (m) FOR DRAINS ON AN IMPERMEABLE BARRIER
1
U)
N)
1
~
4
(•/day)
0.5
0.01
0.005
0.001
0.0005
0.0001
0.00005
io2
157
353
1115
1577
35?7
4989
,o3
50
III
157
353
499
1115
1577
ID'*
•6
35
1 50
1
:>»
i
157
*
353
499
io5
1
5 1
L
II
16
35
50
III
1
157
,o-'
105
235
333
744
105?
235?
3326
h
,o-3
33
1
| 105
1
235
333
744
1052
-4(.J
• o-»
II
23
33
(
i "
|
1 105
I
235
333
h.!
1.)
h-l(')
h-0.5(l)
Mc./itc)
,o-5
3
7
II
23
33
.
1 "
1
1 105
1
io-2 ,
1
1 52
1
j_ "8
166 |
1
371 *
525
1174
1661
17
37
52
118
166
371
525
io-*
5
1?
1?
37
t
1 5?
1
1 118
1
166
io"5 io 2 io"3 io* io 5 io 2 io~3 io*
2
li
5
12
17
37
-1
1 52
1
26
59
83
L___,
186 |
1
263 I
1
588 1
L
831
—
8
19
26
59
83
186
1
263 1
l_
3
6
8
19
26
59
83
_ _
1
2
3
6
13
29
42
93
8 131
1 ,
19 Z94 1
26 416 .
. . - "~
4 1
9 3
13 4
29 9
42 13
93 29
31 42
— _
,o5
> 1
I
1
3
*
9
13
-.-. -
Isobar equal to 122 liters (400 ft.): »a
-------
10001
3 4 367891
2 3 4567891
2 3 4S67891
t.
. i
1001
(I
7
6
I .-:
I
I i
I'M
I
2 3 4567891
0.00005 0.0001 0.0005 .001 0.005 0.01
(q(m/day)
Figure 3-2. Drain Length versus Flow Rates for Head Levels Equal to 1-meter
-------
Leachate generation rate (q) and landfill permeability (1^) are the
most important parameters to determine accurately when designing a
landfill, not because they are the most sensitive, but because they are
the most difficult to determine accurately. Head maintenance levels are
usually predetermined in the landfill design and therefore are not
sensitive even though they potentially can have the greatest effect on
drain spacing.
One aspect of the equation for drains on impervious barriers that is
not readily apparent is that this equation can also be used for
determining the depth of drainage blankets. When the equation is
utilized for this purpose, the height of the water table is designed to
remain within the blanket. Permeabilities of the drainage blanket
materials are also generally known with some accuracy, which is not the
case with most landfill materials. Table 3-2 also shows the combination
of flow rates, hydraulic conductivities, and head maintenance levels that
yield drain spacings that are equal to or less than the maximum spacing
used in the hypothetical site (i.e., 400 ft). Generally, when flow rates
are large and hydraulic conductivities are low, the theoretical upper
limit of 400 feet on drain spacing is too large to accomplish the
intended design. It is, therefore, important to quantify the values
associated with leachate generation rates and landfill permeabilities to
determine drain spacing; drain spacing should not be specified
arbitrarily.
3.2.2 Drains on Sloping Impervious Barriers
Drains that are placed on sloping impervious barriers are governed by:
L = (2h/c°'5)/[(tan2a/c) + 1 - (tan a/cMtan a+c) ' ] (6)
As discussed in the previous section, the underlying drain material is
typically considered impermeable if its permeability is at least ten
times less than the permeability of the overlying material. This
situation is typical of landfills that have compacted clay bases.
-34-
-------
Table 3-3 gives solved values of h/L for selected values of c = q/K
and barrier slope angles, a. Figure 3-3 presents a plot of h/L versus
I = tan ex for selected values of c = q/K . This graph shows how h/L
is indirectly related to the barrier slope angle and directly related to
c = q/K . Generally, the greatest decrease in h/L occurs when the
barrier angle increases from zero to five degrees (i.e., up to
approximately 10% slope). Angles greater than five degrees cause
decreasingly smaller changes in h/L. For design purposes, this means
that increasing the angle above five degrees has little effect on head
maintenance levels (h) and drain spacing (L).
Using the values of h/L presented in Table 3-3, drain spacing was
solved while holding head maintenance levels (h) constant at two meters.
Table 3-4 presents the results of this analysis, and Figure 3-4 shows a
plot of drain length versus c = q/K for barrier angles of 0°, 1° and
5°. These data show that if h is held constant, the barrier angle has
the greatest effect on increasing drain spacing length at lower values of
c = q/K (i.e., low leachate generation rates divided by high
permeabilities).
The equation for drains on an impermeable sloping barrier can also be
utilized for determining the thickness of a drain blanket by substituting
h for the thickness of the drain blanket (i.e., so that the maintenance
head level is designed to be within the blanket) and K for the
permeability of the drain blanket. For example, consider a landfill
without a drainage blanket with the following parameters: q =
0.0005 m/day, K = 0.00864 m/day (i.e., 10~ cm/sec), a = 1", and
h = 2 meters. Based on these figures, drains would have to be spaced at
intervals of L = 18 meters to maintain a 2-meter head level. If a
drainage blanket that has a permeability of K = 0.864 m/day
_3
(10 cm/sec) is installed at the site and the values of q and L remain
unchanged, the thickness of the blanket and hence the corresponding
height of the head levels would be 0.14 meters. This is a substantial
reduction in head levels for a relatively thin drain layer. The
advantage to lowering the head level within the site is that less
leachate is likely to be released from the site.
-35-
-------
TABLE 3-3 VALUES OF h/L FOR VARIOUS C=q/Kj AND ANGLES a
c=q/Kj
5.787
0.5787
0.05787
0.005787
0.0005787
0
1.203
0.380
0.120
0.038
0.012
a (degrees)
0.5 1
1.198
0.376
0.116
0.034
0.009
1.194
0.372
0.112
0.031
0.008
2
1.186
0.364
0.105
0.027
0.007
3
1.177
0.356
0.101
0.024
0.006
5
1.161
0.341
0.092
0.022
0.006
-------
0.10
0.06
0.06
0.04
0.02
.0001
0.1
0.2
to n
0.3
0.4
Figure 3-3. Plot of h/L versus I=tar. a for
drain and sloping in^ervious labors
(Moore, 1980)'
-37-
-------
TABLE 3-4 DKAIN SPACING (m) FOR HEAD MAINTENANCE LEVELS OF 2-METERS
c-q/I^
5.787
0.5787
0.05787
0.005787
0.0005787
0
1.66
5.26
16.67
52.63
166.67
1
1.68
5.38
17.86
64.52
250.00
a (degrees)
3
1.70
5.62
19.80
83.33
317.40
5
1.72
5.87
21.74
90.91
326.60
-38-
-------
3 4 5 6 7 B 9 I
5. 787(10 A)
5.787
c=c|/K
I
o - o
l'"i|Min- T-/I . Drnin T.onpitli (I,) vorsua c=q/K^ for u P(|iinl to 0 , 1 , and 5 .
-------
3.2.3 Drains Above an Impervious Barrier
The governing equation for a drain system above an impervious barrier
is:
.0.5 0)
L =
where the parameters are as defined previously in Chapter 2.
This equation takes into account radial flow to the drains through
the material underlying the drain.
Solutions to the equation are shown in Table 3-5 for a head
maintenance level (h> of two meters, a permeability (K > of the
_5 •"•
landfill material of 0.00864 m/day (i.e., 10 cm/sec), and a depth to
the impermeable layer (D) of 30.4 meters. Figure 3-5 shows a plot of
drain spacing (L) versus inflow rates (q) for various permeabilities of
underlying material (K ). These presentations show that when the
permeabilities of the overlying (K ) and the underlying (K2> material
are the same, the drain spacing (L) increases. This phenomenon is caused
by the introduction of radial flow to the drains rather than straight
lateral flow.
When the permeability of the underlying material (K2> is an order
of magnitude (0.1) less than the overlying material (KI>, the
calculated drain spacing does not differ significantly from a drain on an
impermeable barrier. If the underlying material has a permeability that
is two orders of magnitude (0.01) less than the overlying material, the
drain spacing is identical to a drain on an impermeable barrier. This
occurs because the term 8K dh does not significantly affect the results
of the drain spacing equation. Consequently, the "rule of thumb" for
designing drains is that if the underlying layer has a permeability at
least 10 times less than the overlying material, the underlying material
can be considered impermeable.
3.3 Application
The basic premise behind the use of a landfill located in the zone of
saturation is that if the head maintenance levels within the site are
-40-
-------
TABLE 3-5 DRAIN SPACING (m) FOR DRAINS ABOVE AN IMPERMEABLE
BARRIER (h-2m, K =0.00864 m/day (10-5cm/sec), D=30.4m)
q
(m/day)
0.05
0.01
0.005
0.001
0.0005
0.0001
0.00005
io-5
2
5
7
19
31
114
212
io-6
2
4
5
12
18
43
64
K9 (cm/sec)
ID'7
2
4
5
12
17
37
52
Impermeable
2
4
5
12
17
37
52
-41-
-------
2 3 4 5 fi 7 8 <» 1
7 .1 4
2 3 4367891
Z 3 456789
1,1
t'1'1
0.0005 0.0001
0.0005 0.001
0.005 0.01
0.05
FlRiire 3-5. Drain SpnrfnK ('•) versus inflow races (q) where
li=2 mi-1 ITS and Kj=10~5 cm/sec.
-------
less than the groundwater levels outside the site, then hydraulic
gradients should be into the site, thus minimizing the likelihood of
contaminant release. Basic hydraulic principles show this to be the case
as long as (1) the landfill materials and soils are homogeneous and
isotropic, (2) water tables are drawn down to drain levels, and (3) the
groundwater system is at equilibrium. In reality, these conditions
rarely exist at a landfill site. For these real-world situations, the
Wong (1977) model can be used to predict releases.
3.3.1 Parameter Estimation
In order to utilize the drain spacing equations, the input parameters
must be known with some accuracy. The two parameters which are the most
difficult to estimate accurately are leachate generation rate (q) and
hydraulic conductivity (K). Head maintenance level (h) and barrier slope
angle (a) are typically chosen by the designer and are not estimated.
Total inflow rates to the drains can be estimated through the use of
water balance equations (equation 8) and the method of fragments
(equations 9 and 10). These equations, however, do not take into account
the volume of liquid that is added to the site as landfill material
(e.g., paper waste sludges). Estimating this volume is very specific to
a landfill. Once the total volume is estimated, the value should
probably be increased to take into account variations that were not
anticipated (e.g., acceptance of more liquids, unseasonably high
precipitation) and to add a margin of safety to the design.
Determining the hydraulic conductivity of the landfill material with
any accuracy is very difficult because, typically, the material is a
mixture of wastes and daily covers which tends to create discrete cells.
Waste mixtures tend to cause the landfill to be very heterogeneous and
anisotropic, making estimations of permeabilities difficult. Here, as
before, the hydraulic conductivity selected should probably be the
highest value found for the waste materials. The problems associated
with daily cover can be minimized if the cover is removed each day,
allowing old and new fill materials to be hydraulically connected.
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The use of drainage blankets in a landfill effectively eliminates the
problem associated with determining conductivities. Hydraulic
conductivities for the drainage blanket can be determined easily in the
lab and most blanket materials are relatively homogeneous and isotropic.
A blanket layer covering the bottom of the cell and the sidewalls
provides a hydraulic connection between the leachate generated and the
drains. A drainage blanket also aids in the prevention of leachate
pooling, accelerates leachate removal times, and allows for lower head
maintenance levels with the same drain spacing.
3.3.2 Landfill Size
Technically a landfill is depth-limited in a zone of saturation
setting only by the propensity of the sidewalls for slumping or failure.
Theory shows that lower base grades in the fill result in higher
hydraulic gradients into the site. These higher gradients will result in
larger quantities of groundwater that must be removed from the site.
Problems can arise, however, if the base grades are extended deep enough
to cause quick conditions in the base or if the base intercepts an
aquifer that has a lower head level than the overlying aquifer. If quick
conditions occur, extensive dewatering may be necessary to maintain a
stable base. If the base extends into an underlying aquifer with a lower
head level, contamination of the aquifer is likely when the pit is
filled. The areal extent of the landfill is generally not considered
limiting if the drainage system is functioning properly.
3.3.3 Example Problem
An example of the use of the drain equations for the hypothetical
site developed in Section 3.2 is presented here for illustrative
purposes. Table 3-6 presents the data utilized in the example. For the
example, the recompacted clay base and sidewalls were assumed to have the
same permeabilities as the saturated clay aquifer.
To calculate the drain spacing needed at the site, the equation for
drains on sloping barriers (equation 6) can be used because the
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TABLE 3-6. EXAMPLE DATA SET
Parameter
Value
Cell Dimensions
o Width
o Length
o Depth
1000 ft (304m)
1000 ft (304m)
30 ft (9.1m)
Clay Thickness Below Base
50 ft (15.2m)
Hydraulic Conductivities
o Fill (K1)
o Clay (F^)
- compacted or natural
10 5 cm/sec (0.00864 m/day)
10~7 cm/sec (0.0000864 in/day)
Base Grade
1% (0.6 degrees)
Water Table (b.l.s.)
10 ft (3.0m)
Head Maintenance Levels
10 ft (3.0m)
Inflow (q) From Percolation 10 in/yr (0.0007 m/day)
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underlying clay material has a permeability of 10 times less than the
fill material. Solving equation 6 for drain spacing (L):
L = (2h/c°'5)/[(tan2a/c) + 1 - (tan a/c)(tan o+c) ' ]
c =
= 0. 0007 (m/day)/0. 00864 (m/day)
= 0.08102
L = <2(3.0)/0.08102°'5/[tanZ0.6/0.08102)+l-(tan 0.6/0.08102)
[ ( tan20 . 6+0 . 008102 ) ° ' 5 ]
= (21. 079) /[ (0.00135 )+l-(0. 12926) (0.28483)]
= 21.85m(71.89 ft)
The value obtained for drain spacing using these parameters is
significantly less than the currently specified allowable upper limit
(about 400 ft). However, this solution would not be considered
conservative for an actual site where (1) conductivities of the fill may
not be a uniform 10~ cm/sec, (2) inflow rates may be higher than
expected; for example, when the site is not capped, and (3) materials
with high water content may be landfilled.
An appropriate pipe size can be selected by calculating the system's
expected discharge and considering the factors described in Section 2.
Total flow along any single collector pipe can be calculated by:
Q1 = qA
where q, inflow, is 0.0007 m/day and A, area, is the area drained by each
drain line-22m x 304m (drain spacing times length)
Q = (0.0007 m/day) (22m) (304m)
= 4.7 m3/day (0.002 ft3/sec)
Using Figure 2-6 for plastic pipes with a roughness coefficient of
n = 0.013 and a design velocity of 1.4 ft/sec, a 4-inch drain pipe should
be sufficient to handle the discharge. Total discharge from the main
header pipe is:
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QT = qA
= (0.0007 m/day)(304m) (304m)
= 64.7 m /day (0.026 ft /sec or 17,094 gal/day)
Using Figure 2-6 again, a 4-inch pipe should be sufficient to handle the
flows. A larger size pipe may be selected as a margin of safety.
3.4 Construction Aspects
Previous discussions related the application of the design criteria
for zone-of-saturation landfill leachate collection systems to a
hypothetical site. Some aspects of construction of these sites are
important but do not fall within the focus of the other sections. These
considerations are described in the following sections and include:
o Construction inspection
o Collector drain maintenance
o Future operating conditions
3.4.1 Construction Inspection
Construction inspections are important to ensure the specified design
criteria are implemented in the field. Some criteria which are important
for zone of saturation landfills are:
o Moisture of clays when compacted on bases and sidewalls
o Placement of drain pipe and filter envelopes, and their protection
while exposed
o Compliance with specified design criteria for slopes, and drain
blanket thickness and gradation
The clay tills associated with the zone of saturation landfills can
easily exceed the moisture content at which maximum density can be
achieved. Trying to compact these excessively wet clays on the base and
sidewalls of the landfill is a wasted effort, and in many instances,
causes the soil to shear. Shearing of the clays may result in planes of
weakness and cracks throughout the layer which can easily transmit
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leachate out of the cell. Construction inspections can identify when the
soils exceed the optimum moisture content for compaction and prevent
their placement until the clays have dried to within an acceptable
moisture content range.
Construction inspections should be performed during drain pipe and
filter envelope placement. Inspections can ensure that these operations
are performed as specified in the designs and that:
o Drain pipes are not damaged during placement
o Filter materials are kept free of fines and are graded and
installed properly
o Drains are protected after placement so that fines do not clog
drains before wastes are placed
Slopes specified within a cell are generally just enough to prevent
pooling of leachate on the compacted clay base (i.e., 1% slope). With
these gentle slopes, there is a very small margin for error that would
still ensure that leachate moves freely towards the drain. Inspections
performed during the grading of the base can verify proper construction.
Inspections should also be performed during the placement of a drain
blanket to ensure adequate blanket thickness and proper gradation of the
blanket material.
3.4.2 Drain System Maintenance
Proper maintenance of a drain system is a critical element in
ensuring its continued performance. The initial design of the system
must allow for adequate access to the drain system components both for
inspections and cleanings. Regular maintenance inspections should be
performed to assess the system's performance and to plan cleaning and
repairing activities. Additionally, maintenance inspections should be
performed when an unexplained reduction in flow occurs to sumps or
increased head levels are observed within the cell.
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3.4.3 Future Operating Conditions
Landfill designs should anticipate and account for the type of waste
disposal operations planned. As mentioned previously, hydraulic
conductivities of the wastes placed in a cell are critical to the design
of the leachate collection system. Conductivities used for design
purposes are frequently estimates based on the experience of the designer
or are averages that represent the range of conductivities that could be
found throughout the cell. If the landfill will be accepting balings,
shreddings, large solid objects, or separations of particular wastes
during its operational life, the conductivities used in the drain designs
should be adjusted to reflect the hydraulic conductivity of these
materials and their volume in relation to the remainder of the fill
material.
The total quantity of water that must be extracted from zone of
saturation landfills can vary widely. Water can be derived from
percolating rainfall, groundwater infiltration, and from sludges or
liquids brought into the site. Underestimating the amount of water that
must be removed will result in rising internal head levels and the
eventual release of generated leachate. Therefore, care must be
exercised to ensure that water removal capabilities specified in the
landfill design realistically represent the eventual landfill operations.
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4.0 FLOW NET ANALYSIS
4.1 Introduction
Water flow through porous media is governed by several physical
relationships (e.g., Darcy's Law) that can also be represented
mathematically. The equations used to represent this flow can be
arranged so that they apply to certain physical conditions. The
solutions to the equations can then be obtained through numerical or
graphical means; the graphical solutions are called flow nets. In this
case, the flow nets are graphical representations of two-dimensional
equations of continuity for water flow.
The generation and application of a flow net requires certain
assumptions and simplifications. Most of these are common to any
representation (model) of a natural physical system and include the
assumptions of aquifer homogeneity, water incompressibility and laminar
Darcian flow. Consideration of site conditions at each of three
Wisconsin landfills reveals several features which are common to all, so
that the flow nets can be constructed for a generic site that
incorporates these common features (Figure 4-1) and also includes some
simplifications. These include:
a) elimination of small heterogeneities in the clay around the site,
since they are discontinuous and not well-mapped.
b) treatment of the underlying dolomite as an impermeable boundary.
It is recognized that recharge to the clay probably does occur
from the dolomite; however, the vertical permeability of the clay
is low enough that this recharge can be considered negligible for
the purposes of constructing a flow net.
c) treatment of all landfill surfaces as orthogonal to ground surface.
d) assuming that the landfill excavation can be treated as a circular
well so that water flows radially to it.
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LEACHATE MAINTENANCE LEVEL
WATER TABLE
REFUSE
x10~3cmsec~1
CLAY TILL
K *1 x 10 cm sec
DOLOMITE
DRAINS
O
* 70-80 FEET
h ^50-75 FEET
0 20 40 60 FEET
0
10 FEET
FIGURE 4-1. CROSS SECTION OF TYPICAL ZONE-OF-SATURATION LANDFILL
51
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e) assuming that the water table aquifer can be treated as a line
source of seepage beyond 250 feet from the landfill.
The flow nets are composed of two families of lines that intersect at
right angles. Flow lines show paths along which water can flow;
equipotential lines represent lines of equal hydraulic head. The flow
nets constructed for the generic site indicate the approximate
arrangement of flow lines and equipotential lines around the landfill for
different physical conditions, as explained below.
4.2 Initial Conditions
Three points need to be made about the flow nets before examination.
First, by treating the excavation as a circular well, some idea of the
radius of influence (R ) and equivalent well radius (R ) can be
obtained. These are defined by:
r
LW
where L = length of landfill
* W = width of landfill
Values of L and W were chosen equal to 300 ft. so that R = 169 ft.
rr~
R = C&h-» / where C = a constant ranging from 1.5 to 3
Ah = drawdown expected in "well"
K = hydraulic conductivity in cm/sec
Values chosen for Ah (30 feet) and K (1 x 10 cm/sec )
represent typical values for these parameters at each site. Several
references used suggested setting C = 2.0 for typical results. This
gives R = 19 feet.
o
Second, there is a difference between the horizontal and vertical
hydraulic conductivities at these sites because of the layered nature of
the till deposits. Average values for horizontal K and vertical K used
here were 1 x 10 cm/sec and 2 x 10 cm/sec , respectively.
Construction of flow nets for anisotropic media requires that the
horizontal and/or vertical dimensions be transformed to offset the effect
of different horizontal and vertical hydraulic conductivities.
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y
LANDFILL CELL
300 FEET
~l
AQUIFER ACTS AS LINE SOURCE
OF SEEPAGE BEYOND THESE LINES
FIGURE 4-2. INFLOW TO TYPICAL LANDFILL CELL (PLAN VIEW)
-------
A .3 Parameter Analysis
Examination of the effect of leachate maintenance levels
(Figures 4-3A, B, C) indicate that the most favorable situation in terms
of collecting lower amounts of leachate is when the maintenance level is
relatively high. This causes redistribution of the equipotential lines
in the aquifer around the drains such that the contribution of inflow
from the aquifer is small compared to that from the leachate. However,
this does not leave much of a safety margin and also enhances the
susceptibility for outward leakage along permeable sandy lenses in the
sides of the landfill.
Analysis of the effect of vertical gradients (Figures 4-4A, B)
indicates that, in comparison to sidewall inflow and leachate from the
refuse, the amount of inflow at the base will be small regardless of the
direction of gradient. This is predominantly due to the low vertical
hydraulic conductivity of the clay till in contrast to the more permeable
sidewalls.
Analysis of the effect of drain spacing (Figure 4-5) shows that the
drains tend to depress equipotential lines between their centers,
bringing higher head levels closer to the bottom of the excavation. In
the presence of a local downward gradient, this might enhance the
possibility of diffusion of contaminants through the base of the fill,
although most movement would be horizontal because of aquifer
anisotropy. As the number of drains increases, this effect becomes less
noticeable, and at some point would be offset by inflow through the base
of the excavation.
The effect of the depth of excavation below the water table can also
be examined by using some of the assumptions and site dimensions given
earlier and treating the excavation as a well. If the leachate
maintenance level is set at the base of the fill, the amount of inflow in
relation to excavation depth varies as follows:
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(7) LEACHATE LEVEL AT (H-h)
t
B) LEACHATE LEVEL AT (H-1)
C ) LEACHATE LEVEL AT (H-.5h)
SCALE:
0
10 FT.
VERTICAL EXAGGERATION = 7x
FIGURE 4-3. FLOW NETS WITH DIFFERENT LEACHATE LEVELS
-------
B UPWARD GRADIENT
FIGURE 4-4. FLOW NETS WITH DIFFERENT VERTICAL GRADIENTS
-------
A) TWO CORNER DRAINS
B ) THREE DRAINS
r
C] SIX DRAINS
FIGURE 4-5. FLOW NETS WITH DIFFERENT NUMBERS OF DRAINS
58
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Depth Below Water Approximate
Table Excavated. Ft. Inflow. GPM
10 8.8
20 16.7
30 23.8
40 29.8
These inflows need to be considered in comparison to the capacity of the
drain system and the possible inducement of heaving or buckling because
of the artesian head in the underlying dolomite. Rough calculations
indicate that 40 feet is about the maximum depth that should be
excavated, less if the remaining depth to the dolomite is below about 50
feet.
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5.0 MODELS
This chapter characterizes some commonly accepted models that can be
utilized in the design and performance evaluation of waste disposal
sites, and in the tracking of pollutants released from these sites.
These models are divided into two major groups: (1) release rate models
and (2) solute transport models. Typically, estimates of leachate
quantity and quality released from a site are obtained from a release
rate model and are used as input to a solute transport model. The theory
behind some of the models is very complex and readers should refer to
other sources such as Bachmat et. al. (1980), Mercer and Faust (1981),
Anderson (1979), and Weston (1978) for in-depth discussions of modeling.
5.1 Release Rate Models
The first and probably the most crucial step in waste site modeling
is to obtain accurate estimates of the quantity and quality of leachate
that will be released into the subsurface environment. Only after
adequate determination of leachate release can a solute transport model
be performed. This section briefly describes the theory behind release
rate models and presents those models that can potentially be utilized in
obtaining release rates from landfills.
5.1.1 Fundamentals
Most release rate models are based on dividing the problem of
prediction into three separate components—leachate generation,
constituent concentrations, and leachate release rates from the site.
Combining the three separate components allows for prediction of the
quantity and quality of leachate that can be expected to be released from
the site.
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5.1.1.1 Leachate Generation
Leachate generation refers to the quantity of fluid within the site
available to leach and transport waste constituents. The major factors
that directly influence leachate generation are listed in Table 5-1.
Probably the most important factors for zone-of-saturation landfills are
infiltrating precipitation and groundwater intrusion. There are
currently two main approaches for predicting leachate generation—the
water balance approach and the use of bounding assumptions.
Fenn, Hanley, and DeGeare (1975) pioneered the use of water balances
to predict leachate generation from solid waste disposal facilities based
on the earlier work of Thornthwaite (1955). Several authors have since
updated and modified Fenn, et al.'s., work for application to other types
of waste disposal sites. Basically, water balances numerically partition
the amount of fluid moving into, around, and through the cap of a land
disposal facility by utilizing the equation:
Perc = P - RO - ST - ET
where:
Perc = percolation rate; the portion of precipitation which infiltrates
the surface and is not taken up by plants or evaporated
P = precipitation rate
RO = surface water runoff; the portion of the precipitation which
does not infiltrate into the ground but instead moves overland
away from the site
ST = change in soil moisture storage
ET = Actual evapotranspiration; the combined amount of water returned
to the atmosphere through direct evaporation from surfaces and
vegetative transpiration.
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TABLE 5-1 MAJOR FACTORS AFFECTING LEACHATE GENERATION
Primary Factors
Secondary Factors
Precipitation
Liquid Content of Wastes
Liquids from Waste Decomposition
Groundwater Intrusion
Soil Moisture Storage
Evapotransp iration
Runon/Runoff Control
Operation Mode
Surface/Cap Conditions
quantity, intensity, duration,
frequency, seasonal distribution
type, quantity, moisture content,
and moisture storage capacity
(field capacity)
waste composition, waste environ-
ment, and micro-organism populations
flow rates into pit, seasonal
distribution of water table, head
levels, liner materials
field capacity of materials,
seasonal fluctuations
temperature, wind velocity,
humidity, vegetation type, solar
radiation, soil characteristics
diversions, crowning of surface
cap, permeability and integrity
of cap, depression storage
open versus closed, coverage
permeability, integrity, surface
contour, runoff underdrain
systems, subsidence
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Values for the parameters needed for such an analysis can be found in
a variety of sources or estimated using a variety of techniques.
Precipitation values for a given location are available from a number of
sources including the National Weather Service. Runoff is very site
specific and difficult to measure. Most release rate models use one of
the following methods to estimate runoff:
• Rational Formula—utilizes empirical runoff coefficients based on
vegetative type, soil type, and slope.
• SCS Curve Numbers—utilizes empirical coefficients which relate
runoff to soil type, land use, management practices, and daily
rainfall.
• Green-Ampt Equation—approximates runoff based on soil properties,
initial water content and distribution, surface conditions, and
accumulative infiltration.
Both evapotranspiration and soil moisture storage can be estimated using
empirical soil moisture retention relationships such as those developed
by Thornthwaite (1955). Some models require that evapotranspiration be a
measured site specific input, while others do not specify a method to
obtain values. Some models relate evapotranspiration to physical
parameters such as temperature, solar radiation, and the leaf area index
(LAI), while others store evapotranspiration and soil moisture
information for various locations on a national data base that can be
accessed by the model. One model also makes simplifying assumptions to
estimate soil moisture storage either by apportioning soil moisture into
a "wet" zone and a "dry" zone or by using the method of depth-weighted
fractional water content within the soil profile.
Release rate models also allow the user to set surface conditions and
cover liner characteristics for the site with varying degrees of
flexibility. Some methods allow multiple clay-synthetic liners, others
only clay liners, while still another can only be applied to open sites
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without covers. Cover vegetation, slope, contour, and soil properties
can also be specified in all but the most simplistic models.
When cover liners are used to impede percolation to waste cells,
excess water moves away from the site through subsurface lateral drainage
above the liner. Models estimate this lateral drainage by:
• Approximate methods (utilizing correction factors) derived from
the Boussinesq equation for lateral saturated flow.
• Empirical methods development by Moore (1980) which calculate the
maximum hydraulic head above the liner and then the upper bound of
the quantity of liquid flowing into tile drains. The liner is
assumed to be impermeable for these calculations.
• Empirical methods which calculate percolation through the liner
and soil moisture storage; then extrapolate lateral drainage as
the remaining excess water.
The next step is to predict the flow rate through the top liner.
This is ultimately the major contributing factor in leachate generation.
Numerous methods are used to predict this percolation rate and they can
be divided into methods for clay liners and those for synthetics.
Methods used for clay liners include:
• Darcy's Law for saturated conditions, which relates flow velocity
to hydraulic conductivity, effective porosity, hydraulic head, and
travel distance using the following general equation:
V = Kh/nx
where: V is flow velocity, K is hydraulic conductivity, n is
effective porosity, h is hydraulic head difference, and
x is travel distance
• Approximations of saturated Darcy flow as proposed by Wong (1977).
• Soil storage routing techniques through multiple soil layers which
relate liner permeability to inflow rate, time interval, hydraulic
conductivity, soil water storage, and evapotranspiration.
• Darcy's Law with provisions to arbitrarily increase liner
permeability assuming that certain events occur (e.g., burrowing
animals or equipment breach of the liner).
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o Prediction of unsaturated flow driven by capillary forces
utilizing the Green-Ampt approximation of the wetting front
assuming a constant capillary head.
Methods to predict flow through synthetic liners include:
o Darcy's Law as described above and based on hydraulic head and
liner thickness.
o Power Law relationships for estimating the aging of a liner based
on the life expectancy of the liner.
o Arbitrary methods such as assuming that the liner will be
impermeable for 20 years and then will fail completely (i.e.,
after 20 years the model treats the facility as if it were
unlined).
o Stochastic (Monte Carlo) simulation for liner failure due to aging
and installation problems.
The amount of water percolating through the cover liners and into the
waste cells is either adjusted according to the moisture content of the
wastes and fill materials in the facility, or the wastes and fill
materials are assumed to be at field capacity and, therefore, the amount
of water percolating into the waste cells is also the total quantity of
leachate generated.
Water balances for waste disposal sites produce only relative
solutions to leachate generation for comparing different designs or
sites. The high degree of uncertainty that exists in these solutions has
led to the use of bounding assumptions. Bounding assumptions are based
on the knowledge that the quantity of leachate generated at a given
facility falls between 0% and 100% of the maximum potential amount (based
on total possible leachate), such that upper and lower bounds for
leachate generation volume can be established. Using assumptions and
empirical data, the bounds can be narrowed to produce best- and worst-
case scenarios which can, in turn, be used to design the landfill based
upon performance goals.
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5.1.1.2 Leachate Constituent Concentrations
For a release rate model to be useful, it must not only estimate the
quantity of leachate produced, but it also must estimate the quality
(i.e., leachate constituent concentrations). The quantitative simulation
of the processes and interactions occurring within a landfill to produce
leachate are very complex, and therefore most available models do not
attempt to simulate all these processes. Table 5-2 lists some of the
factors affecting leachate constituent concentrations that would have to
be considered.
Because of the complexity of the interdependent interactions
occurring within a disposal site and our inability to accurately
characterize these interactions, simulations of the processes are
extremely difficult, if not impossible. Consequently, release rate
models do not address the factors which govern constituent
concentrations. Rather, the models make assumptions to greatly simplify
the complexities of the real world. These assumptions are:
• Constituents are at the saturation solubility concentration levels
in leachate.
• Constituents exist at equilibrium concentrations between the
aqueous and sorbed phases.
• Bounding assumptions are used in a similar manner as described for
leachate generation.
5.1.1.3 Leachate Release
Leachate release is defined as the escape of any contaminants beyond
the containment boundary of a land disposal facility. The type and
magnitude of release depends upon the presence of a liner system, the
type of liner employed, the presence and efficiency of a leachate
collection system, and the occurrence and magnitude of any system
failures. Table 5-3 lists some of the factors affecting leachate release
from a land disposal site.
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TABLE 5-2 FACTORS AFFECTING LEACHATE CONSTITUENT CONCENTRATIONS
Major Conponents
Primarv Factors
Secondarv Factors
Waste Composition
Physicochemical
Properties
Contact Time
Chemical Reactions
and Interactions
Chemical Reactions
and Interactions
(continued)
Volume
Constituents
Constituent
Concentrations
Solubility
Mobility
Persistence
Volatilization
Phase/State
Conditions of Waste
Environment
Facility Age
Hvdrolysis
Oxidation
Reduction
photolysis
Microbial Degradation
Microbial Acclimation
Changes in Waste
Environment
pH; temperature; composition of
liquid phase
Viscosity; temperature; density;
sorption; complexation
?H; temperature; presence of catalysts;
chemical degradation (e.g. oxidation,
reduction, hydrolysis, photolysis);
biological degradation
tugacity; constituent vapor pressure;
temperature
Temperature; pressure
Flow rates through wastes, fill
materials and drain layers; waste
permeability; waste porosity;
particle size: site heterogeneities:
capillary action; piping through
wastes: ponding in waste cells:
plugging of pore spaces
pH; temperature; soil pH, catalysts
Presence and type of oxidants:
catalysts: oxygen concentration; pH:
temperature
Oxygen concentration; conpiexation
state; concentration and type of
reducing agents: pH; temperature
Solar radiation; transmissivity 01
water: presence of sensitizens and
quenchers
Microbial population; soil aoisture
content; temperature; pH, oxygen
concentration, redox potential
oh: temperature; removal of nost
soluble constituents
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TABLE 5-3 FACTORS AFFECTING LEACHATE RELEASE
Major Components
Primary Factors
, Secondarv Factors
Synthetic Liners
Physical Factors
Chemical Factors
Clav Liners
Leachate Collection
Phvsicochemical Factors
Chemical Factors
Phvsical Factors
Biological Factors
Leachate Collection
Aging: human activities; internal
loading stresses: hydrogeology;
bathtub effect; weather resistence;
deep root growth, burrowing animals;
installation and design problems
(e.g., subsidence from improper
siting, improperly prepared seams);
uplifting by gasses or liquids
under pressure; impingment rate;
temperature
Chemical disintegration; weather,
ultraviolet radiation, chemical
and microbial attack from the soil
atmosphere; waste-linear compata-
bility: nature soil chemistry; pH,
temperature
Efficiency, maintenance; design
Chemical dehydration, flocculation/
dispersion, alteration of shrink/
swell properties; soil piping;
leachate characteristics; pore size
distribution
Dissolution of c.hemicai species,
adsorpsion properties; chemical
disintegration, native soil chemistry-
Internal loading stresses; dehydration;
hydrogeology; weathering; erosion:
bathtub effect; aging; iapingner-t
rate; hydraulic head: structural and
design considerations (e.g., proper
siting and design to handle
differential subsidence
Microbial population; burrowing
animals; deep root growth, human
activities
Efficiency; maintenance; design
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Prediction of leachate releases involves the estimation of leachate
quantity escaping from the site over time, which is combined with
constitutent concentration. The major drawbacks in predicting releases
are obtaining realistic estimates of liner lifetime, estimating the
probability of liner failure, and establishing cause and magnitude of
failure if it occurs. The methods employed to predict releases parallel
those used to estimate flow rates through cover liners (described
previously as part of the water balance approach).
5.1.2 Selected Release Rate Models
Five release rate models and one series of simple calculations chosen
for their potential applicability to Wisconsin zone of saturation
landfills are discussed below and summarized in Table 5-4.
DRAINMOD (1980) is a computer model developed to predict the response
of water in both the unsaturated and the saturated zones to rainfall,
evapotranspiration, specified levels of surface and subsurface drainage,
and the use of water table control or subirrigation practices. DRAINFIL
(1982) is an adaptation of DRAINMOD for landfills which considers
drainage from a sloping layer underlain by a tight clay liner and seepage
through the cap. DRAINFIL can also quantify drainage to the leachate
collection system and through the underlying clay liner during the time
the landfill is open. A water balance for the soil water profile is used
to calculate the infiltration rate, vertical and lateral drainage,
evapotranspiration, and distribution of soil water in the soil profile
using approximate solutions to nonlinear differential equations. The
prohibitive cost of using numerical methods to finding solutions to
equations of this sort requires that approximate methods be used. Checks
of solutions obtained through these methods suggest, however, that
satisfactory results can be consistently obtained.
The minimum data required for these models include precipitation
(amount, distribution, intensity, and duration), water table elevation,
daily potential evapotranspiration (PET), net solar radiation,
temperature, humidity, wind velocity, soil moisture content, soil profile
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TABLE 5-4
RELEASE RATE MODELS
MODEL
DRAINMOD/
DRAlNKiL
HELP/
HSSWDS
LSIPE
PCLTF
LTTM
Release
Rate
Computa-
tions
ADVANTAGES
Predicts response
of water in un-
saturated zones
to rainfall
Requires minimum
amount of data.
Estimates vertical
and lateral
percolation.
Evaluation
efficiency of
liner design in
controlling
leachate release.
Addresses all
necessary to predict
a mass load release.
Accounts for
moisture content
of wastes and fill
material. Includes
both gravitational
and capillary forces,
Simple and easy to
use.
A series of simple
calculations
DISADVANTAGES
High cost
Ignores rainfall
intensity, duration,
and distribution.
Accuracy is
questionable
Estimates release
in a single
modular waste cell.
Based on assump-
tions of good
engineering design
(rare in older
landfills)
Unsaturated zone
Currently being
developed. All
elements have not
been tested.
REFERENCE
Skaggs (1982)
Perrier and
Gibson (1980)
Moore (1980)
Some assumptions
are questionable.
U.S. EPA (1982)
Pope-Reid Assoc.
(1982) (Unpub.)
SCS Engineers
(1982)
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depth, surface compaction, vegetation, depth of root zone, hydraulic
conductivity (saturated and unsaturated), and pressure head. Host of the
data are readily available, though some difficulty may exist in obtaining
reliable unsaturated hydraulic conductivity and pressure head data. In
addition, to simplify calculations, the models assume:
• One-dimensional, saturated flow in the bottom liner.
• Infiltration rates for uniform deep soils with constant initial
water content expressed in terms of cumulative infiltration alone,
regardless of the rate of application.
• Drainage is limited by the rate of soil water movement to the
lateral drains and not by the hydraulic capacity of the drain
tubes or outlet.
DRAINMOD is currently being used in assessing agricultural drainage
systems and has been field-verified in a variety of locations. DRAINFIL,
however, has not yet undergone the final changes needed for its use in
assessing infiltration at waste disposal sites, and therefore remains
untested.
These models are similar to other release rate models in that they
use a water balance approach, do not consider leachate constituent
concentrations, and do not consider any processes occurring within the
waste cell that may affect leachate quantity or quality. Some unique
features of DRAINMOD/DRAINFIL are their ability to predict the upward
movement of water, and the precision of their hydraulic head estimates.
Hydrologic Evaluation of Landfill Performance (HELP/HSSWDS)
(Perrier and Gibson. 1980)
The hydrologic evaluation of landfill performance (HELP, formerly
HSSWDS) is a one-dimensional, deterministic water balance model modified
and adapted from the CREAMS (Chemical Runoff and Erosion from
Agricultural Management Systems) soil percolation model for use in
estimating the amount of water that will move through various landfill
covers. This model can simulate daily, monthly, and annual values for
runoff, percolation, temperature, soil-water characteristics, and
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evapotranspiration with a minimum of data (e.g., percipitation, mean
temperature, solar radiation, leaf area index, and characteristics of the
cover material). Should data be unavailable, the model provides default
values for such parameters as soil-water characteristics, precipitation,
mean monthly temperature, solar radiation, vegetative characteristics and
climate based on the location of the site. The model is portrayed by its
developers as "no more complex than a manual tabulation of moisture
balance."
The HELP model ignores rainfall intensity, duration, and distribution
and considers only mean rainfall rates, which could somewhat limit the
accuracy of the estimates. It also does not evaluate leachate quality.
However, it can estimate percolation through up to eight drainage layers
including through the waste cell itself, and estimate lateral drainage
through any or all of these layers. Some other features of the HELP
model include the ability to provide estimates of the impingement rate of
leachate entering the bottom liner collection system, predict the seepage
rate through a saturated clay liner, and estimate evapotranspiration and
runoff using a minimum of data.
The HSSWDS model has been successfully field verified by Gibson and
Malone (1982) and many others. Those HSSWDS users contacted for comments
and opinions believed that HSSWDS was very useful in comparing sites or
cover designs, but that the accuracy or validity of the outputs could not
be determined. HELP is currently undergoing refinement and has not yet
been tested.
Landfill and Surface Impoundment Performance Evaluation (Moore. 1980)
The Landfill and Surface Impoundment Performance Evaluation (LSIPE)
model attempts to determine the adequacy of designs of hazardous waste
surface impoundments and landfills in controlling the amount of fluid
released to the environment. LSIPE utilizes a series of linearized
equations and simplified boundary conditions to evaluate the efficiency
of a proposed liner design in terms of:
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• Horizontal flow through sand and gravel drain layers.
• Vertical flow through low permeability clay liners.
• Efficiencies of liner-drain layer systems.
• Seepage through the bottom liner.
LSIPE has the advantage of allowing for nonlinear equations and more
complex boundary conditions to be employed if needed. Only transport of
liquids through a single modular waste cell can be estimated; however,
modules can then be arranged in the proposed configuration for analysis.
The LSIPE approach also possesses the unique capability of allowing for
leachate releases to be measured indirectly through the efficiency of the
leachate collection system.
In order to provide estimates of the above mentioned parameters, this
approach requires:
• Liquid routing diagram for the site.
• Water balance for the site.
• Slopes in the routing system.
• Hydraulic conductivities.
• Service life of any synthetic liners.
The LSIPE model also makes the general assumption that the operating
conditions for a waste landfill or surface impoundment meet the basic
requirements of good engineering design, including:
• Surface water runon has been intercepted and directed away from
the site so that only the rainfall impinging directly on the
landfill needs to be accounted for.
• Proper precautions have been taken to prevent erosion of the cover
soils which would degrade cover performance.
• Synthetic liners have been installed properly to ensure their
integrity for design life.
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Post-Closure Liability Trust Fund Model (PCLTF) (U.S. EPA 1982)
This model is being developed to assess the adequacy of the
Post-Closure Liability Trust Fund established as the result of the
passage of CERCLA in 1980. The fund will provide for liability claims
resulting from failure of RCRA permitted sites after proper closure. The
analytical approach involves:
• Assessing failure probabilities of facilities.
• Determining environmental exposure.
• Assessing damage potential.
• Quantifying damages, and assessing costs for clean-up, remedial
action, damage to natural resources, personal injury and economic
loss.
PCLTF is the only model reviewed which addresses all three components
necessary to predict a mass load release from a land disposal site. The
model can be applied to open or closed facilities with both clay and/or
synthetic liners. The user can specify one of seven generic site types
from a variety of cover and bottom liner and leachate collection
configurations. The components of the model consists of:
• User-supplied inputs which characterize the site design and
operation, and identify the wastes placed within the fill.
• A data base of physical and chemical characteristics of waste
constituents which relate to their solubility, toxicity,
persistance, and mobility as well as their effect on synthetic
liner performance.
• A data base of climate, soils, and the geology of various regions
of the U.S.
• A baseline analysis with which to set initial site conditions.
• Water movement simulation which uses Monte Carlo simulation
techniques to generate values for seepage velocity, effective
porosity, dispersion, and liner failure to route leachates through
the layers of the landfill, including liners and drain layers.
Adjustments to leachate quantity are made based on moisture
content of wastes and fill materials.
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This model's output is a two-dimensional, uniformly distributed, leachate
discharge estimate (concentration and flux) to the unsaturated soil
column beneath the site. The output provides the source term for a mass
transport module for the unsaturated zone.
Leachate Travel Time Model (LTTM) (Pope-Reid Associates. 1982)
The Leachate Travel Time Model (currently under development by
Pope-Reid Associates) combines several analytical techniques and
previously developed models to evaluate the performance of landfills of
various designs in a variety of climatic settings. The model consists of
a monthly, quarterly, or annual hydrologic and waste budget which is used
to calculate leachate volume in the active fill area, leachate head in
drain layers, containment time and seepage rate through the bottom liner,
and travel time and seepage rate in the unsaturated zone below the
landfill. The model possesses the unique feature of accounting for the
moisture content of wastes and fill materials. Actual measured values or
estimations of moisture content can be input. Also, the model includes
both gravitational and capillary forces to calculate seepage rates
through liners.
The Leachate Travel Time Model does not include a specific cover
liner option, although the user can incorporate a cover liner by altering
the hydrologic budget. Like other models, the current program does not
address constitutent concentrations of contaminant mass transport, but
the authors do intend to incorporate constituent transport in the
future. The model does, however, address both leachate generation and
release in a relatively simple and easy-to-use program which incorporates
many interesting features.
Release Rate Computations for Land Disposal Facilities (SCS Engineers.
1982)
This approach consists of a series of simple calculations to predict
the quantity of leachate generated and released from landfills and
surface impoundments which will be incorporated into EPA's RCRA Risk/Cost
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Policy Model Project (ICF Incorporated, Clement Associates, Inc., and SCS
Engineers, Inc., 1982). The approach assumes that:
• Synthetic liners last for 20 years, after which liquid moves
freely through them.
• Clay liners retain their integrity for longer than 100 years.
• The only sources of liquids are infiltration from the surface and
free liquids in waste. Only saturated flow takes place through
the liner in the absence of free liquids.
• Infiltration through the cover system after closure is less than
or equal to leachate movement through the liner system.
• Synergistic effects do not occur.
The time required for leakage to appear beneath the bottom of a clay
liner is given by:
t = T*d2/4D
where:
t = time to first appearance of leakage (sec)
d = thickness of clay liner (cm)
2
D = linearized diffusivity constant, (cm /sec) assumed to be
5 2
10 cm /sec.
The volume of leachate release over time is given by:
q = Ks(dh/d )A(At)
z
where:
q = volume of leachate released over time
Ka = saturated permeability coefficient
dh = hydraulic gradient
dz
A = Area at base of facility
At = length of time over which leachate releases.
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5.2 Solute Transport Models
Once the quantity and quality of a release from a land disposal site
has been determined, this estimate can be used as the input for a solute
transport model. Solute transport models predict the migration of
chemical constituents away from a source over time in one, two, or three
dimensions. A brief discussion of the principles used in transport
modeling and descriptions of several transport models are contained in
the following sections.
5.2.1 Fundamentals
The transport models presented in this section are all mathematical
models, rather than rating or ranking type models (e.g., the MITRE
Model). The mathematical approach to modeling involves applying a set of
equations, based on explicit assumptions, to describe the physical
processes affecting pollutant transport from a site. These models can be
divided into two types—deterministic and stochastic. Deterministic
models attempt to define the shape and concentration of waste migration
using the physical processes (e.g., groundwater flow) involved, while
stochastic models attempt to define causes and effects using probablistic
methods. Models presented in this report are generally deterministic.
Deterministic mathematical models can be further divided into
analytical models and numerical models. Analytical models simplify
mathematical equations, allowing solutions to be obtained by analytical
methods (i.e., function of real variables). Numerical models, on the
other hand, approximate equations numerically and result in a matrix
equation that is usually solved by computer analysis. Both types of
deterministic models address a wide range of physical and chemical
characteristics but the analytical models usually simplify the
characteristics by assuming steady state conditions. The physical and
chemical characteristics considered by these models include:
• Boundary Conditions—hydraulic head distribution, recharge and
discharge points, locations and types of boundaries.
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• Material Constants — hydraulic conductivity, porosity,
transmissivity, extent of hydrogeologic units.
• Attenuation Mechanisms — adsorption-desorption, ion exchange,
complexing, nuclear decay, ion filtration, gas generation,
precipitation-dissolution, biodegradation, chemical degradation.
• Hydrodynamic Dispersion — diffusion and dispersion (transverse and
longitudinal) .
• Waste Constituent Concentration — initial and background
concentrations, boundary conditions.
Both mathematical model types incorporate two sets of equations to
define transport; a groundwater flow equation and a mass transport
equation. Figure 5-1 illustrates the relationship between these
equations.
A general form of the water momentum balance equation for
nonhomogeneous anisotropic aquifers (Pope-Reid Associates (1982)) is:
a ,„ ah . 0 ah
IT (Kij 1x7 > - s IT + w
where:
h = hydraulic head
K = hydraulic conductivity
S = storage coefficient
W = volume flux per unit area (e.g., pumping or injection wells,
infiltration, leakage)
z = distance
t = time
The Darcy equation is generally represented as:
V. =
i
where:
V = groundwater velocity
n = porosity
K = hydraulic conductivity
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Mass Transport
Eauations
Groundwater Flow
Equations
Mass Balance
for Chemical
Species
Water
Momentum
Balance
Darcv's
Equation
Transport
Eauation
Figure 5-1. Major Components of Groundwater Transport Equation
(after Mercer and Faust, 1981)
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The mass transport side of the model, which describes the
concentrations of a chemical species in a flow pattern in general form is:
ac a ac a cfw
tt - ^7 (Dij • ^ - aT7 (CV - V + R
where:
C' = concentration of solute in the source or sink fluid
C = chemical species concentration
D = dispersion tensor (i.e., hydrodynamic dispersion)
V = groundwater velocity (i.e., convection transport)
R = rate of chemical species attenuation/transformation
These equations are coupled to provide predictions of solute
transport in the groundwater system with chemical reactions considered.
For analytical models, these equations are simplified to explicit
expression. For either type of model, a sensitivity analysis of model
results can be performed by varying the input characteristics singularly
or in combination. One type of sensitivity analysis that could be
performed involves changing single parameters (within known values of
occurrence) to demonstrate the effects that variations in individual
parameters have on model output. This analysis helps identify those
parameters which have the greatest influence on model results. A second
type of sensitivity analysis involves a series of trial runs of the model
using an array of input parameters which vary in accordance with the
expected errors associated with each parameter (i.e., Monte Carlo
simulation techniques). This method provides a general assessment of the
overall model sensitivity and intrinsic precision by providing a range of
variations of the model outputs as a function of the error bars
associated with the input parameters (e.g., mean values, maximum values,
minimum values).
5.2.1.1 Analytical Models
Analytical models provide estimates of waste constituent
concentrations and distributions using simplified, explicit expressions
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generated from partial differential equations. The mathematical
expressions are usually simplified by assuming steady state conditions
relative to fluid velocity, dispersion dynamics, and other physical
parameters. For example, groundwater flow equations can be simplified if
the aquifer is assumed to have infinite extent. Governing equations
characterize both groundwater flow and mass transport, and may also
address dilution, dispersion, and attenuation. These models can simulate
plume migration from the source to a utilized groundwater system allowing
for attenuation and dispersion. The method provides a quick and
inexpensive solution with minimal amounts of data as long as the
simplifying assumptions do not render results invalid.
5.2.1.2 Numerical Models
Numerical models characterize groundwater contamination processes
without the simplification of complex physical and chemical
characteristics required by analytical models. The numerical models
reduce the partial differential equations to a set of algebraic equations
that define hydraulic head at specific points (i.e., grid points). These
equations are solved through linear algebra using matrix techniques.
The numerical methods most commonly used to simulate groundwater
transport problems can be divided into four groups: finite difference
-------
form a matrix equation which is solved numerically. The FE method is
better suited for solving complex two- and three-dimensional boundary
conditions than the FD method. When using FD or FE methods for solving
contaminant transport problems, results are subject to numerical
dispersion or numerical oscillation. Numerical dispersion causes answers
to be obscured because of accumulated round-off error at alternating time
steps. Numerical oscillation causes answers to overshoot and undershoot
the actual solution at alternating time steps. Numerical oscillation is
generally associated with FE methods, while numerical dispersion is
generally associated with FD methods.
The method of characteristics and discrete parcel random-walk models
were developed to minimize the numerical difficulties associated with the
FE and FD methods. Both the method of characteristics (MOO and discrete
parcel random walk method analyze temporal changes in concentrations by
tracking a set of reference points that flow with the groundwater past a
fixed grid point. In the HOC method, points are placed in each finite
difference block and allowed to move in proportion to the groundwater
velocity at the point and the time increment. Concentrations are
recalculated using summed particle concentrations at the new locations.
The random-walk method varies from the HOC method because, instead of
solving the transport equation, a random process defines dispersion.
Reference points move as a function of groundwater flow, consistent with
a probability function related to groundwater velocity and dispersion
(longitudinal and transverse). The methods provide comparable results
but the HOC method is time consuming, expensive, and requires
considerable computer storage.
5.2.2 Selected Solute Transport Models
Eight analytical models and nine numerical models are presented in
the following sections. Each has characteristics that make it unique;
therefore, selection of a model should be based on making the best use of
available data given the desired output. These models are summarized in
Table 5-5.
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TABLE 5-5 SOLUTE TRANSPORT MODELS
MODEL
(Analytical)
SESOIL
PESTAN
PLUME
Leachate
Plume
Migration
Model
Cleary
Model
ADVANTAGES
"User-friendly",
addresses numerous
processes.
Calculates pollutant
velocity, length of
pollutant slug,
contamainant concentra-
tions. Easy to use,
inexpensive, can be
used as a screening
model. Can be coupled
with PLUME.
A saturated zone model.
Provides 2-dimension-
al plume traces.
DISADVANTAGES
Field and analytical
verifications not
yet performed.
Designed for un-
saturated zone
simulations.
One dimensional
through un-
saturated zone.
REFERENCE
Bonazountas and
Wagner (1981)
Enfield, et
al., (1982)
Effects of adsorp-
tion and degreda-
tion ignored in
testing.
Predicts plume migra-
tion and mixing in
saturated zone.
Methods of use are
simplified.
10 different models using a variety
of boundary conditions. Little
information available.
Wagner (1982)
Kent, et al.,
(1982)
Cleary
(1982)
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TABLE 5-5 SOLUTE TRANSPORT MODELS (Continued)
MODEL
(Analytical)
AT123D
Screening
Procedure
PATHS
ADVANTAGES
Estimates waste trans-
port in saturated and
unsaturated zones.
Computer coded,
available, valuable
for preliminary
assessments.
Assesses transport
and degradation
in saturated zone
of specific chemicals.
Provides quantitative
and qualitative
screening based on
estimates of exposure.
Used for saturated
flow. Estimates
single contaminant
transport. Fast,
inexpensive.
DISADVANTAGES
Appears to have
not been field
verified.
REFERENCE
Yeh (1981)
Does not address
synergistic effects.
Unpublished and not
available to public.
Falco, et.
al. (1980)
Ignores dispersion
effects. Has not
been field verified
Nelson and
Sheen (1980)
(Numerical Models)
MMT/VTT/ Used for saturated or
UNSAT I D unsaturated zone.
Computer package
available for
graphic displays.
As with all
numerical models,
costs are high as
are requirements
for accurate
geohydrologic data.
Battelle
(1982)
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TABLE 5-5 SOLUTE TRANSPORT MODELS (Continued)
MODEL
ADVANTAGES
DISADVANTAGES
REFERENCE
(Numerical Models)
CFEST/
UNSAT ID
Pollutant
Movement
Simulator
FEMWASTE
Random Walk
Solute
Transport
Model
SWIFP
Solute
Transport/
Groundwater
Flow Model
Leachate
Organic
Migration
and Attenua-
tion Model
CFEST predicts fluid
pressure, temperature,
and contaminant con-
centrations in
saturated zone.
Three-dimens ional
model for coupled
saturated-unsaturated
zone.
Two-dimensional model
for coupled saturated-
unsaturated zones.
Utilizes FEMWATER.
Saturated zone model.
Concentrations can be
specified in any segment
of model. Documented,
available to public,
and verified.
Saturated zone models
three-dimensional. Field
and analytically verified,
well documented.
Presently being field Battelle
verified. Model (1982)
documentation in
preparation.
Has not been
tested for
landfills.
Khaleel and
Reddell,
Yeh (1981)
Prickett, et
al., (1981)
USGS (1982)
Saturated zone.
Analytically and
field verified.
Coupled saturated-
unsaturated zones.
Specifically designed
for landfills.
Ignores dispersion.
Currently being
revised.
Colder
Associates
(1982)
Sykes, et
al. (1982)
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5.2.2.1 Analytical Models
The eight analytical models characterized in this section do not
address cases involving secondary porosity, immiscible liquids, or more
than one contaminant. Only one model, AT123D developed by G.T. Yeh
(1981), considers both the saturated and unsaturated hydrologic zones;
the other models are restricted to modeling only one hydrologic zone.
SESOIL (Bonazountas and Wagner. 1981)
SESOIL, a seasonal soil compartment model, was developed by A.D.
Little Inc. for the U.S. EPA Office of Toxic Substances. The model is
described as a "user-friendly" statistical/analytical mathematical model
designed for long term environmental pollutant fate simulates.
Simulations are performed for the unsaturated zone and are based on a
three-cycle rationale—the water cycle, the sediment cycle, and the
pollutant cycle. SESOIL addresses numerous processes including
diffusion, sorption, chemical degradation, biological degradation, and
the complexation of metals. The model is presently being updated and is
available for limited use although field or analytical verifications have
not yet been performed.
PESTAN (Enfield. et al.. 1982)
PESTAN was developed at the EPA Robert S. Kerr Environmental Research
Laboratory. The model calculates the movement of organic substances in
one dimension through the unsaturated zone based on linear sorption and
first order degradation (i.e., hydrolysis and biodegradation).
Calculated outputs include pollutant velocity, length of the pollutant
slug, and contaminant concentrations. Pollutant application rates to
the soil surface can be changed to determine the effect of the number of
applications, application period, and number of days before
reapplication. This model is best classified as a screening model
because it provides for a rapid evaluation of chemicals without the
sophistication of numerical models. The model is also easy to use and
inexpensive. PESTAN can be coupled with PLUME, a saturated zone
analytical model. PESTAN has been field verified for the chemicals DDT
and Aldicarb, and the model is being used by EPA-Athens.
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PLUME (Wagner. 1982)
PLUME is a steady state analytical model developed at Oklahoma State
University to model contaminant transport in the saturated zone.
Themodel provides two-dimensional plume traces from a continuous source
and allows for first order degradation and linear sorption (i.e., organic
pollutants) with dispersion. The model was verified using a case history
of groundwater contaminated with hexavalent chromium, although the
effects of adsorption and degradation were ignored.
Leachate Plume Migration Model (Kent, et. al. 1982)
The Leachate Plume Migration Model was developed as an analytical
technique for the hazard evaluation of existing and potential, continuous
source waste disposal sites by predicting plume migration and mixing in
the saturated zone. Predictions can be made from nomographs, hand-held
calculators, or a large scale computer. The model allows for degradation
(i.e., radioactive and biological) of constituents and for the effects of
dispersion and diffusion. The predictive methods presented are
simplified so that a strong background in mathematics and computer
programming are not required for their use. The model has been verified
using data from a chromium plume at Long Island and is presently being
tested against other case studies.
Clearv Model (Clearv. 1982)
The Cleary Model consists of ten different analytical models that
describe mass transport and groundwater flow, with dispersion, under a
variety of boundary conditions. The model addresses conservative
constituents (i.e., without degradation). The ten available models are:
• 1-dimensional, mass transport; 1st type boundary conditions.
• 1-dimensional, mass transport; 3rd type boundary conditions.
• 2-dimensional, mass transport; strip boundary, finite width.
. 2-dimensional, mass transport; strip boundary, infinite width.
• 2-dimensional, mass transport; Gaussian source, infinite width.
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• 3-dimensional, mass transport; patch source, finite dimensions.
• 3-dimensional, mass transport; 5 area Gaussian source.
• 2-dimensional, groundwater flow; infinite dimensions, recharge
boundary.
• 2-dimensional, groundwater flow; finite dimensions, recharge
boundary.
• 2-dimensional, groundwater flow; infinite dimensions, no recharge.
These models were not available for review and R. Cleary (developer)
could not be contacted; information concerning these models was,
therefore, very limited.
AT123D (Yeh. 1981)
AT123D, developed by G. T. Yeh at Oak Ridge National Laboratory, is a
generalized transient, one-, two-, or three-dimensional analytical
computer model for estimating waste transport in both the unsaturated and
saturated zones. The model is flexible, providing 450 options: 288 for
the 3-dimensional case; 72 for the 2-dimensional case in the x-z plane;
72 for the 2-dimensional case in the x-y plane; and 18 for the
1-dimensional case in the longitudinal direction. AT123D models all of
the following options:
• Eight sets of source configurations (i.e., point source; line
source parallel to x-, y-, or z-axis; area source perpendicular to
the x-, y-, or z-axis; and a volume source).
• Three kinds of source releases (instantaneous, continuous, and
finite duration releases).
• Four variations of the aquifer dimensions (finite depth and width,
finite depth and infinite width, infinite depth and finite width,
infinite depth and infinite width).
• Modeling of radioactive wastes, chemicals, and head levels.
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The transport mechanisms addressed are advection, hydrodynamic
dispersion, adsorption, decay/degradation, and waste losses to the
atmosphere from the unsaturated zone. The model is computer coded and
publicly available, making it a potentially valuable tool for preliminary
assessment of waste disposal sites. Fifty sample problems (simulations)
have been performed but actual field verification appears to be lacking.
Screening Procedure (Falco et al.. 1980)
A screening procedure for assessing the transport and degradation of
solid waste constituents in the saturated zone as well as surface waters
was developed by Falco et al., (1980). The procedure estimates the
movement and degradation of chemicals released from landfills and lagoons
based on the physical and chemical properties of the compound and a
defined range of environmental conditions that the compound would be
expected to encounter in groundwater. The procedure developed involves
two parts, a mathematical model to obtain quantitative estimates of
exposure and a logic sequence that assigns qualitative descriptors of
behavior (e.g., low, significant, high) based on the quantitative
estimates of exposure. Quantitative estimates are based on hydrolysis,
biological degradation, oxidation, and sorption. The results of using
this procedure indicate that it provides a means of qualitatively
screening organic chemicals when specific process rates are available.
PATHS (Nelson and Schur. 1980)
The PATHS groundwater model is a hybrid analytical/numerical model
for two-dimensional, saturated groundwater flow that estimates single
contaminant transport under homogeneous geologic conditions. The model
also considers the effect of equilibrium ion exchange reactions for a
single contaminant at trace ion concentrations. Dispersion effects are
not considered by the model. The model provides a fast, inexpensive,
first-cut evaluation consistent with the amount of field data usually
available for a site. Analytical verifications have been performed but
field verifications have not.
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5.2.2.2 Numerical Models
The nine numerical models characterized in this section generally
address the following characteristics:
• Coupled saturated/unsaturated zones.
• Primary porosity.
• Heterogeneous, anisotropic aquifers.
• Miscible constituents.
• Dispersion.
• Attenuation/degradation.
This group of models represents the most flexible approach to
modeling a wide range of hydrogeologic conditions and contaminant types
because the governing equations are not simplified as they are for the
analytical models. These models generally involve greater costs and
require accurate geohydrologic data for a given site.
MMT/VTT/UNSAT1D (Battelle. 1982)
MMT (Multicomponent Mass Transport) is a one- or two-dimensional mass
transport code for predicting the movement of contaminants in the
saturated or unsaturated zone. The MMT model utilizes the discrete
parcel random-walk method and was originally developed to simulate the
migration of radioactive contaminants. The model accounts for
equilibrium sorption, first-order decay and n-members radioactive decay
chains. A velocity field (i.e., groundwater flow equations) must be
input to the model and this is generally accomplished by coupling with
the VTT (Variable Thickness Transient) model for the saturated zone and
UNSAT1D (One-Dimensional Unsaturated Flow) model for the unsaturated
zone. A computer package facilitates interpretation of results by
providing graphic data displays. The model has been used at the Hanford,
Washington, site to predict tritium concentrations.
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CFEST/UNSAT1D (Battelle. 1982)
CFEST (Coupled Fluid Energy and Solute Transport) predicts fluid
pressure, temperature, and contaminant concentrations in saturated
groundwater systems. Coupling the model with UNSAT1D allows for modeling
the unsaturated zone. The model applies finite element techniques to
solve equations. The flow system may be complex, multi-layered,
heterogeneous and anisotropic with time-varying boundary conditions and
time-varying areal sources and sinks. Sorption and contaminant
degradation are presently being incorporated into the model. The model
is presently being field verified for EPA at the Charles City, Iowa, site
for arsenic and pharmaceutical chemical waste (organics). Model
documentation is in preparation.
Pollutant Movement Simulator (Khaleel and Reddell. 1977)
The Pollutant Movement Simulator is a three dimensional model
describing the two-phase (air-water) fluid flow equations in a coupled
saturated-unsaturated porous medium. Flow equations are solved by finite
difference methods. A three dimensional convective-dispersive equation
was also developed to describe the movement of a conservative,
noninteracting tracer in nonhomogeneous, anisotropic porous medium.
Convective-dispersive equations are solved by the method of
characteristics. Attenuation processes have been incorporated into the
model since its original release. The model has been tested for salt
(NaCL) movement in sample plots and is presently being used in coal mine
contamination studies.
FEMWASTE (Yen. 1981)
FEMWASTE, developed by G. T. Yeh at Oak Ridge National Laboratory, is
a two dimensional, finite element, mass transport model for the coupled
saturated-unsaturated hydrologic zones. This model utilizes FEMWATER,
also developed by Yeh, to provide the groundwater flow field allowing for
a variety of boundary conditions and initial moisture conditions.
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Additionally, FEMWASTE incorporates the effects of convection,
dispersion, chemical sorption and first order decay in the mass transport
equations. FEMWASTE/FEMWATER is computer coded and available to the
public. This model has been field verified and is presently being used
by the Carson City office of the USGS.
Random Walk Solute Transport Model (Prickett et al.. 1981)
The Random Walk Solute Transport Model, developed by Prickett,
Naymik, and Lonnquist (1981), predicts the transport of chemical species
(e.g., organics, metals, inorganics) in the saturated zone by the random
walk or particle-in-a-cell method. Mass transport equations include
provisions for dispersion and chemical reactions (attenuation). The
model also accounts for time varying pumpage, injection by wells, natural
or artificial recharge, water exchange between surface water and
groundwater, and flow from springs. Chemical constituent concentrations
in any segment of the model can be specified. Flow equations are solved
by finite difference methods. The model has been documented and made
available to the public. Analytical and field (i.e., fertilizer plant,
Meredosia, Illinois) verifications have been performed.
Solute Transport j.nd Dispersion Model (Konikow and Bredehoeft. 1974)
The Solute Transport and Dispersion Model simulates the movement of
conservative chemical species in a two dimensional, coupled
unsaturated-saturated hydrologic zone. Flow equations are solved using
the finite difference method while mass transport equations are solved by
the method of characteristics. The model allows for the incorporation of
pumping or recharging wells, diffuse infiltration, and for varying the
transmissivity, boundary conditions, contaminant concentrations, and
saturated thickness. Analytical and numerous field verifications have
been performed for the model (e.g., Hanford Reservation, Washington for
radioactives; Rocky Mountain Arsenal, Colorado for pond leachate).
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SWIFF (USGS. 1978)
The SWIFP model simulates the movement of nonconservative
constituents through the saturated zone in three dimensions. The model
incorporates dispersion processes and also allows for deep well injection
predictions. Flow and transport equations are solved by the finite
difference method. This model is well documented and maintained, and has
been field and analytically verified. Presently field verifications are
being performed in New Jersey for sea-water intrusion, in Carson City for
geothermal transport, and in Minnesota for coal tar residues. A version
of SWIFP has also been developed for the Nuclear Regulatory Commission to
handle radioactive materials.
Solute Transport/Groundwater Flow Model (Colder Associates. 1982)
This model simulates the movement of multiple conservative
constituents without dispersion in the saturated zone. Flow and mass
transport equations are solved using finite element techniques. The
model has been analytically and field verified.
Leachate Organic Migration and Attenuation Model (Svk.es et al.. 1982)
The Leachate Organic Migration and Attenuation Model simulates the
movement of nonconservative organic solutes through the
saturated-unsaturated zone. The model is generally run in one or two
dimensions but can be modified for three dimensional analysis. Flow and
mass transport equations are solved by finite element techniques. This
model is specific to sanitary landfills because it measures organics as
chemical oxidation demand, and addresses biodegradation, adsorption,
convection, and dispersion processes. The model is currently being
revised. Field verification has been performed for the model at the
Borden Landfill, Ontario, Canada for chloride and potassium, at granite
sites for nuclear wastes, and for aldicarb.
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5.3 Model Limitations
The following issues provide a context for considering the
limitations inherent in the application of models to evaluate or predict
groundwater problems.
• Modeling results represent approximations of the actual movement
of contaminants and groundwater; results should be used to
estimate the comparative magnitude of a problem and to assign
priorities.
• Models should be verified against actual field observations to
determine how closely they simulate real world situations;
verification should be performed in the actual hydrogeologic
system to which the model is to be applied.
• Model accuracy may vary dramatically when models are applied to
situations for which they have not been verified.
• Models presently do not simulate all the processes that control
contaminant movement; the equations that describe attenuation and
dispersion are especially weak in most models.
• Generally, the capacity of a model to simulate field situations is
a function of its complexity; the more complex the model, the more
data are required. Model reliability becomes a function of data
accuracy, i.e., "garbage in, garbage out".
• Models for which sensitivity analyses have not been conducted may
generated mathematical errors when parameters are changed and
assumptions modified.
Because of the complexity and limitations of models, assertions
determined through the use of models should not be interpreted as actual
values but only as estimates.
For Wisconsin zone of saturation landfills, those models which
address leachate production, such as DRAINMOD/DRAINFIL and LSIPE, are
applicable. LTTM may be useful when fully documented and verified.
Solute transport models, such as SWIFP and the Random Walk. Solute
Transport Model, may be applicable if leachate migration from existing
landfills is detected or suspected.
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6.0 RECOMMENDATIONS
Zone of saturation landfills are by nature very susceptible to
contaminant releases because of the large amounts of water, both
groundwater inflows and leachate, that must be removed by the drainage
system. If the only sites available for construction of the landfills
are in the saturated zone, the following recommendations may help limit
the probabilities of leachate release:
• Determine as accurately as possible the values of inflow rates (q)
and hydraulic conductivities (K) of both landfill material and
soils because these values have a major impact on drain system
design, especially drain spacing.
• Determine the hydrogeology of the site accurately so that
groundwater level variations throughout the year are known and
conservative head maintenance levels can be specified (i.e.,
always maintaining inward hydraulic gradients).
• Design the landfill base so that it slopes toward the drains, thus
allowing for lower head maintenance levels and more rapid leachate
removal.
• Specify low head maintenance levels within the fill, thus reducing
the hydraulic head capable of discharging leachate (i.e., for
hazardous waste sites, EPA regulations require heads less than
1-foot).
• Select the proper drain equation for system design and allow for a
margin of error in the results obtained (i.e., design
conservatively).
• Design filters and envelopes for drain pipes that prevent silting
and allow for free flow of liquids.
• Remove daily fill covers from cells or ensure by some other means
that the cells are hydraulically connected to the drainage system
thus allowing free flow of water between cells and drains.
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• Incorporate drain blankets into landfill designs to reduce head
maintenance levels without decreasing drain spacing and improve
the overall efficiency of leachate collection.
• Compact clay base and sidewall layers at optimum working moisture
conditions so that high density, low permeability barriers are
created without shearing or fracturing the clay.
• Provide construction inspections to ensure that critical
operations such as placement of leachate collection drain and
filter envelopes, base gradings, and clay recompaction are
performed as specified.
• Design leachate collection system so that routine maintenance and
inspections can be performed to adequately maintain flows.
• Provide for continuous monitoring of leachate collection volumes
and head levels so that problems can be identified quickly.
• Design landfill drainage systems to incorporate anticipated future
operations such as the acceptance of large quantities of liquids.
Understanding the drain equations and the theory behind them and
incorporating the above recommendations into the original landfill design
can substantially reduce the chances of leachate release. However,
unless the designs are incorporated properly during construction, the
system will fail to meet its intended purpose.
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7.0 BIBLIOGRAPHY
7.1 Background
Fenn, D.G., K.J. Hanley, and T.V. Degeare. 1975. Use of the water
balance method for predicting leachate generation from solid waste
disposal sites. U.S.EPA-530/SW-168, Cincinnati, Ohio. 40 pp..
Fungaroli, A.A. 1971. Pollution of subsurface water by sanitary
landfills. U.S.E.P.A. Solid Waste Management Series, SW-12rg. Vol. 1.
132 pp.
Fungaroli, A.A. and R.L. Steiner. 1979. Investigation of sanitary
landfill behavior. Volume 1. U.S.E.P.A. 600/2-79-053a.
Gerhart, R.A. 1977. Leachate attenuation in the unsaturated zone
beneath three sanitary landfills in Wisconsin. University of Wisconsin.
Wisconsin Geological and Natural History Survey. Info. Circular No. 3.
93 pp.
Green, J.H. et al. 1965. Groundwater pumpage and water level changes in
Milwaukee-Waukesha area, 1950-61. USGS Water Supply Paper 1809-1.
Harr, C.A., L.C. Trotta, and R.G. Borman. 1978. Ground-water resources
and geology of Columbia County, Wisconsin. University of Wisconsin -
Extension. Geological and Natural History Survey. Info. Circular
No. 37. 30 pp.
Holt, C.L.R., Jr. 1965. Geology and water resources of Portage County,
Wisconsin. USGS Water Supply Paper 1796. 77 pp.
Hughes, G.M. et al. 1971. Hydrogeology of solid waste disposal sites in
Northeastern Illinois. U.S.E.P.A. Publication No. SW-12d. 154 pp.
LeRoux, E.F. 1957. Geology and ground water resources of Outagamie
County. USGS Water Supply Paper 1604.
Newport, T.G. 1962. Geology and ground water resources of Fond du Lac
County, Wisconsin, USGS Water Supply Paper 1604.
Olcott, P.G. 1966. Geology and water resources of Winnebago County,
Wisconsin. Geological Survey Water Supply Paper 1814.
Roudkivi, A.J. and R.A. Callander. Analysis of groundwater flow. John
Wiley and Sons, N.Y. 214 pp.
-97-
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Sherrill, M.G. 1978. Geology and ground water in Door County,
Wisconsin, with emphasis on contamination potential in the Silurian
Dolomite. Geological Survey Water Supply Paper 2047.
Soil Conservation Service. 1980. Ground water resources and geology of
Washington and Ozaukee Counties, Wisconsin. University of Wisconsin -
Extension. Geological and Natural History Survey. Inform Circular
No. 38, 37 pp.
Young, H.L. and Batten, W.G. 1980. Soil Survey of Winnebago County,
Wisconsin. U.S. Department of Agriculture.
7.2 Drains
Bureau of Reclamation. 1978. Drainage Manual. Water Resource Technical
Publication. United States Government Printing Office, Washington, D.C.
286 pp.
DuPont Company. 1981. Designing and Constructing Subsurface Drains.
DuPont Company, TYPAR Sales, Wilmington, Delaware.
Hooghoudt, S.B. 1940. Bijdragen tot de kennis van enige natuurkundige
grootheden van de grond. No. 7. Versl. Landbouwk. 46:515-707.
Konet, P., K.J. Quinn and C. Slavik. 1981. Analysis of design
parameters affecting the collection efficiency of clay lined landfills.
In: Proceedings of Fourth Annual Madison Conference of Applied Research
and Practice on Municipal and Industrial Waste. September 28-30,
University of Wisconsin Extension, Madison, WI.
Moore, C.A. 1980. Landfill and Surface Impoundment Performance
Evaluation. U.S. Environmental Protection Agency, SW-869, Office of
Water and Waste Management, Washington, D.C.
Moulton, L.K. 1979. Design of subsurface drainage systems for the
control of groundwater. Presented at: 58th Annual Presentation of the
Transportation Research Board, Washington, D.C.
Powers, J.P. 1981. Construction Dewatering. John Wiley and Sons. New
York. 484 pp.
Sichardt, W. and W. Kyrieleis. 1940. Grundwasserabenkungen bei
Fundierunesarbeiten. Berlin, Germany.
Soil Conservation Service. 1973. Drainage of Agricultural Land. Water
Information Center, Inc. Syosset, New York. 430 pp.
Van Schlifgaarde, J. 1974. Drainage for Agriculture. American Society
of Agronomy 17, Madison, Wisconsin.
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Wesseling, J. 1973. Theories of Field Drainage and Watershed Runoff:
Subsurface Flow into Drains. In: Drainage Principles and Applications.
International Institute for Land Reclamation and Improvement. P.O.
Box 45, Wageningen, The Netherlands.
Winger, R.J. and W.F. Ryan. 1970. Gravel Envelopes for Pipe
Drain-Design. American Society of Agricultural Engineers. Chicago,
Illinois, December 8-11.
7.3 Flow Nets
Bennett, R.R. 1962. Flow Net Analysis. In: Ferris, J.G., D.B.
Knowles, R.M. Brown, and R.W. Stallman. 1962. Theory of Aquifer Tests:
U.S. Geological Survey Water Supply Paper 1536-E, pp. 139-144.
Bennett, R.R. , and R.R. Meyer. 1952. Geology and Groundwater Resources
of the Baltimore Area. Maryland Department of Geology, Mines, and Water
Resources. Bulletin 4, 573 p.
Cedergren, H.E. 1977. Seepage, Drainage, and Flow Nets (2nd Edition).
John Wiley and Sons, New York. 534 p.
Freeze, R.A. and J.A. Cherry. 1979. Ground Water. Prentice-Hall,
Englewood Cliffs, New Jersey, pp. 168-191.
Lohman, S.W. 1979. Ground Water Hydraulics. U.S. Geological Survey
Professional Paper 708. 70 p.
Mansur, C.I. and R.I. Kaufman. 1962. Dewatering. In: Leonards, G.A.
(Ed.). 1962. Foundation Engineering. McGraw-Hill, New York.
pp. 241-350.
Powers, J.P. 1981. Construction Dewatering - A Guide to Theory and
Practice. John Wiley and Sons, New York. 484 pp.
7.4 Models
7.4.1 Release Rate Models
All, E.M., C.A. Moore, and I.L. Lee. 1982. Statistical Analysis of
Uncertainties of Flow of Liquids Through Landfills. Proceedings of the
Eighth Annual Research Symposium: Land Disposal of Hazardous Wastes.
EPA-600/ 9-82-002; U.S. Environmental Protection Agency, Cincinnati, OH.
pp. 26-52.
Anderson, D. , K.W. Brown and T. Green. 1982. Effect of
on the Permeability of Clay Soil Liners. Proceedings of the Eighth
Annual Research Symposium: Land Disposal of Hazardous Wastes.
EPA-600/9-82-002; U.S. Environmental Protection Agency, Cincinnati, OH.
pp. 174-178.
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Bailey, G. November 1982. Personal communication.
Barrier, R.M. 1978. Zeolites and Clay Minerals as Sorbents and
Molecular Sieves. Academic Press, New York. 497 pp. As cited in Haxo
et al., 1980.
Brady, N.C. 1974. The Nature and Property of Soils. Macmillan
Publishing Co., Inc. New York, N.Y.
Brenner, Walter and Barry Rugg. 1982. Exploratory Studies on the
Encapsulation of Selected Hazardous Wastes with Sulfur Asphalt Blends.
Proceedings of the Eighth Annual Research Symposium: Land Disposal of
Hazardous Waste. EPA-600/9-82-002; U.S. Environmental Protection Agency,
Cincinnati, OH. pp. 315-326.
Burns, J., and G. Karpinski. August 1980. Water Balance Method
Estimates How Much Leachate Site Will Produce. Solid Wastes Management.
pp. 54-86.
Chou, Sheng-Fu J., Robert A. Griffin, and Mei-In M. Chou. 1982. Effect
of Soluble Salts and Caustic Soda on Solubility and Adsorption of
Hexachlorocyclopentadiene. Proceedings of the Eighth Annual Symposium:
Land Disposal of Hazardous Waste. EPA-600/9-82-002; U.S. Environmental
Protection Agency, Cincinnati, OH. pp. 137-149.
D'Appolonia, D.J. and C.R. Ryan. 1979. Soil-Bentonite Slurry Trench
Cutoff Walls. In: Geotechnical Exhibition and Technical Conference
Proceedings, Engineered Construction International, Inc., Chicago, IL.
Dragun, James and Charles S. Helling. 1982. Soil and Clay Catalyzed
Reaction: I. Physicochemical and Structural Relationships of Organic
Chemicals Undergoing Free - Radical Oxidation. Proceedings of the Eighth
Annual Research Symposium: Land Disposal of Hazardous Waste.
EPA-600/9-82-002; U.S. Environmental Protection Agency, Cincinnati, OH.
pp. 106-121.
Falco, J.W., L.A. Mulkey, R.R. Swank, Jr., R.E. Lipcsei, and S.M. Brown.
A Screening Procedure for Assessing the Transport and Degradation of
Solid Waste Constituents in Subsurface and Surface Waters. (Unpublished
paper)
Fenn, D.G., K.J. Hanley, and T.V. Degeare. 1975. Use of the Water
Balance Method for Predicting Leachate Generation from Solid Waste
Disposal Sites. EPA/530/SW-168, Solid Waste Information, U.S.
Environmental Protection Agency. Cincinnati, OH. 40 pp.
Freeze, R.A. and J.A. Cherry. 1979. Groundwater. Prentice-Hall, Inc.,
Englewood Cliffs, N.J. 604 pp.
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Fuller, W.H. 1982. Methods for Conducting Soil Column Tests to Predict
Pollution Migration. Proceedings of the Eighth Annual Research
Symposium: Land Disposal of Hazardous Wastes. EPA-600/9-002; U.S.
Environmental Protection Agency, Cincinnati, OH. pp. 87-105.
Garrett, B.C., J.S. Warner, M.P. Miller, and L.G. Taft. 1982.
Laboratory and Field Studies of Factors in Predicting Site Specific
Composition of Hazardous Waste Leachate. Proceedings of the Eighth
Annual Research Symposium: Land Disposal of Hazardous Wastes.
EPA-600/9-82-002; U.S. Environmental Protection Agency, Cincinnati, OH.
pp. 67-86.
Gibson, A.C. and P.G. Malone. 1982. Verification of the U.S. EPA HSSWDS
Hydrologic Simulation Model. Proceedings of the Eighth Annual Research
Symposium: Land Disposal of Hazardous Wastes. EPA-600/9-82-
002; U.S. Environmental Protection Agency, Cincinnati, OH. pp. 13-25.
Giroud, J.P. and J.S. Goldstein. 1982. Geomembrane Liner Design. Waste
Age, September 1982.
Glaubbinger, R.S., P.M. Kohn, and R. Ramirez. 1979. Love Canal
Aftermath: Learning from a Tragedy. Chemical Engineering. October 22.
Guerero, P., November 1982. Personal Communication.
Hardcastle, J.H. and J.K. Mitchell. 1974. Electrolyte Concentration
Permeability Relations in Sodium Illite-Silt Mixtures. Clays and Clay
Minerals. 22(2) -.143-154. As cited in. Haxo, et al. , 1980.
Haxo H E. et al. September 1980. Lining of Waste Impoundment and
Disposal Facilities. EPA/530/SW-870c, U.S. Environmental Protection
Agency, Cincinnati, OH. 385 pp.
Haxo HE Jr. 1981. Testing Materials for Use in the Lining of Waste
Disposal Facilities, Hazardous Solid Waste Testing: First Conference
ASTM STP 760, American Society for Testing and Materials, pp. 269-292.
Haxo H E 1982. Effects on Liner Materials of Long-Term Exposure in
Waste Environments. Proceedings of the Eight Annual Research Symposium:
Land Disposal of Hazardous Waste. EPA-600/9-82-002; U.S. Environmental
Protection Agency, Cincinnati, OH. pp. 191-211.
Huck PJ 1982. Assessment of Time Domain Reflectrometry and Acoustic
Emission Monitoring; Leak Detection Systems for Landf"1 jj»««-
Proceedings of the Eighth Annual Research Symposium: Land Disposal of
Hazardous Waste. EPA-600/9-82-002; U.S. Environmental Protection Agency,
Cincinnati, OH. pp. 261-273.
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Hughes, J. 1975. Use of Bentonite as a Soil Sealant for Leachate
Control in Sanitary Landfills. Soil Lab. Eng. Report Data 280-E.
American Colloid Co., Skokie, IL. As cited i.n Haro et al., 1980.
Hung, Cheng Y. 1980. A Model to Simulate Infiltration of Rainwater
Through the Cover of a Radioactive Waste Trench Under Saturated and
Unsaturated Conditions. Draft submitted to AGU for "Role of the
Unsaturated Zone in Radioactive and Hazardous Waste Disposal," to be
published by the Ann Arbor Science Publishers in 1983.
ICF Incorporated, Clement Associates, Inc., and SCS Engineers, Inc.,
1982. RCRA Risk/Cost Policy Model Project, Phase 2 Report. Submitted
to: Office of Solid Waste, U.S. Environmental Protection Agency,
June IS, 1982. Draft internal agency document.
JRB Associates, Inc. 1982. Techniques for Evaluating Environmental
Processes Associated with the Land Disposal of Specific Hazardous
Materials, EPA Contract No. 68-01-5052, DOW No. 36, Task 1.
Kinman, Riley N., Janet I. Rickabaugh, James J. Walsh, and W. Gregory
Vogt. 1982. Leachate from Co-Disposal of Municipal and Hazardous Waste
in Landfill Simulators. Proceedings of the Eighth Annual Research
Symposium: Land Disposal of Hazardous Waste. EPA-600/9-82-002; U.S.
Environmental Protection Agency, Cincinnati, OH. pp. 274-293.
Krisel, W.J., Jr., Editor. 1980. CREAMS, a Field Scale Model for
Chemical Runoff and Erosion from Agricultural Management Systems. Vols.
I, II, and III. Draft copy. USDA-SEA, AR, Cons. Res. Report 24.
Kumar, J. and J.A. Jedlicka. 1973. Selecting and Installing Synthetic
Pond Linings. Chemical Engineering. February 5, 1973.
Lee, J. 1974. Selecting Membrane Pond Liners. Pollution Engineering.
6(1):33-40.
Lentz, J.J. 1981. Apportionment of Net Recharge in Landfill Covering
Layer into Separate Components of Vertical Leakage and Horizontal
Seepage. Vol. 17, No. 4, Water Resources Research, American Geophysical
Union, pp. 1231-1234.
Lu, J.C.S., R.D. Morrison, and R.J. Stearns. Leachate Production and
Management from Municipal Landfills: Summary and Assessment. Calscience
Research, Inc. (Unpublished paper)
Lyman, W.J., W.F. Reehl, and D.H. Rosenblatt. 1982. Handbook of
Chemical Property Estimation Methods: Environmental Behavior of Organic
Compounds. Prepared by Arthur D. Little, Inc. for the U.S. Army
Bioengineering Research and Development Laboratory, Fort Detrick, MD. As
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Mabey, W. and T. Mill. 1978. Critical Review of Hydrolysis of Organic
Compounds in Water Under Environmental Conditions. J. Phys. and Chem.
1(2):383, 1978. As cited in. JRB Associates, 1982.
Mielenz, R.C. and M.E. King. 1955. Physical-Chemical Properties and
Engineering Performance of Clays. Bulletin 169. California Division of
Mines, pp. 196-254. As cited in Haxo, et al., 1980.
Mingelgren, U. et al. 1977. A Possible Model for the Surface-Induced
Hydrolysis of Organophosphorus Pesticides on Kaolinite Clays, Soil
Science Society of America J. 41.. As cited in JRB Associates, 1982.
Mill, X. 1980. Data Needed to Predict the Environmental Fate of Organic
Chemicals. In: Dynamics, Exposure and Hazard Assessment of Toxic
Chemicals. Rizwanul and Hague, Eds. As cited ill JRB Associates, 1982.
Montague, P. 1982. Hazardous Waste Landfills: Some Lessons from New
Jersey. Civil Engineering. ASCE. September 1982.
Moore, C.A. September 1980. Landfill and Surface Impoundment
Performance Evaluation. SW-869, U.S. Environmental Protection Agency,
Cincinnati, OH. 63 pp.
Moore, C.A. and M. Roulier. 1982. Evaluating Landfill Containment
Capability. Proceedings of the Eighth Annual Research Symposium: Land
Disposal of Hazardous Wastes. EPA-600/9-82-002; U.S. Environmental
Protection Agency, Cincinnati, OH. pp. 174-178.
Moore, C.A. and E.M. Ali. 1982. Permeability of Cracked Clay Liners.
Proceedings of the Eighth Annual Research Symposium: Land Disposal of
Hazardous Wastes. EPA-600/9-82-002; U.S. Environmental Protection
Agency, Cincinnati, OH. pp. 174-178.
Perrier, E.R.. and A.C. Gibson. September 1980. Hydrologic Simulation
of Solid Waste Disposal Sites. EPA/530/SW-868c, U.S. Environmental
Protection Agency, Cincinnati, OH. Ill PP-
Peters WR DW Shultz, and B.M. Duff. 1982. Electrical Resistivity
£ch"ques for Locating L^ner Leaks. Proceedings of the Eight Annual
Research Symposium: Land Disposal of Hazardous Wastes.
EPA-600/9-82-002; U.S. Environmental Protection Agency, Cincinnati, OH.
pp. 250-260.
Pohi«nrf Frederick G Joseph P. Gould, R. Elizabeth Ramsey, and Daniel!
C Waters 1982 The Behavior of Heavy Metals During Landfill Disposal
of Hazardous Wastes. Proceedings of the Eight A™
Symposium: Land Disposal of Hazardous Wastes^ EP
Environmental Protection Agency, Cincinnati, OH. pp
-103-
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Pope-Reid Associates, Inc. 1982. Hazardous Waste Landfill Design, Cost
and Performance Modelling. Unpublished draft report.
Prickett, T.A. July 1982. Personal Communication.
Rosenbaum, M.S. 1976. Effect of Compaction on the Pore Fluid Chemistry
of Montmorillonite. Clays and Clay Minerals. 24:118-121. As cited ill
Haxo, et al., 1980.
SCS Engineers, Inc. 1982. Release Rate Computations for Land Disposal
Facilities. Currently under development for EPA.
Shuckrow, Alan J. and Andrew P. Pajak. 1982. Studies on Leachate and
Groundwater Treatment at Three Problem Sites. Proceedings of the Eighth
Annual Research Symposium: Land Disposal of Hazardous Waste.
EPA-600/9-82-002; U.S. Environmental Protection Agency, Cincinnati, OH.
pp. 346-359.
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Skaggs, R.W. 1982. Modification to DRAINMOD to Consider Drainage from
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Skaggs, R.W. 1982. Field Evaluation of a Water Management Simulation
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SCS Engineers, Inc. 1982. Release Rate Model. Currently under
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Siefken, D.L. and R.J. Starmer. 1982. NRC-Funded Studies on Waste
Disposal in Partially Saturated Media. AGU Spring Meeting, 4 June 1982.
Silka, L.R. and T.L. Sweringen. 1978. A Manual for Evaluating
Contamination Potential of Surface Impoundments. U.S. EPA, Groundwater
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Smith, L. and F.W. Schwartz. 1981. Mass Transport. 3. Role of
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in Fractured Porous Media: Analytical Solution for a Single Fracture.
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U.S. EPA. 1982. Post-Closure Liability Trust Fund Model Development.
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Leachate and Soil Interactions in an Aquifer. Proceedings of the Third
Annual Municipal Solid Waste Research Symposium on Management of Gas and
Leachate in Landfills. EPA/600/9-77-026, p. 95-103.
Wagner, J. 1982. Personal communication.
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Assessment. U.S. EPA, Office of Solid Waste. EPA/513/684-8491.
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in Unsaturated Flow. 1. The Analytical Method. Water Resources
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L.B. Baskin. 1982. Transport and Fate of Selected Organic Pollutants in
a Sandy Soil. U.S. EPA, RSKERL, Ada, OK. (Unpublished report)
Wyrick, G.G. and J.W. Borchers. 1982. Hydrologic Effects of
Stress-Relief Fracturing in an Appalachian Valley. USGS Water Supply
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Yeh, G.T. and D.S. Ward. 1980. FEMWATER: A Finite-Element Model of
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