United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-87/050 Apr. 1988
v>EPA Project Summary
Verification of the Hydrologic
Evaluation of Landfill
Performance (HELP) Model
Using Field Data
P. R. Schroeder and R. L. Peyton
The study described was conducted
to verify the lateral drainage compo-
nent of the Hydrologic Evaluation of
Landfill Performance (HELP) computer
model using laboratory drainage data
from two large-scale physical models
of landfill liner/drainage systems.
Drainage tests were run to examine the
effects that drainage length, slope,
hydraulic conductivity and depth of
saturation have on the lateral drainage
rate. The drainage results were com-
pared with HELP model predictions and
numerical solutions of the Boussinesq
equation for unsteady, unconfined flow
through porous media.
This Project Summary was devel-
oped by EPA's Hazardous Waste Engi-
neering Research Laboratory, Cincin-
nati, OH, to announce key findings of
the research project that is fully doc-
umented in a separate report of the
same title (see Project Report ordering
information at back).
Purpose and Scope
This study was performed to verify
Version 1 of the Hydrologic Evaluation
of Landfill Performance (HELP) model
using existing field data. Mathematical
simulations of 20 landfill cells at seven
sites across the United States were made
using the HELP model and were com-
pared to measured field data. Measure-
ments of leachate drainage were avail-
able from all 20 landfill cells, while data
on runoff were available only from about
half of the cells. Measurements of
percolation were available only from one
cell and no data on evapotranspiration
were available. These landfills included
a wide variety of conditions for which the
HELP model was tested. The cells ranged
in size from 0.04 to 24 acres and the
simulation periods ranged from 2.5 to 8
years. This report summarizes the results
of these simulations and evaluates the
verification that has been achieved. In
addition, the report presents a sensitivity
analysis for the input parameters used
in the HELP model and a review of landfill
design regulation and guidance in light
of the results of the verification studies
and sensitivity analysis.
The HELP model was developed to help
hazardous waste landfill designers and
evaluators estimate the magnitudes of
components of the water budget and the
height of water-saturated soil above
barrier soil layers (liners). This quasi-
two-dimensional, deterministic
computer-based water budget model
performs a sequential daily analysis to
determine runoff, evapotranspiration,
percolation, and lateral drainage for the
landfill (cap, waste cell, leachate collec-
tion system, and liner) and obtain esti-
mates of daily, monthly, and annual
water budgets. The model does not
account for lateral inflow or surface
runon.
The HELP model computes runoff by
the Soil Conservation Service (SCS)
runoff curve number methpd. Percolation
is computed by Darcy's law, modified for
unsaturated flow. Lateral flow is com-
puted by a linearization of the Boussinesq
equation, and evapotranspiration is
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determined by a method developed by
Ritchie. The vertical percolation and
evapotranspiration components of the
HELP model originated with the Chem
ical, Runoff, and Erosion from Agricul-
tural Management Systems (CREAMS)
model. The development presented here,
however, reflects a significant advance
beyond the CREAMS model in both of
these areas.
In the course of the development of
any model, provisions should be taken
to verify that the model accurately
represents reality. Laboratory tests have
been performed to verify the lateral
drainage portion of the HELP model, but
prior to this study the other portions of
the model had only been calibrated and
not verified. This report presents the
results of efforts to verify the model using
existing data collected at landfills, test
cells, and lysimeters.
This study consists of three parts. The
main part is an attempt to verify the
model using existing data. The purpose
of this part was to assess the adequacy
of the model to simulate reality and to
validate the use of the model to evaluate
designs (generally with minimum infor-
mation). The second part of this study
is a sensitivity analysis of the principal
input parameters used in the HELP
model. The purpose of this part was to
determine which parameters need to be
well-defined and what are the likely
effects of a change in the value of a
parameter. This analysis also provided
much insight for interpreting and
explaining the verification results. The
third part of the study consists of an
evaluation of the technical guidance
developed to support the regulations
regarding design and operation of a
landfill. The purpose of this part was to
examine the results of the laboratory and
field verification studies and the sensi-
tivity analysis to determine whether the
technical guidance was practicable and
achieved its objectives (particularly that
of minimizing potential percolation
through the liner at the base of the
landfill) in the best practicable manner.
Field Verification
Comparisonswere performed between
simulations and measured field data for
seven sites:
(1) University of Wisconsin-
Madison. From 1970 to 1977, eight
large lysimeter cells filled with either
shredded or unprocessed refuse were
monitored for surface runoff and
leachate production. The general
purpose of the study was to determine
the effect on landfill performance of
shredding the refuse prior to place-
ment and covering the refuse with a
soil layer.
(2) Sonoma County, CA. A solid
waste stabilization project was spon-
sored by the U.S. Environmental
Protection Agency and Sonoma
County, California from 1971 to 1974.
The purpose of the project was to
investigate the stabilization of solid
waste in five municipal sanitary
landfill test cells by analyzing leach-
ate, gas, temperature, and settlement
parameters and to determine the
effect on solid waste stabilization of
applying excess water, septic tank
pumpings, and recycled leachate.
Leachate production was measured
for all five cells and runoff was
measured from three of the cells.
(3) Boone County. KY. Two field-
scale test cells and three small-scale
cells were studied from 1971 to 1980
in Boone County, Kentucky under the
sponsorship of the U.S. Environmen-
tal Protection Agency. The study
objectives were to evaluate the
amount and characteristics of leach-
ate, the composition of gases, the
temperature conditions, the settle-
ment of the cells, the clay liner
efficiency, and to compare the behav-
ior between the field-scale and small-
scale cells. The data collected from
one of the field-scale cells was used
in this study.
(4-6) Brown, Eau Claire, and
Marathon Counties, Wl. The State of
Wisconsin Bureau of Solid Waste
Management has reported on the
geologic setting, major design fea-
tures, construction experience, leach-
ate production and operational perfor-
mance of these three large landfills
in Wisconsin. These landfills were
started between 1976 and 1980;
however, none of these landfills has
yet been completely filled and capped
so that each data set reported thus
far represents the conditions of a
continuously expanding landfill. For
example, at any given time, the cover
could range from the daily cover of
a 6-inch-thick blanket of sand or silty
clay to a final cover of clay and topsoil.
(7) Niagara Falls, NY. Since 1976,
a chemical waste management com-
pany has filled and capped three
landfill cells in Niagara Falls, NY. The
surface areas of the cells range from
2 to 5 acres. Records of leachate
pumpage have been kept from 1983
and indicate annual withdrawals
ranging from 1 to 11 inches. An
evaluation of the performance of the
facility during 1984 was reported to
the USEPA Region II by Recra
Research, Inc.
The data used in the simulations were
obtained from a variety of sources. In
most cases, daily rainfall and monthly
temperature data were obtained from the
nearest National Oceanic and Atmos-
pheric Administration weather station.
Solar radiation values stored in the HELP
model were used for all simulations.
Model input values for design data and
soil and waste characteristics were
determined from published reports des-
cribing the construction and operation of
each landfill. In general, the information
available on soil and waste characteris-
tics, surface vegetation, runoff curve
numbers, or evaporative depths was
descriptive and sketchy, not quantitative;
therefore, extensive use was made of
default values stored in the HELP model.
The measured data used for compar-
ison with the HELP model simulations
were primarily lateral leachate drainage
volumes. Measured runoff data was
available from 11 landfill cells. Barrier
soil percolation was measured at one
landfill, although its suitability for model
verificaiton was limited. The measured
data from very similar cells at the same
landfill varied greatly from cell to cell.
For four practically identical cells the
range in total runoff was about 50
percent of the mean total runoff, and for
lateral drainage the range was greater
than 100 percent of the mean.
Runoff
Measured runoff data existed for eight
cells at the University of Wisconsin and
for three cells at Sonoma County, CA.
Runoff was overpredicted for five cells
by an average of 30 percent of the
measured runoff, and underpredicted for
six cells by an average of 20 percent of
the measured runoff. Overall, runoff was
overpredicted by 3 percent. Following
these initial simulations, the curve
numbers were varied to determine their
effect on the overall model prediction of
landfill performance. Five simulations
were improved by a change in curve
number—all had originally underpre-
dicted runoff.
For the three cells at Sonoma County,
it was obvious that the evapotranspira-
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tion and/or soil characteristics were
controlling runoff volume and not the
curve number. Because of this close
interaction, it was difficult to assess the
accuracy of the curve number method in
the HELP model based on the field data
in this report. However, the predicted
runoff volumes appear overall to be in
reasonable agreement with the mea-
sured results.
A comparison of measured and pre-
dicted runoff on a monthly basis for the
University of Wisconsin cells indicated
that the assumptions used in the HELP
model for snowmelt runoff may not be
appropriate. The model stores all precip-
itation on the surface when the mean
daily temperature interpolated from the
mean monthly temperature is below
freezing. When this mean daily temper-
ature rises above freezing, the precipi-
tation is allowed to either run off or
infiltrate. Since mean daily temperatures
are computed in the HELP model based
on mean monthly temperatures which
are generally below freezing in Wiscon-
sin for several consecutive months, no
runoff was predicted by the HELP model
during the winter. Instead, a large runoff
volume was predicted during April of
each year when temperatures warmed.
This compared to measured results
which showed significant runoff
throughout the winter without an exces-
sively large runoff in April. This discrep-
ancy probably contributed to the overpre-
diction of runoff for several cells.
Evapotranspiration
No suitable evapotranspiration field
data from landfill sites was found for
model testing. This was not unexpected
due to the complexities involved in
collecting this type of data. Yet, evapo-
transpiration is typically the single
largest outflow component of the landfill
system; therefore, small changes in
evapotranspiration can have major
impacts on volumes of lateral drainage
and barrier soil percolation.
For those cells which had runoff data
available, a surrogate variable for evapo-
transpiration was identified, and compar-
isons were made between measured and
predicted results. The variable consisted
of the sum of the water balance com-
ponents which were not directly mea-
sured. In the case of the University of
Wisconsin cells, the variable was the
sum of evapotranspiration and change in
moisture storage, ET+DS. For the
Sonoma County cells, it was the sum of
evapotranspiration, change in moisture
storage, and percolation, ET+DS+PERC.
The ET+DS variable was found to be
underpredicted by an average of 4
percent of the measured values, whereas
the ET+DS+PERC variable was
underpredicted by an average of 25
percent. It is obviously rather complex to
discern the meaning of these results
since evapotranspiration, change in
moisture storage, and percolation are all
interrelated. The evidence suggests that
values chosen for evaporative depths
may have been too small.
Lateral Drainage and
Percolation
Since measurements of barrier soil
percolation volumes and leachate pond-
ing depths were not available, the lateral
drainage and barrier soil percolation
submodel could only be evaluated using
measured leachate collection data. One
exception was the Boone County, KY cell
where barrier soil percolation volumes
were measured. However, the configu-
ration of the clay liner and percolation
collection pipe was such that vertical
percolation did not actually occur; rather,
the percolation flow paths were forced
to converge radially toward the collection
pipe. The attmept to simulate this
percolation using the HELP model
resulted in an overprediction of approx-
imately 35 percent.
Lateral drainage was overpredicted by
10 percent of the measured drainage in
two cells where very high leachate
collection rates were observed. In three
cells where very small quantities of
leachate were collected, lateral drainage
was underestimated by 97 percent of the
measured drainage, although this differ-
ence only amounted to 1.4 inches per
year. Of the remaining nine cells, lateral
drainage was overpredicted by an aver-
age of 4 percent of the measured
drainage in five covered cells and over-
predicted by an average of 53 percent
of the measured drainage in four per-
manently uncovered cells with a weath-
ered waste surface that supported dense
vegetation. Small errors in the hydraulic
conductivities of the cover soils can
cause large differences in. the leachate
production when the leachate production
is small. Also the overpredictions may
have been partially related to the manner
in which the HELP model estimates
unsaturated hydraulic conductivities. To
linearly relate unsaturated hydraulic
conductivity to moisture content
between field capacity and saturation
tends to overpredict unsaturated hydrau-
lic conductivity. Thus, moisture is routed
more quickly through the evaporative
zone, contributing to larger leachate
volumes and smaller evapotranspiration
volumes.
The poor reproductions of lateral
drainage for the uncovered cells at the
University of Wisconsin and the three
cells without subsurface liquid addition
at Sonoma County were probably caused
by poor estimates of the hydraulic
conductivity of the surf ace layer. The field
results could be reproduced by adjusting
only the hydraulic conductivity of the
surface layer by less than a factor of 10,
within the range of its probable value.
This result is understandable since
cumulative lateral drainage is dependent
on two main factors: the rate of infiltra-
tion into the lateral drainage layer and
the rate of percolation through the liner
beneath the drainage layer. The rate of
percolation was very small in these cells;
therefore, the rate of infiltration was the
source of error.
Summary
The lack of adequate site description
and measured water budget components
affected the verification study in two
ways. First, the lack of descriptive landfill
information required the frequent use of
default values in the HELP model which
introduced additional uncertainty into
the verification. Second, the lack of water
balance overflow measurements limited
the number of HELP outflow predictions
that could be verified. These limitations
restricted the ability of the study to isolate
and test mathematical characterizations
of specific physical processes, such as
soil mositure storage and routing, evapo-
transpiration demand and its distribution
through the soil profile, unsaturated
vertical drainage, and details of the
apportioning of leachate production
between lateral drainage to collection
systems and vertical percolation through
the clay liner.
In addition, the variable degree of field
measurement precision and reliability
presented challenges in interpreting the
data which did exist. None of the field
data used in this report were collected
specifically for verifying the HELP model;
therefore, the field data were not always
consistent with the needs of this study.
For instance, the data available for the
three largest landfills were collected
while they were simultaneously under-
going expansion. In other cases, there
was large variability in measured results
between otherwise identical landfill
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cells. In general, the error in estimates
of water budget components were much
smaller than the variability in the field
measurements for similar landfill cells.
These results are very good in light of
the fact that the precipitation data used
in this study, which is known to be
spatially highly variable, were not mea-
sured at most of the landfill sites. All of
this required a significant amount of
engineering judgment in interpreting the
data for the HELP model comparisons.
Although a detailed verification of
specific model components was not
always possible, the data did confirm the
model's overall utility in estimating a
landfill water balance even without
extensive knowledge of specific landfill
characteristics. This was an important
finding since the HELP model is typically
used without a large amount of detailed
landfill information.
The following conclusions are made.
The field data verified the utility of the
HELP model for estimating general
landfill performance. However, not all
model components were well tested due
to the limited field data available. It is
concluded that a laboratory and field
monitoring program explicitly designed
for HELP verification would be necessary
for further refinement of specific model
components. In addition, studies are
needed to examine lateral drainage and
percolation for small infiltration rates and
flow through synthetic liners and in
leakage detection of double liner
systems.
The overall data base of long-term
water budget measurements at landfills
is poorly organized and too small to
continually advance the state of the art
in understanding landfill leachate gener-
ation and migration. More extensive
monitoring activities are required to fill
this gap.
Improvements to the HELP model are
warranted in the areas of snowmelt,
winter runoff, unsaturated hydraulic
conductivities, and the selection of
evaporative depths based on the results
of this study.
Sensitivity Analysis
A sensitivity analysis of the HELP
model was performed to examine the
effects of the major design parameters
on components of the water budget for
landfills. The analysis examined the
effects of cover design, topsoil thickness,
topsoil characteristics, vegetation, runoff
curve number, evaporative depth, drain-
able porosity, plant available water
4
capacity, hyhdraulic conductivity, drain-
age length, and liner slope on the water
budget. Hydraulic conductivity values for
the topsoil, lateral drainage layers and
clay liners are the most important
parameters in determining the water
budget components. These parameters
are particularly important in estimating
the percolation through the landfill.
Other design parameters tend to affect
the apportionment between runoff,
evapotranspiration and lateral drainage
from the cover.
The interrelationship between
parameters influencing the hydrologic
performance of a landfill cover in the
HELP model is complex. It is difficult to
isolate one parameter and exactly predict
its effect on the water balance without
first placing restrictions on the values of
the remaining parameters. With this
qualification in mind, the following
general summary statements are made.
The primary importance of the topsoil
depth or thickness is in controlling the
extent or existence of overlap between
the evaporative depth and the head in
the lateral drainage layer. Surface
vegetation has a significant effect on
evapotranspiration from soils with long
flow-through travel times (low hydraulic
conductivity) and large plant available
water capacities; otherwise, the effect of
vegetation on evapotranspiration is
small. The general influence of surface
vegetation on lateral drainage and barrier
soil percolation is difficult to predict
outside the context of an individual cover
design. Clayey soils yield greater runoff
and evapotranspiration, and less lateral
drainage and barrier soil percolation.
Simulations of landfills in colder climates
and in areas of lower solar radiation are
likely to show less evapotranspiration
and greater lateral drainage and barrier
soil percolation. An increase in the runoff
curve number will increase runoff and
decrease evapotranspiration, lateral
drainage, and barrier soil percolation. As
evaporative depth, drainable porosity or
plant available water increase, evapo-
transpiration tends to increase while
lateral drainage and barrier soil perco-
lation tend to decrease; the effect on
runoff is varied.
The sensitivity analysis shows that the
ratio of lateral drainage to percolation is
a positive function of the ratio of KD/KP
and the average head above the liner.
However, the average head is a function
of QD/KD and L/a. The quantity of lateral
drainage, and therefore also the average
head, is in turn a function of the infil-
tration. Therefore, the ratio of lateral
drainage to percolation increases with
increases in infiltration, and the ratio of
KD/KP for a given drain and liner design.
The ratio of lateral drainage to
percolation for a given ratio of Ko/Kp
increases with increases in infiltration
and decreases in L/a. The percolation
and average head above the liner are
positive functions of the term L/a.
Review of Technical Guidance
The information from the sensitivity
analysis and the verification results were
used to evaluate RCRA landfill design
guidance and regulation. This evaluation
showed that saturated hydraulic conduc-
tivity is the most important design
parameter for minimizing percolation.
Care should be taken to recommend the
highest hydraulic conductivity that is
commonly available for drainage media.
Similarly, the lowest saturated hydraulic
conductivity practically obtainable
should be used as guidance for soil liners.
Changes in other design parameters
yield much smaller effects on percolation
or leakage volumes if the values of these
parameters are kept in a reasonable
range.
P. R. Schroeder and R. L Peyton are with U.S. Army Engineer Waterways
Experiment Station, Vicksburg, MS 39180.
Robert Landreth is the EPA Project Officer (see below).
The complete report, entitled "Verification of the Hydrologic Evaluation of Landfill
Performance (HELP) Model Using Field Data," (Order No. PB 87-227 518/
AS; Cost: $18.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
U.S. GOVERNMENT PRINTING OFFICE: 1988/548-158/67117
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