oEPA
United States
Environmental Protection
Agency,
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81-016 July 1981
Project Summary
Planning Study to
Model and Monitor
Coal Pile Run-Off
G. T. Brookman, J. A. Ripp, P. B. Katz, B. C. Middlesworth, and D. K. Martin
This report describes a planning
study for predicting and monitoring
the hydrologic and chemical charac-
teristics of drainage from the open
storage of coal with focus on the
developments of a mathematical model.
The project report contains four main
sections: 1) The results of a question-
naire survey of members of the Edison
Electric Institute in which recent
information was gathered concerning
coal pile characteristics and coal pile
run-off treatment systems, 2) a report
on the development of a coal pile
drainage model with both hydrologic
and qualitative components, 3) a
detailed field program with work plan
which itemizes those tasks necessary
to describe the physical, chemical and
hydrologic characteristics of a coal
pile, the meteorological conditions
relative to that pile, and the framework
for conducting an intensive run-off
sampling program, and 4) a field
procedures manual which details all
the steps necessary to monitor coal
pile run-off from the training of field
personnel to the analysis of run-off
samples. This field program will be
used to gather data for calibration and
verification of the model.
This Project Summary was develop-
ed by EPA's Industrial Environmental
Research Laboratory, Research Tri-
angle Park, NC to announce key find-
ings of the research project which is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
In August 1979, TRC Environmental
Consultants, Inc. began Phase I of a
multiphased effort to predict the quanti-
ty and quality of coal pile run-off from
utility sites by development of a verified
mathematical model. This predictive
tool will be used todesign collection and
treatment facilities for coal pile run-off
without use of arbitrary and often overly
conservative storm frequency parameters.
In addition, the resulting pollutant
loads, under different precipitation
events, can be assessed.
Phase I began with a literature search
and industry survey to research the
desired capabilities'for the model,
review the most recent developments in
treatment and characterization of coal
pile run-off, and assess the theoretical
research in quantifying and qualifying
coal pile run-off. Based on the informa-
tion gathered, TRC developed a prelimi-
nary model structure able to predict
both volume and quality of drainage
from any coal storage area. Along with
the model format an outline was devel-
oped for the field monitoring program
and data collection procedures which
will provide input as well as verification
data to the model.
Utility Industry Survey
To compile recent (1979-1980) infor-
mation necessary to both plan a field
study and develop a predictive model,
TRC designed an independent poll of
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coal-burning utilities owning plants
with a generating capacity greater than
25 megawatts (MW).
TRC received completed or partially
completed questionnaires from 81 one
plants with a total of 109 bituminous
coal piles.
Data was tabulated using a total of 81
responses. From the data received,
mean plant operating characteristics,
coal pile characteristics, and treatment
methods were determined. A list of the
parameters tabulated is given in Table
1.
Coal Pile Drainage Model
Development
Initial Model Research
In 1977, TRC completed a monitoring
and modeling study of non-point sources
at two coal-fired utilities. In this pro-
gram, TRC utilized the Short Storm-
water Management/RECEIV-ll Model
(SSWMM-RECEIV-II) to address sheet
wash-off from coal storage piles. At the
completion of this modeling program a
number of shortcomings in the SSWMM-
RECEIV-II Model were identified. For
example, the model could not address:
• storm erosion of material from the
coal storage pile
• stormwater percolation through
the coal pile
• pyrite oxidation/acid production in
the coal pile.
It was decided to try to include these
aspects in a newly developed coal pile
drainage model.
In addition to the industry survey, TRC
undertook a literature search of coal pile
drainage. In this search, TRC determined
what physical/chemical phenomena
associated with coal piles would be
valuable in characterizing coal pile
drainage in a modeling effort. The
phenomena researched can be divided
into quantitative and qualitative aspects.
Table 1. Parameters Tabulated In
Project Report
Sulfur and pyritic sulfur content of
active and reserve coal piles
Sizes, densities, heights, and slopes
of active and reserve coal piles
Active and reserve piles with multi-
state coal sources
Percentage ash and moisture in
active and reserve piles
BTU values of active and reserve
coals
Generating capacities of respond-
ing utility plants
Percentage of coal usage
Types of coal pile construction
bases
Treatment methods used for run-
off and year of system start-up
Design storms and run-off coeffi-
cients used in system design
Number of plants treating specif fed
chemical parameters
Precipitation
Run-Off,
Stream
Run-Off
Stream
To Deep Storage
Figure 1. Schematic of hydrologic cycle of coal storage pile.
2
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1) Quantitative
a) Precipitation (rain/snowfall)
b) pile surface run-off/infiltration
c) percolation through the pile
d) snow-melt
e) infiltration into ground water
beneath the pile
2) Qualitative
a) oxidation of pyrite in coal,
production of acid
b) freeze/thaw of coal to expose
surface areas
c) erosion of gullies down the
sides of the coal pile
d) wash-off of pollutants from
surface of the pile
Few models have been developed to
describe industrial stormwater run-off
situations and none have been develop-
ed specifically for material storage piles.
The existing run-off models were re-
searched to ascertain which one could
best be utilized as the basis for the coal
pile drainage simulation effort.
The Ohio State University (OSU)
version of the Stanford Watershed
Model was selected as the base model.
The OSU model simulates overland run-
off in a rural area. TRC also used the
OSU co-model for acid production in
coal refuse piles as the basis for the
qualitative modeling work.
A number of modifications were
made to the OSU models to make them
applicable to coal storage piles. The
physical/chemical reactions included
in the models are described in more
detail in the project report. In addition,
the models were altered so that a
standard NOAA magnetic tape of histor-
ical meteorlogical data can be utilized as
data input instead of requiring the user
to create his own data file.
The resulting TRC coal pile drainage
model is actually two models - a hydro-
logical model, termed TRCH2 0, and a
qualitative model, TRCCOAL
Quantitative Model - TRCH2 0
The TRCH2 0 model reads hourly pre-
cipitation data in the form of rain or
snow from the input meteorological
tape. The phenomena of direct infiltra-
tion, gully erosion, delayed infiltration,
direct run-off, snow accumulation, and
snowmelt are hydrologic surface re-
actions to precipitation. The relation-
ship is shown in Figure 1.
The amount of rainwater which im-
mediately enters the pile is known as
direct infiltration. It is dependent upon
input factors of pile moisture and pile
moisture storage capacity as well as
time and adjustment factors. Some
rainwater is retained by depressions in
the coal pile surface and this infiltration
is delayed. The depression storage is
considered to be the upper zone of the
coal pile. The amount of depression
storage on the pile surface is estimated
by the model user. The entire coal pile is
considered pervious.
The rainwater which does not infiltrate
into the coal pile becomes direct run-off.
The Chezy-Manning equation for turbu-
lent flow was utilized in the model to
derive this relationship. Input variables
are slope of the coal pile, length of the
slope, and roughness coefficient.
In addition to rainwater, the model
considers the effect of snow on the coal
pile. The model reads the meteorological
tape and determines the amount of
snow which falls on the coal pile. The
snow pack accumulates over time, and
snow is converted to its liquid compo-
nent by snow-melt. In this model snow-
melt is estimated by the air temperature
above freezing and a degree-day melt
factor. Once snow is melted, it is handled
in the model as rainwater for infiltration
and run-off purposes.
The impact of rain on the coal pile
causes solids to erode and creates
gullies on the side of the coal pile. Using
measured data on pile slope, length, and
rain intensity, as well as erosion coeffi-
cients, the model calculates the pounds
of coal solids moved to the base of the
pile per day during a day with rainfall.
The moisture which percolates from
the upper zone of the pile is stored in the
lower zone. Some of the moisture
erupts from the side of the coal pile and
is termed inter-flow. In addition, some
moisture percolates through the pile to
the ground water. The amount of rain-
water which emerges as inter-flow is
proportional to the amount of rainwater
which infiltrates from the surface, and
the current in-pile moisture storage.
While some water is retained in the
coal pile, some infiltrates into the
interface that consists of the layer
immediately beneath the coal pile. For
the purposes of the model, moisture in
this layer is termed ground water, and
moisture below the interface is deep
storage. The moisture in the ground
water can be routed to deep storage or
emerge from the base of the pile as a
flow stream. The amount of seepage or
base-flow from the pile is dependent
upon the ground water storage, the
ground water slope, and flow recession
constants. Infiltration into the ground
water increases the slope and the re-
sulting base-flow emerging from the
coal pile.
Percolation to deep, inactive ground
water storage or ground water flow out
of the basin is modeled by allowing a
fixed portion of inflow to ground water
storage to bypass the active storage that
contributes to base-flow.
Qualitative Model - TRCCOAL
Qualitatively, during dry weather the
surface of the coal pile undergoes the
physical/chemical processes of pyrite
oxidation: acid, iron, and sulfate produc-
tion as well as the dissolving of trace
materials. During wet weather these
materials are washed off the surface
and out of the interior of the coal pile.
Seepage is generated during both wet
and dry weather. These phenomena are
simulated in TRCCOAL using the hydro-
logic balance developed in TRCH2 0.
The block diagram for TRCCOAL is
shown in Figure 2.
Dry Day
Pyrite Oxidation
Acid Production
Sulfate Production
Wet Day
Distribute
Dissolved
Materials From
Surface to Zones
of Coal Pile
Wash-Out Materials
From Coal Pile
Dissolving of Trace
Materials
Subtract Removed
Materials FromlJotal
Dissolved Materials
Freeze-Thaw
Acceleration
of Reactions
Figure 2.
Block diagram of qualitative model
for coal pile drainage.
During non-rainfall periods the coal
pile is subjected to atmospheric condi-
tions which break up the coal lumps and
the moisture and oxygen in the surface
of the pile cause oxidation of the pyrite
in the coal. The products of pyrite
oxidation are acid, sulfates, and iron.
The acid further acts to dissolve trace
materials.
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In TRCCOAL the user inputs data on
coal characteristics and reaction rates.
The model simulates the total amount of
pollutants in the coal and the total
amount of pollutants in a dissolved state
during a dry weather period. The number
of freeze/thaw cycles calculated in
TRCH2 0 is used to simulate the breaking
up of coal and the subsequent pyrite
oxidation in TRCCOAL. The dissolved
material is then available for wash-off
during the next storm event. Also during
dry weather, the model simulates mois-
ture being emitted from the lower zone
as seepage.
The model conducts a wet/dry test to
determine if the wet weather mode of
wash-off or the dry weather mode of
acid production, and seepage will be
utilized for a given day. If the total
rainfall is less than 0.1 inches, then the
model considers it a dry day and only
acid production and seepage generation
takes place.
During wet weather, rainfall entering
the pile distributes a portion of the
dissolved metals, sulfates, and the acid
on the surface to the interior of the pile
and to direct run-off. In addition, the
acidity of the rain is added to the
available acid in the pile. The amount of
pollutants that is distributed to the
upper zone (depression storage), lower
zone (pile interior), direct run-off, and
inter-flow is proportional to the solubility
of the pollutants and an adjustment
factor. The material stored in the lower
zone is further distributed to deep
storage and the base-flow.
The amount of acid, metals, and
sulfate in storage zones that is washed
out of the pile can be linearly or expo-
nentially related to the available mate-
rial. EXPO-1 simulates an exponential
decay rate. This relationship is best
demonstrated by the "first flush" effect
and considers the removal of both
dissolved and suspended material. The
transport of suspended solids is simu-
lated either by the exponential wash-off
function in TRCCOAL or by the erosion
routine of TRCH2 0, but not both.
Input and Output of Models
The input data required by TRCH2 0
consists of a card deck of site specific
parameters, a magnetic tape of meteoro-
logical data and a card deck of data for
the plotter. TRCCOAL uses the output
disk file of hydraulic data created by
TRCH2 0 and a small card deck of
qualitative site specific parameters. The
output of the coal pile drainage modeling
I
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Figures. Example output plot-acid
effort is the simulation of daily, monthly,
and annual run-off flows and loadings
of pollutants such as acid, iron, sulfate,
and trace metals. In addition, there is
the optional output of detailed storm
events with subhourly flow and load-
ings. Figure 3 is an example output plot
showing run-off flow and acid loadings
for a three-month period.
Following preliminary model develop-
ment a sensitivity analysis of selected
input parameters was conducted. In
TRCH2 0 the most sensitive parameter
analyzed was the infiltration parameter,
CB. Increasing the coefficient CB, in-
creased the pile moisture and signifi-
cantly reduced run-off flows. For the
qualitative model the most sensitive
parameters were those dealing with the
wash-out of pollutants during wet
weather, not the accumulation of dis-
solved pollutants in the pile.
The report also discusses, in detail,
limitations of the coal pile drainage
model as designed. For example, the
hydrologic model, TRCH2 0, concerns
the flow from one run-off drainage point
from the coal pile under consideration.
When there is more than one run-off
stream, the simulation must be run
multiple times. In addition, the modeling
of snow-melt is an approximation based
on limited input data.
Field Work Plan and Procedures
In order to obtain representative data
to modify the model as well as to ,
generate a data base for study purposes, I
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i well designed field program must be
jarried out. As part of Phase I the pro-
posed field program strategy and proce-
dures were outlined.
The field program is divided into two
phases and is based on monitoring coal
pile run-off from 12 different coal fired
utility plants. The selection matrix of 12
sites is based on testing 3 sizes of
generating plants and 4 coal sources
representing different coal character-
istics.
The first phase of the field program
will involve sampling two medium size
plants, one using low sulfur coal and
one using high sulfur coal. These sites
will serve as "test" sites to fine tune
sampling procedures and modify and
calibrate the mathematical models. The
sampling program at each "test" site
will be approximately 10 weeks dura-
tion.
The remaining 10 sites will be used to
test the run-off model for a variety of
climatic regimes, coal pile configura-
tions, and coal characteristics, as well
as to generate a substantial data base
on coal pile run-off.
All data collected in the field will be
put i n a form which is compatible for use
in the run-off model. In addition, this
data will be digitized and made easily
ccessible through magnetic tape. The
data tape will contain information on
site description, meteorological condi-
tions, run-off flows and pollutant load-
ings as well as statistical summaries.
The field program plan detailed in the
report, addresses the following compo-
nents:
1) Selection of 12 utility sites.
2) Initial site visit including back-
ground data acquisition. Examina-
tion of pile run-off streams and
existing weirs and flumes.
3) Preliminary work, including ac-
quisition of coal samples, and coal
pile and ground water testing.
4) Laboratory testing of the coal
samples to determine variability.
5) Choice of pollutant parameters for
analysis.
6) Determination of sampling fre-
quency.
7) Selection of sampling, flow moni-
toring and meteorological equip-
ment.
8) Development of a plan for shipping
samples for analysis.
.9) Laboratory analysis of the run-off
samples.
The sampling strategy will reflect
greater frequency during wet weather
periods than sampling during dry weath-
er seepage. Sampling will also represent
the 2-hour, 4-hour, 12-hour, and 24-
hour frequency storms.
Two other uses for the field data will
be to form a referral bank for utility use
and to develop simpler tools, such as
nomograms, charts, and/or tables to
assist those without computer resources
in designing treatment for coal pile run-
off.
The field program at the 12 utility
sites should take 2-3 years to complete.
Recommendations
The collection of coal pile run-off data
should begin soon at two test sites. This
data will be used for model modifications
and calibration.
Following model verification, the
model can be used to simulate run-off
using local site specific data. Initial runs
should be made with several days of
precipitation data. Following any pa-
rameter adjustments simulations can
be run for several years of historical
meteorological data or a specially for-
mulated cjesign storm.
Conclusions
From the survey of EEI member utili-
ties it is apparent that, in order to meet
the effluent limitations for pH and sus-
pended solids, most utilities have incor-
porated treatment for run-off from coal
piles. In fact, from the questionnaire, it
is shown that 85% of the plants respond-
ing have some form of treatment. The
design criteria for collecting the coal pile
run-off for the majority of plants is the
10 year, 24-hour storm taken from
either local data or the National Weather
Services Technical Paper #40. This
design storm is an arbitrary parameter
used by many states without regard to
the dynamic nature of coal pile run-off.
It appears from the survey that most
utilities are relying on such rudimentary
techniques to design a coal pile run-off
treatment system.
The coal pile drainage model, after
calibration and verification, can become
an effective tool to design collection
basins and treatment systems. For ex-
ample, it may be possible to collect and
treat only the first flush of the storm run-
off with higher level of contaminants and
discharge the subsequent "clean" run-
off which meets NPDES limitations.
Using historical meteorological data or
simulated storms the model could be
used to determine when the transition
from collection of run-off to bypassing of
run-off could take place during a storm
event.
The quantitative and qualitative models
in this program must now be calibrated
and verified by a field data program.
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G. T. Brookman, J. A. Ripp, P. B. Katz, B. C. Middlesworth. andD. K. Martin are
TRC-Environmental Consultants, Inc.. Wethersfield, CT 06109.
D. B. Harris is the EPA Project Officer (see below).
The complete report, entitled "Planning Study to Model and Monitor Coal Pile
Run-Off," (Order No. PB81-152 530; Cost: $ 17.00, 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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
» US GOVERNMENT PRINTING OFFICE. 1961-757-012/716
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Penalty for Private Use $300
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