v°/EPA
United States
Environmental Protection
Agency
Industrial Environmental Researe
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S7-82-055 Jan. 1983
Project Summary
Procedures for Predictive
Analysis of Selected
Hydrologic Impacts of
Surface Mining
D. B. McWhorter
This report presents a methodology
for the prediction of selected hydrolog-
ic impacts of surface coal mining.
Procedures are provided for esti-
mating the chemical and hydrologic
parameters required by an algebraic
water quality model. The model
predicts the long-term mean dissolved
solids concentration in combined
direct and subsurface runoff from a
watershed partially disturbed by
mining. The computational procedure
is demonstrated in a step-by-step
calculation for a mine site in Colorado.
The predicted results are in satisfactory
agreement with short-term (2 and 3
year) observations.
Procedures for determining the
transmissivity of coal and overburden
aquifers from single-hole aquifer tests
are provided. The procedures permit
the analysis of recovery data, affected
by well-bore storage, following a
prolonged pumping period. Well-bore
storage is an important effect in the
recovery of low transmissivity aquifers
often encountered in coal mining
related hydrology. Several approxi-
mate, closed-form formulas for esti-
mating selected impacts of surface
mining on groundwater are provided.
Among them are formulas for estimat-
ing groundwater inflows to an advanc-
ing pit and to a pit advancing parallel
to an alluvial valley. Formulas for
calculating the extent of the depressed
piezometric surface as a function of
time and distance from the pit are
developed. These formulas can be
used to assess the probable severity of
corresponding impacts and to judge
the need for additional data and more
detailed models in site specific situa-
tions.
This Project Summary was devel-
oped by EPA's Industrial Environmen-
tal Research Laboratory, Cincinnati,
OH, to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Federal and State regulations require
an analysis of the potential influence of
coal mining upon the hydrologic balance
in the area affected by mining. Poten-
tial effects of coal mining upon the hy-
drologic balance include changes m the
quality of ground and surface waters and
a modification of the relative quanti-
ties of direct and groundwater runoff.
Other possible effects are the modifi-
cation of recharge to regional and lo-
cal aquifers, a change in the pattern
of groundwater flow, and a shift in the
magnitude and peak of the runoff hydro-
graphs. The changes that may be antici-
pated are different in the active mining
phase than in the long-term, post-mining
phase.
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Implicit is the requirement that the
influence of a particular mining project
upon the hydrologic balance be predicted
before mining is initiated. This can be
accomplished only through the use of
models, even if they are conceptual
Each component of the hydrologic
balance is a complex phenomenon that
exhibits all of the vagaries of natural
processes. Models range from simple,
non-quantitative concepts through
sophisticated stochastic models to
detailed, physically-based descriptions.
Those who are faced with the prepara-
tion and review of predictions relative to
the hydrologic consequences of mining
must select methods or models upon
which to draw conclusions. The most
useful set of models provides results
in the desired, suitably reliable form,
commensurate with the experience,
technical knowledge, resources, and
data that can be reasonably obtained by
the user.
In keeping with this perception, this
report presents a set of methods by
which the influence of surface coal
mining upon the hydrologic balance can
be analyzed. The methods presented in
this report are not applicable to all
situations, of course, nor are they
intended to be. The application of the
methods is demonstrated through
examples. It is anticipated that interested
readers will devise ways to modify the
procedures for site specific needs. It is
hoped that a reasonable balance has
been struck between the degree of rigor
and realism in the methods and the
knowledge, resources, and data required
to apply them. The emphasis throughout
the report is on guidelines for application
rather than on theoretical justification.
A Combined Water and Salt
Balance
Of interest is the change in the water
quality hydrology that results from
disturbing a portion of a watershed by
surface mining. Based upon a simple
water and dissolved solids balance, the
long-term mean concentration of dis-
solved solids in total runoff (direct and
subsurface) from a watershed partially
disturbed by mining can be expressed as
p _ KRPn "*" Pm
1 + KR
In this model, Pt is the mean concentra-
tion of dissolved solids in total watershed
runoff, Pn is the mean concentration in
combined direct and subsurface runoff
from the undisturbed (natural) portion of
the watershed, and Pm is the correspond-
ing quantity for the mined portion. R is
the ratio of the area of the natural land
to the area of the mined land, while K is
a hydrologic parameter that character-
izes the relative quantity of total runoff
on the undisturbed and disturbed
portions of the watershed Both the
relative quantity and quality of direct
and subsurface runoff from the mined
land are important determinants of the
parameter Pm. The relationship is
Pm — fs
(1 ~ fsm) Pg
where fsm is the fraction of the total
runoff from the mined land that is direct
runoff, Psm is the dissolved solids
concentration in direct runoff, and Pgm is
the dissolved solids concentration in the
subsurface runoff.
It is anticipated that pre-mine moni-
toring will establish the value of Pn, and
the appropriate value for R is determined
from the mine plan. The remaining
parameters to be estimated are PSm, Pgm,
fsm and K Probably the most reasonable
estimate of Pgm can be made from
a judicious study of the quality of spoil
water from nearby mines in a similar
hydrogeochemical environment. Sam-
pling of springs formed on the interface
between the spoil and the undisturbed
underburden and/or of wells completed
in the spoil aquifer is recommended. In
the absence of this possibility, present
experience suggests that the dissolved
solids concentration in extracts from
saturated drill cuttings will provide a
reasonable lower limit for Pgm.
The hydrologic parameter K, being the
ratio of total unit area runoff on the
undisturbed ground to that on the mined
land, depends directly on the relative
consumptive use of water on the two
portions of the watershed. The quantity
of water consumptively used depends,
in turn, upon the type and quality of
vegetal cover, the potential evapotran-
spiration, and the timing and volume of
infiltration into the soil. In arid and
semi-arid climates, the potential annual
evapotranspiration is larger than the
mean annual precipitation. Considering
the fact that a fraction of precipitation is
lost by direct runoff instead of entering
the root zone, it becomes apparent that
the potential evapqtranspi ration is an
even greater multiple of the volume of
soil water available for plant use. At first
glance it would seem, therefore, that no
subsurface runoff would occur under
such circumstances. However, the
timing and volume of infiltration maybe
such that, at particular times, the water
holding capacity of the soil is exceeded
and percolation through the root zone
occurs. This is especially true where a
large fraction of the annual precipitation
is in the form of snow that accumulates
through the winter and melts quickly in
the spring. Subsurface runoff may occur
in response to percolation below the
root zone during this period, even
though there exists a deficit of available
soil moisture on the average over the
year. Thus, both K and fsm are directly
dependent upon the partitioning of
precipitation into infiltration and direct
runoff components.
The procedures used to estimate the
long-term values for K and fsm are based
upon long-term mean water balance
computations made for the surface and
the root zone. The surface water
balance is used to compute infiltration
by subtracting direct runoff from
precipitation. The infiltration is then
used as input to the soil-water zone
balance. The subsurface runoff is
computed as the residual required to
maintain a soil-water zone balance.
The first step in this procedure is to
compute long-term mean monthly
direct runoff The Soil Conservation
Service Curve Number method is used
to estimate daily direct runoff by month
using the historical precipitation record
as input. A histogram procedure is
provided that minimizes the required
computations. The mean monthly direct
runoff is subtracted from the mean
monthly precipitation to yield the mean
monthly infiltration Table 1 shows the
results of one such computation.
Infiltration is used as input to the soil-
water zone balance computation. An
accounting is kept of the available water
in storage in the root zone as a means of
determining when the evapotranspira-
tion demand exceeds the quantity of
water available. By this method, the
actual evapotranspiration is calculated
as being equal to the demand or to the
quantity available, whichever is limiting.
Percolation below the root zone occurs
when infiltration is sufficient to exceed
the evapotranspiration demand plus any
deficit in available water storage. Table
2 shows the results of a computation on
mined land.
The mean annual direct runoff,
together with the mean annual subsur-
face runoff, are used to compute K and
fsm directly from the definitions of these
parameters. The procedures outlined
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Table 1 . Summary of Surface Water Balance
Tntnl In^fmm Avat/able Prec
Prec Snow Pack
Total (cm) (cm)
Jan 4.6 1.5
Feb 4.5 1.5
Mar 5.6 1.5
Apr 5.0 1.5
May 3.6 0
Jun 3.8 0
Jul 3.3 0
Aug 43 0
Sep 44 0
Oct 4.2 0
Nov 4.0 1.5
Dec 5.1 1.5
52.4 9.0
briefly above and given in detail in the
report were used to predict Pt for a
mined area where measured values of
Pt were available for comparison The
comparison is shown in Table 3. It is
believed the agreement is satisfactory
considering that the measured values
are not long-term averages
Single-Well Aquifer Tests in
Coal Hydrology
Aquifer tests are the primary means
of determining the hydraulic param-
Snow
(cm)
0
0
0
17.5
0.8
0
0
0
0.6
1.3
0
0
20.2
Rain
(cm)
0
0
0
1.3
2.8
3.8
3.3
4.3
38
2.9
1.0
0
23.2
Direct Runoff
Spoil
(cm)
0
0
0
0.58
0.04
0
0
0
0.05
0
0
0
0.7
Natural
(cm)
0
0
0
0.29
0.03
0
0
0
0
0
0
0
0.3
Table 2. Soil- Water Balance in Spoil*
Month AW} Deficit} 1 5,p
Oct
Nov
Apr
May
Jun
Jul
Aug
Sep
(cm)
0
1.6
2.6
66
30
0
0
0
(cm)
6.6
5.0
4.0
0
3.6
66
6.6
6.6
(cm)
42
1 0
18.2
36
38
3.3
4.3
43
42.7
(cm)
5.3
0
5.4
10.0
14.8
16.4
14.1
9.6
75.6
f,
(cm)
2.6
0
3.2
7.2
10.7
11.8
9.2
4.9
496
Infiltration
Spoil
(cm)
0
0
0
18.2
3.6
3.8
3.3
4.3
4.3
4.2
1.0
0
42.7
Ft.
(cm)
2.6
0
3.2
7.2
6.8
3.3
4.3
4.3
31.7
Natural
(cm)
0
0
0
18.5
3.6
3.8
3.3
4.3
4.4
42
1.0
0
43.1
AS
(cm)
+ 1.6
+ 1.0
+4.0
-3.6
-3.0
0
0
0
0
W
(cm)
0
0
11.0
0
0
0
0
0
11.0
eters of water-bearing strata that are re-
quired for projecting the effect of mining
on the groundwater regions and for esti-
mating the quantities of groundwater
inflow that can be anticipated in the
mine workings Single-well aquifer
tests have found substantial use in coal
hydrology where permeabilities are low
and drawdown cones are excessively
steep.
Single-well aquifer tests may be
performed by "instantaneously" chang-
ing the water level in the well and
monitoring the recovery or by pumping
the well for a prolonged period before
monitoring the recovery The first
method is a slug test andthe response is
reflective of the aquifer properties in a
small volume of aquifer in the immediate
vicinity of the well bore. This disadvan-
tage is offset to some degree by
pumping for a prolonged period prior to
monitoring the recovery This report
presents two methods by which the
recovery data collected after a prolonged
pumping period can be analyzed. The
first method is an extension of existing
theoretical response functions for the
pumping period to application to the
recovery period. Full consideration is
^Evaluated at beginning of month
given to the effects of afterflow caused
by non-zero well-bore storage. Figure 1
shows the theoretical response func-
tions superimposed on a set of recovery
data collected after a prolonged pump-
ing period.
An algebraic method applicable to
recovery analysis was developed also.
This method is based on superposition
of the familiar line-sink solutions to
account for the variable aflerflow
discharge. The algebraic method is an
approximate procedure easily adaptable
for desk-top computer calculations. This
method does not require the somewhat
subjective matching of type curves The
range of applicability and accuracy of
the algebraic method were investigated
by comparison with the exact solution
used by the first method; guidelines for
use are provided.
Analysis of Selected Flow
Problems
Aspects of groundwater hydrology
that may be important during the mining
phase include: 1) the quality and
quantity of inflows to pits, shafts, or
other excavations, 2) the resultant
lowering of the piezometric surface in
the affected aquifers, 3) inflow to the
mine from fault zones, 4) the lowering of
water levels in infrequently recharged
alluvial aquifers adjacent to the mine,
and 5) sustained inflows from frequently
recharged alluvial aquifers This report
presents analyses and solutions that
are specifically oriented toward such
problems that are known to have been
encountered in surface mining projects.
Flow to an advancing pit that incises
one or more aquifers is treated The
method of succession of steady states is
used to calculate the inflow as affected
by the rate of advance of the pit and the
conversion of the aquifer from confined
to unconfined in the vicinity of the pit.
Figure 2 shows the cross section
through the pit that is used in the
analysis and is typical of the degree of
idealization utilized in all of the develop-
ments. The results of an example
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Table 3. Comparison of Predicted Pt with Measured Values
Watershed
No.
C 3
C 5
C 9
CIO
C 9 + C 10
R
0.47
0
1.86
1.27
1.44
Predicted
Pt
(mg/l)
2220
2860
1450
1670
16OO
Avg. Meas.
Pt
(mg/l)
1840
2910
1240
1850
1550
Range of
Meas. Pt
(mg/l)
1610-2030
2830-3080
1190-1290
1850-1860
1520-1580
1-0
0-8
0-6
5 o
CO CO
0-4
0-2
(0-40M4-61/
T - — = 5-5 cm /mm
= 25O
0-1
1-0 10-0
Recovery Time, Minutes
100-0
computation of inflows to an advancing
pit are shown in Figure 3.
A similar analysis was used to
develop formulas for inflow to a mine
that is initiated on a crop line. The
method accounts for the fact that
successive pits constructed in the
down-dip direction will induce incremen-
tally greater draw-downs in the affected
aquifer. The problem of inflow to a pit
advancing parallel to an alluvial valley is
treated. The results can be used to
estimate the quantity of alluvial ground-
water induced into the pit as affected by
the width and hydraulic properties of
the buffer zone. Also, a formula is
developed for prediction of the lowering
of the water table in an alluvial aquifer
as the result of nearby mining. Finally,
an analysis of the groundwater buildup
and discharge from spoil banks subjected
to periodic recharge is provided. Example
applications and computations for each
of these problems are presented.
Figure 1. Superposition of response functions on data plot for Example 1.
Ground Surface
High wall
Water Table
Figure 2. Definition sketch for flow to the first cut.
4
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600
500
400
300
200
100
0
0 70 20 30 40 50 60 70 80 90 10O
Time, days
Figure 3. Calculated inflows to box cut for Example 1.
DavidB. McWhorter is with Colorado State University, Fort Collins, CO 80523.
Roger C. Wilmoth is the EPA Project Officer (see below).
The complete report, entitled "Procedures for Predictive Analysis of Selected
Hydrologic Impacts of Surface Mining," (Order No. PB 82-258 476; Cost:
$ 11.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
. S. GOVERNMENT PRINTING OFFICE: 1983/659-095/0570
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Environmental Protection
Agency
Center for Environmental Research
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