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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