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OCR for page 81
6
—t~ ~
- -
This chapter describes the basic techniques for
quantifying ground water recharge used by soil
scientists, engineers, hydrologists, and
hydrogeologists. The standard methods available to
quantify ground water recharge are discussed, and
their applications to ground water recharge of
surface mining sites are assessed.
TECHNIQUES FOR ESTIMATING GROUND WATER RECHARGE
Ground water recharge at a mine site can be
estimated, in principle, by a variety of
techniques. Tracing water that enters the surface
soils as it percolates through the soils and
underlying material (vadose zone) to the water
table is one example of a direct measurement
technique. Other indirect techniques involve such
approaches as evaluation of water budgets of root
zones and/or aquifers, analysis of
surface-water-discharge hydrographs, and
interpretation of soil water and ground water
chemistry.
Attempts at determining recharge in small
research plots typically require measurement of
many climatic, geologic, soil, and ground water
parameters (Table 6.1~. For mine scale
-81-
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TABLE 6.1 Experimental Measurements Required to
Determine Ground Water Recharge
Measurement
Instrument or Technique
Onsite data--atmospheric
Precipitation
Atmospheric pressure
Air temperature
Relative humidity
Wind speed
Net radiation
Onsite data--subsurface
Hydraulic head
Water table
Water content of soil
Bulk density
Soil temperature
Water characteristic
Hydraulic conductivity
Laboratory data
Soil texture
Particle density
Tipping-bucket rain gauge
Barometric-pressure
transducer
Thermistor probe
Relative-humidity probe
Anemometer
Fritschen-type net
radiometer
Piezometer and duplicate
tensiometer nests
Water-table well
Neutron probe, gravimetric
method, and gypsum blocks
Gamma probe and core method
Thermocouples
Tensiometers and neutron
probe
Determined from changes
in water content and
tension of a bounded
soil volume during
drainage and/or
evaporation.
Hydrometer method
Pycnometer method
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Table 6.1 continued
Bulk density
Water characteristic
Hydraulic conductivity
Clod method
Hanging column, pressure
plate, pressure
membrane
Constant-head
permeameter, water-
characteristic-based
methods
SOURCE: Adapted from Sophocleous and Perry, 1985.
evaluations, spatial and temporal variability in
recharge and differences between pre- and post-mining
site hydrology are important considerations in the
selection of measurement techniques (Allison, 1988~.
Interpretation of recharge data collected under
ideal conditions requires an appreciation of possibl
sources of measurement error and uncertainty.
Measurement error can usually be estimated or
controlled, but uncertainty is more difficult to
quantify. For instance, uncertainty is introduced
when short-term hydrologic conditions are assumed to
be a valid representation of the long-term site data
base, which in fact may be quite different (Court,
e
1960; McKay, 19659. Uncertainty is also introduced
when recharge is measured for a small area and then
extrapolated to a larger area. The sources of these
uncertainties are temporal and spatial variability.
Comparative studies have shown that different methods
can give different estimates of recharge even when
the study site and time period are the same
(Johansson, 1987; Uma and Egboka, 1988~.
Direct measurement and observation of the migration
of water from the land surface to the water table
require installation of instruments to detect
variations in the water content of soil with depth,
from the land surface to the water table, over an
extended period of time. Generally, these methods
OCR for page 84
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require banks of instrumentation and a large
data-measurement program (Sophocleaus and Perry,
1985~. Interpretation of the fluctuation of a water
table can also be direct evidence of net recharge.
Hydrologic-budget calculations are based on the
continuity equation, which states that all water
entering and leaving a system must be accounted for
(i.e., inflow minus outflow equals change in
storage). This concept is applied over a selected
area (i.e., a mine site or basin) for a specific time
interval. A budget equation written in terms of
ground water recharge from the vadose zone is
where
interflow;
P = ET + RO + GWR + ASoil Storage,
p
ET
RO
precipitation;
evapotranspiration;
surface runoff and lateral vadose
GWR = ground water recharge; and
Roil Storage = change in soils water storage.
An equation written in terms of ground water recharge
for the saturated zone is
where
GWin
GWR
GWout
AGW Storage
GWin + GWR = GWOut + ~GW Storage
ground water inflow rate;
ground water recharge;
ground water outflow rate; and
change in ground water storage.
(The units of all above terms are length/time.) Use
of the above equations assumes that other system
inputs and outputs are negligible.
Accurately quantifying ground water recharge by
water-budget calculation is more difficult than it
may appear because it requires measurement of all the
terms in the equation except ground water recharge.
Most hydrology and hydrogeology text books--including
Kirkby (1978), USDI (1977a, b), and Fetter
(1988~--address the method for calculating the water
OCR for page 85
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budget. Knott and Olimpio (1986) compiled
water-budget estimates of the average annual recharge
rate for Nantucket, Massachusetts, and also made
estimates from water table fluctuation and isotope
data (Table 6.2~. Their recharge rate calculations
have a standard deviation of 39 percent.
Typically, the parameter most difficult to estimate
in water-budget calculations is evapotranspiration
(ET) (Bouwer, 1989~. Lysimeters have been used to
document the seasonal changes of ET and the resulting
downward flux of water that escapes ET (Bouwer,
USDI, 1977a, b; Williams and Hammond, 19881.
_ _ ~
1989;
However, given the relatively shallow placement of
most lysimeters, the assumption that all the draining
water would make it to the water table should be
carefully considered.
Another method for estimating ground water recharge
involves the measurement of water flow through the
vadose zone. The flux of water through the vadose
zone is a function of the ability of the medium to
transmit water, its unsaturated hydraulic
conductivity, and the driving force, the hydraulic
gradient. In order to use this method the
unsaturated hydraulic conductivity and total head
distribution must first be quantified in three
dimensions and through time. The hydraulic
conductivity may be measured using field or
laboratory techniques.
Hydraulic gradients are
estimated from measurements of total head in a series
of vertical profiles (Wilson, 1979~. These
techniques require sophisticated instrumentation and
the expertise of trained personnel, and therefore
these techniques are most commonly applied to small
research plots and are not typically employed in
large basin studies.
Some studies have used infiltration rates to infer
the occurrence of ground water recharge. These
infiltration rates have been found to be helpful in
assessing rainfall-runoff relationships (Wells et
al., 1982~. However, field measurement techniques
may not provide values representative of natural
rates (Bouwer, 1989~. Infiltration values alone will
not allow calculation of recharge without
OCR for page 86
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TABLE 6.2 Comparison of Recharge Rates
(Centimeters Per Year) Derived from Tritium, Water
Table Fluctuation, and Water-Budget (Thornthwaite)
Methods for Southeastern Massachusetts
Study
Method
Water Water
Location Tritimn Table Budget
This study [Knot and Site 1 66.3 -- --
Ol~mpio, 1986] Site 2 >42.4 -- --
Site 3 -- 52.1 --
Guswa and LeBlanc (1985) Cape Cod -- -- 45.7
LeBlanc (1984) Falmouth, Cape Cod -- -- 53.3
Olimpio and de Lima (1984) Mattapoisett -- -- 40.4
G. J. Larson (1982) Truro, Cape Cod 27.9-40.6 -- --
Walker (1980) Nantucket ~~ __ 46.0
Delaney (1980) Martha' s Vineyard -- ~- 56.4
Williams and Tasker (1974) Mattapoisett -- -- 45.7
Delaney and Cotton (1972) Truro, Cape Cod -- -- 46.2-49.3
43.9-46.7
Magnusen and Strahler (1972) Tours, Cape Cod
Strahler (1972) Cape Cod
30.5 - -
-- 44.4
aMichigan State University, written
communication, 1982.
bRanges based on values of the water-holding
capacity of the root zone between 5 and 10 cm,
respectively.
SOURCE: Knot and Olimpio, 1986.
OCR for page 87
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corresponding measurement of ET, changes in soil
water storage, and rates of vertical movement
through the vadose zone.
Measurement of a change in the position of a
water table in response to precipitation events or
snowmelt is evidence of ground water recharge, but
other possible influences on water table elevations
must first be eliminated (Table 6.3) (Freeze and
Cherry, 1979; Todd, 1980~. - ~ ~ ~
annual or storm-event
To estimate the total
recharge from water table
hydrographs, both the quantity of water added to
storage during the period of rise and that quantity
v ~ ,
of water flowing away from the water table during
the event must be determined (Johansson, 1987~.
Rasmussen and Anderson (1959) developed a method to
estimate seasonal recharge using an estimated
recession level from which to calculate the water
table rise and the ground water recharge (Figure
6.1~. To compute recharge from such plots, the
estimated total change in the water table, Ah,
is multiplied by the specific yield and the surface
area over which the change is estimated to occur.
Analysis of stream baseflow recession curves has
also been utilized to estimate basin ground water
recharge (Fetter, 1988; Figure 6.2~. This method
requires streamflow hydrographs for two or more
consecutive years and assumes that all recharge is
reflected in the stream hydrographs. The method
should not be used for cases where the stream
recharges the ground water system (losing streams)
Ground water recharge rates have also been
.
inferred from geochemical studies of water in the
unsaturated and saturated zones (Stone, 1985;
Bouwer, 1989; Knott and Olimpio, 1986; Colville,
1984; see Table 6.2~. Measurement of thermonuclear
tritium, chlorine-36, and chloride mass balance in
vadose zone water profiles have been used to infer
recharge.
Numerical modeling of ground water systems can
also be used to estimate areal recharge rates. The
inverse method is used to determine a recharge
necessary to calibrate the model to measured
fluctuations in ground water level (Wang and
OCR for page 88
-88-
TABLE 6.3 S,~mmary of Mechanisms That Lead to
Fluctuations in Ground Water Levels
Uncon- Hunan- Short- Long- Climatic
fined Confined Natural induced lived Diurnal Seasonal term influence
Ground water recharge X X X X
(infiltration to the
water table )
Air entrapment during X X X X
ground water rechargo
Evapotranspiration and X X X X
phreatophytic
con~umption
Bank-storage effects X X X X
near streams
Tidal effects near X X X X
oc eans
Atmospheric pressure X X X X X
effects
External loading of X X X
confined aquifers
Earthquakes X X X
Ground water pumpage X X X X
Deep-well injection X X X
Artificial recharge; X X X
leakage from ponds,
lagoons, and land-
ft lls
Agricultural irriBati on X X X X
and drainaSe
Geotechnical drainaBe X X X
of open pit mines,
slopes, tunnels
SOURCE: Freeze and Cherry, 1979.
OCR for page 89
-89-
_1 ~ .
it:
At:
u,
c
~Q
¢
u,
;z 4
it
>
At:
3
20
10
~ 1]
:,'1'
11
11
U
l
Well NEW 228 ~
~\1\'
1
Estimated
1',4, '::recession level
-
~\
11 \ -
~ ma_
J ~ J O
1978
J i J~O~ J ~ ~ O ~ ~ J O
. 1979 1 1980 1981
J O
1 982
L
J A J O
1 983
FIGURE 6.1 Hydrograph of monthly ground water
levels and bar graph of monthly precipitation.
SOURCE: Knot and Olimpio, 1986
.
OCR for page 90
-9o-
2(10c'
1 5(~,
10~}
9
70t~
;00
4()O
300
20()
100
q()
7()
A)
4(}
JO
'art
~1
.L
., O ~
f
.:
.K
It
'.N
10 ~ ~ . 1 . .
1 1 . · I . I I
.u I I ~ S O i~ D I F `\ ~ `, I I ~ S O ~ ~ I f ~ ~ ~ I I ~ S O ~ D I f
Runoff Year 1
Runoff Year 2
Runoff Year 3
FIGURE 6.2 Semilogarithmic stream hydrographs
showing base flow recessions.
SOURCE: Fetter
, 1988.
OCR for page 91
-91-
Anderson, 1982~. Such ground water modeling
requires input of aquifer geometry, boundary
conditions, initial conditions, aquifer parameters,
and other inflows and outflows at each model node.
Water level data spanning the time period of
interest are required for calibration.
APPLICATION OF TECHNIQUES TO SURFACE MINING SITES
Attempting to quantify ground water recharge at
mine sites requires an appreciation for the
strengths and limits of monitoring techniques, the
impact of the selected mining technique on the
hydrologic system, and the constraints of the
climatic and geologic setting. Successful
evaluation of recharge in western settings, such as
those found in the Northern Plains Coal Province,
requires an assessment of recharge parameters
associated with thick coal seams covering thousands
of hectares, semiarid climates (observations likely
to be highly variable over short distances),
seasonally frozen ground, thin soils overlying
bedrock, coal aquifers, and clinker outcroppings in
potential recharge areas. In contrast, recharge
evaluations in the Appalachian Coal Province
usually encompass sites of a few hundred hectares,
steep wooded terrains, small perennial streams,
temperate climates, thin soils overlying fractured
bedrock, and small recharge areas. Mining in
either of these coal provinces produces a different
topography, soil profile, and stratigraphy than
existed prior to mining. The mining process also
disrupts the local ground water system, which can
make identifying post-mining ground water recharge
trends difficult (Western Water Consultants, Inc.,
1985~. It also has been demonstrated that ground
water recharge varies spatially and temporally in
both pre-mining and post-mining areas (see Figure
3.5) (Rehm et al., 1982; Van Voast and Reiten,
1988; Kipp et al. ? 1983~. The choice and success
of a particular method or combination of techniques
OCR for page 92
-92-
to estimate recharge will depend on site conditions
and the desired post-mining hydrology, and on the
desired degree of resolution--criteria that have
not yet been set.
Water-Budget Methods
The water-budget techniques for both unsaturated
and saturated zones are generally applicable in
principle to pre- and post-mining conditions. The
equations appear to be simple, with the requisite
number of input parameters limited. In practice,
however, accurately quantifying these few
parameters, such as soil water storage and ground
water flow rates (input and output), poses a
formidable task. The uncertainty associated with
estimating recharge using water-budget techniques
can be quite significant and must be assessed on a
site-by-site basis. Further, if included,
water-budget measurements for the vadose zone are
data intensive and have been predominantly applied
in research situations only.
Vadose Zone Flux Measurements
Most mine sites in the United States exist in
areas of thin unconsolidated soils that overlie
coal, sandstone, shale, and limestone. Such sites
do not lend themselves to detailed vadose zone
monitoring, principally because unsaturated
consolidated formations and secondary fracture
porosity and permeability make standard
instrumentation unuseable or unreliable. While
there may be special situations where the technique
would be valuable (e.g., monitoring the
effectiveness of spoil-segregation techniques),
mine-scale problems generally are not amenable to
the use of this type of instrumentation and
analysis.
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Water Table Fluctuation Measurements
Water table recession analysis requires
monitoring recessions and quantifying specific
yield. Analysis is complicated by several
factors. The specific yield and the points of
system recharge (natural and post-mining) and
discharge are spatially variable and are influenced
by mining. Post-mining analysis is further
h~mn~red he the time ~ ~
delay in the ground water
system recovery period, which may mask recharge
events. Implementation requires measurement of the
level of the water table, which may be quite
difficult to obtain in zones of secondary
fractures.
Stream Hydrograph Separation
Stream hydrograph separation may have the widest
applicability in the eastern United States. This
method is only appropriately applied to areas large
enough to support perennial streams draining mined
watersheds. The data-collection techniques use
well-established methods. Data analysis is
complicated by climatic trends and variability, so
meaningful analysis demands either a prolonged
data-acquisition phase or comparisons limited to
similar climatic conditions. Stream hydrographs
are sensitive to both the size and type of mining
disturbance. Increases in the post-mining baseflow
component of the hydrograph can be attributed to
enhanced water storage in reclaimed spoils
accompanied by slow release (Minear and Tschantz,
1976).
Geochemical Techniques
Geochemical techniques have some applicability to
pre-mining conditions, particularly in arid
regions, because they reflect long-term averages
(Bouwer, 1989; Stone, 1985; Knott and Olympia,
OCR for page 94
-94-
1986). Because of the long-term averaging aspect
of this technique, it is generally not applicable
to post-mining evaluations. Also, leaching of
chemicals from the mine spoils may interfere with
the chemistry of ground water recharge.
Numerical Modeling
Numerical methods can be used for comparison or
optimization of proposed techniques. Because of
the typically data-intensive nature of ground water
models, they are usually employed only in
conjunction with other techniques. In the absence
of an extensive data-collection program, numerical
methods to infer recharge are not particularly
useful.
CONCLUS ION
In conclusion, recharge estimation is fraught
with difficulty and uncertainty. Recharge cannot
be measured directly. Uncertainty in the
measurement of relevant parameters results in an
uncertainty in recharge estimates that likely
exceeds changes in long-term recharge due to mining
if proper reclamation practices are followed.
Also, it must be recognized that at some mine
sites that are small or are located in dry
climates, reliable recharge quantification may be
outside the realm of current technology. As an
example, consider Van Voast and Reiten's (1988)
comment that at some wells associated with mine
sites in the Montana coal fields ". . . no local
recharge has been observed over 15 years of (water
level) record; at others, recharge has been evident
only during occasional periods of unusually high
snow melt or springtime precipitation."
Representative terms from entire chapter:
water recharge
