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OCR for page 24
3
HYdrolomr of Ground Water Recharge
_
HYDROLOGIC CYCLE
This chapter deals with those parts of the
hydrologic cycle associated with the occurrence of
ground water in surface-mined areas. This water
may move rapidly through the part of the hydrologic
cycle of interest, going from precipitation to
percolation into a rock fracture where it may
quickly flow to a spring or stream and out of the
system. Other water may infiltrate and percolate
into slowly permeable formations and aquifers. It
may then take many years, decades, or even longer
before the water emerges and reenters the more
active parts of the hydrologic cycle through
surface water flow, ground water pumping, or
evapotranspiration (Figure 3.1~.
Surface mining activities may alter many
hydrologic processes, including infiltration,
overland flow, surface runoff, surface storage and
detention, interception, evapotranspiration,
percolation, vadose zone storage, ground water
storage, ground water flow, streamflow, and water
quality. In fact, about the only part of the cycle
not generally considered to be influenced
potentially is precipitation.
The flow and storage processes shown in Figure
3.1 are unsteady--that is, the flow rates and
- 24 -
OCR for page 25
- 2 5 -
Precipitation
Surface Divide
Infiltration
1
Percola:
Vadose Zone
Overland Flow
Surface
Runoff /
\\~/ / Transpiration
~~ /
Channel
Precipitation
InterRow
Surface
~~ Storage
Evaporation \ 7/
Rae ~ Interception
\
Ground Water ~
Divide Ground ::
Ground Water Water Flow
Zone
FIGURE 3.1 Schematic of hydrologic cycle
SOURCE: Barfield et al
.., 1981.
OCR for page 26
-26-
volumes of water in any particular form of storage
are constantly changing with time. The time rate
of change of some processes such as precipitation
and surface runoff may be rapid, measured in
minutes and hours, while the rate of change of
ground water storage and discharge may be very
slow, measured in days, weeks, and months.
The principle of conservation of mass relates the
fl law and Tar r~rnr~c:.c:~.~
-a- a--------. Over a selected time
interval and area, the difference in the volume of
water entering and leaving a control element must
equal the change in the volume of water stored in
the element. In other words, inflow volume less
the outflow volume equals the change in storage.
Over short intervals of time (up to a few years),
the change in inflow, outflow, or the volume of
water stored in the control element may be
alihet~r~ti~l Try = - 1l=Hi c!~llrh~r] rmntrr`1 "1 "meant
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ _ . _ ~ ~ ~ ~ ~ ~ ~ ~ ~ _ ~ ~ ~ _ ~ ~ ~ _ _ ~ . ~ _ ~ _ _ _ ~ A. _ . . ~ ,
for long time intervals (several years), the change
in the average value of these quantities is
relatively small and the inflow and outflow volumes
are nearly equal. Under these conditions, a state
of dynamic equilibrium exists. The relationship
between the inflow, outflow, and change in storage
for any control element refers to the hydrologic
budget of the element. Any factors that alter
inflows, outflows, or storage characteristics
potentially alter the hydrologic budget.
Ground water is generally considered to be water
contained in underground formations in a saturated
or near-saturated condition at a pressure greater
than atmospheric. Water in the unsaturated
(vadose) zone is at a pressure less than
atmospheric. Permeable formations that contain and
transmit ground water in useable quantities are
known as aquifers. Aquifers are classified as
unconfined, confined, and perched (Figure 3.2~. An
unconfined aquifer, also known as a water table
aquifer, has as its upper boundary the ground water
table. A capillary fringe exists immediately above
the water table. A confined aquifer has a
relatively impervious layer as its upper boundary.
The pressure potential of the water in contact with
_
, _ _ _
—- — 1
, _ _
OCR for page 27
- 2 7 -
R - 8r8e k"
1
l ~tm
~ ~ Scat ~
_~
Recherp ken
Tabb ~ _
_ i_ ~ ~ ~ A,~
\~\—~
~jr~ing Layers u'~confined
_,~l~ ~
FIGURE 3 . 2
K. Card bum
,_` ~
Idealized aquifer settings.
OCR for page 28
-28-
this confining layer is greater than atmospheric.
The confining layer may be an aquitard or an
aquiclude. Confined aquifers are also known as
artesian aquifers. A perched aquifer is an
unconfined aquifer of limited areal extent that
retains water because of an underlying restricting
layer, which in turn is above an unsaturated zone.
Perched aquifers may be seasonal or permanent.
For a given section or finite element of an
aquifer, inflow or water accretion may consist of
downward-moving water from the vadose zone, flow
through semiconfining layers, or lateral flow from
upgradient portions of the aquifer. Outflow of
water, also called depletion, may consist of
pumping from wells, flow from seeps and springs,
evaporation, leakage through semiconfining layers,
and lateral flow to downgradient portions of the
. ~
aquifer .
Water in the vadose zone and ground water zone is
governed by the same basic chemical and physical
relationships. For unconfined aquifers, water may
transfer freely between the zones. A zone occupied
by ground water may become a part of the vadose
zone as the water table is lowered.
time, as the water table rises, the
At a later
zone may again
become a part of the ground water zone. Virtually
the same water may be ground water at some time,
vadose water at a later time, and ground water once
again at still a later time. The chemical and
physical properties of the material that water
comes in contact with and the rate of movement of
underground water both have an affect on the
quality of that water.
Ground water becomes surface water when it
emerges as a spring or seep. These can be located
on the land surface or below the water level of a
stream, pond, or lake. Ground water discharge to
streams, springs, and seeps generally forms the
base flow for streams between major
runoff-producing events. The pathways taken by
water, as it infiltrates and percolates to become
ground water and then emerges as baseflow to become
surface water, have a major impact on the quality
of that water.
OCR for page 29
-29-
The interchange between ground water, vadose
water, and surface water points to the need to
consider the entire hydrologic system in assessing
the impact of alterations within a catchment on any
aspect of the hydrology of that catchment.
OCCURRENCE AND MOVEMENT OF GROUND WATER
The material that constitutes the earth's outer
mantle is composed of solid material and void
spaces. The solid material may be in the form of
individual particles or more massive rock
formations. The void spaces occur between the
particles and as cracks, fractures, or solution
channels in the rock.
They are occupied by gases
or liquids, most commonly air and water. In the
ground water zone, the voids are filled with water
(some entrapped air may be present), and a
condition of saturation or near saturation exists.
In the vadose zone, there is more air in the void
spaces.
Water-bearing formations may be either
consolidated (rock) or unconsolidated (clay, sand,
gravel). Except for rock outcroppings, the earth's
surface is covered by a layer of unconsolidated
material that may range in thickness from a few
centimeters to several thousand meters.
Consolidated material always underlies the
unconsolidated material. Alternating strata of
consolidated and unconsolidated material may exist
above the basement consolidated rock.
Unconsolidated material consists of individual
particles derived from the breakdown of
consolidated rock. Individual particles may range
from clay-sized particles measuring two micrometers
or less to rocks and boulders measuring several
meters across.
Consolidated material consists of mineral
particles that have been fused together by heat and
pressure or by chemical reactions to form solid
masses. They generally consist of sedimentary,
, and igneous rocks. Consolidated rocks
metamorphic
OCR for page 30
-30-
of importance relative to ground water are
limestone, dolomite, shale, siltstone, sandstone,
conglomerate, granite, and basalt. These rocks can
only become aquifers if they are fractured or, in
the case of limestone, have solution openings
(Figure 3.31.
Porosity is defined as the percentage of the
volume of a material that is occupied by voids.
Materials with high porosity contain considerable
water when saturated, typically from 15 to 30
percent for coarse materials and 50 percent for
clays on a volume basis (Table 3.11. All of the
water contained in a formation will not drain
solely due to gravity. The amount of water that
will drain from a saturated material due to gravity
is referred to as the specific yield, while the
amount of water retained is known as the specific
retention. For saturated material, the sum of the
specific yield and the specific retention is the
porosity (Table 3.11.
Darcy's equation (see, for example, Bouwer, 1978)
indicates that the rate of water movement in a
porous medium is proportional to the hydraulic
gradient. The proportionality factor is known as
the hydraulic conductivity. For saturated systems,
hydraulic conductivity depends, among other things,
on the size, shape, and connectedness of pores and
fractures and varies over a wide range (Figure
3.4~. For unsaturated systems, the hydraulic
conductivity is additionally dependent on the water
content, which is in turn a hysteretic function of
the metric water potential. For a given material
the hydraulic conductivity may vary by several
orders of magnitude as the water content ranges
from very dry to saturation.
Flow in fractured systems is dependent on the
extent of fracturing, the interconnectedness of the
fractures, and the mechanisms available for water
to enter the fracture systems. All of these
factors are highly variable and site specific.
Highly fractured rock may have quite high hydraulic
conductivities and thus be able to rapidly transmit
water.
OCR for page 31
-31
PRIMARY OP[N1N#$
"ELI-SORTED S^ND
~~%
AL
`~.'/`1 ^~:
_- r ',
;/~: ;~
~ ~ ~ / `~ ~
fR^CTuRES 1W
GRA~TE
FIGURE 3.3 Exampl
porosity.
POORLY SORTED SAND
SECONDARY OPENINGS
i.
Ha'
- 1 -
C^VE ANS I%
L 1" E STON E
of primary and secondary
SOURCE: Adapted from Heath, 1982.
OCR for page 32
-32-
TABLE 3.1 Typical Values (Percent by Volume) for
Porosity, Specific Yield, and Specific Retention
Material
Specific Specific
Porosity Yield Retention
Soil 55 40 15
Clay 50 2 48
Sand 25 22 3
. .
Gravel 20 19 1
Limestone 20 18 2
Sandstone 11 6 5
(semicon-
solidated)
Granite 0.1 0.09 -0.01
Basalt (young) 11
8
3
SOURCE:
Heath, 1982.
OCR for page 33
- 33 -
IGNEOUS ANO METAMORPHIC Rig
Urfractured Fnctund
~LT
. .
Unhactwed Fed Lava flaw
SANDSTONE
Fractured S~ldated
SHALE
Unfractured Fractured
CARBONATE ROa
OCR for page 34
-34-
Spatial variability in hydraulic properties of
both the vadose and ground water zones makes
quantification of underground flow processes
difficult. Measurements at many points are
required, with the number of measurements depending
on the variability present and the desired degree
~ ~ . (1982) found that ground
water recharge varied by more than a factor of 10
in a limited area due to spatial variability in
hydraulic properties of the material overlying an
aquifer (Figure 3.5~.
Of accuracy. Rehm et a]
naturally.
'=^h='rrm hack - e
GROUND WATER RECHARGE
Ground water recharge is the addition of surface
or precipitation water to the ground water
reservoir, and is expressed as volume per unit time
or depth of water per unit time. Natural recharge
occurs as a result of the natural movement of water
through the vadose zone. Artificial recharge
occurs when water is added to the ground water
reservoir that would not have reached the reservoir
Artificial recharge can result from
I, ~~;,.~, water spreading, artificial
impoundments, recharge wells, applying water to the
land surface through irrigation, waste disposal,
and other means. Recharge enhancement refers to
activities that increase the rate of natural
recharge. Such activities as land treatment to
increase infiltration or vegetation management to
reduce evapotranspiration could constitute recharge
enhancement.
The combinations of hydrologic and geologic
settings that contribute to natural ground water
recharge are many and varied. Some of the major
settings include general infiltration of
precipitation water and percolation over large
areas, percolation from bodies of surface water,
and rapid movement of water from the surface
through fractures, solution channels, and other
highly pervious areas.
OCR for page 35
- 35-
[13W low 1 ;
Or ~ ~~
1 ~ ~ f.'',,'2',~..',''.~
i ~ ~-
1~ ~ ~ ~ T-
146N
r 145N
RECHARGE (~3e ,;1 - it)
)0.e
Oft
E] 0.1 to O.o.
O <0.04
0 1000 2000 - ~~e
FIGURE 3.5 Spatial distribution of ground water
recharge rates based on field data for pre-mining
conditions .
SOURCE: Rehm et al., 1982.
OCR for page 36
-36-
Recharge depends on the availability of water for
recharge, the physical characteristics of soil and
' ~ ~ ~ ~ r --- through, and the
ability of the ground water reservoir to accept the
recharge water. Any one of these three major
factors may be limiting and thus define the actual
recharge. The actual recharge rate cannot exceed
the rate at which water is available to supply the
recharge process.
Deep percolation of infiltrated precipitation is
a common and widespread means of natural recharge.
Hydrologic processes at the surface and in the
vadose zone largely determine the quantity of water
that becomes deep percolation. Rainfall amounts,
timing, and intensities are influential. Large
quantities of rainfall occurring at low intensities
during periods when surface conditions are such
that high rates of infiltration can be sustained
will maximize the water available for recharge via
percolation. Surface runoff prevents precipitation
water from infiltrating where it landed. The finer
the soil texture (including crusting), the sparser
the vegetation, the steeper the slope, the higher
the rainfall intensity, and the smoother the
surface, the more water will flow off laterally as
surface runoff. Evapotranspiration also removes a
large fraction of the infiltrated water before it
can become deep percolation. Climatic, plant, and
soil factors govern evapotranspiration rates. In
arid and semiarid regions evapotranspiration is
nearly equal to precipitation. There only large,
normally infrequent precipitation events may then
contribute to deep percolation. In humid regions,
annual deep percolation can be a significant part
of the hydrologic budget, accounting for about half
of the annual precipitation.
Thus recharge via deep percolation is governed to
a large extent by the hydrologic processes that
take place in the near-surface zone. This zone
generally constitutes the root zone of any actively
growing vegetation. The character of the
vegetation can significantly affect the amount of
recharge. In evaluating evapotranspiration, type
rock material the water must mass
/
OCR for page 37
-37-
of vegetation, density of vegetation, leaf-area
index, root density, ~
must be considered. Factors that reduce
evapotranspiration would tend to increase recharge
if all other factors remained the same.
The permeability of the material in the vadose
zone is often an important determinant of the
recharge rate. Highly permeable materials that
allow rapid infiltration and movement of water
vertically are primary contributors to recharge.
If the aquifer is overlain by a layered system,
very slowly permeable layers may restrict the
recharge rate. ~ - - -
root depth, and growlug season
Perched ground water may then form.
fracture zones and solution channels through rock
material may increase recharge if they reach the
surface or are otherwise located so that they come
in contact with water at atmospheric pressure or
act as localized
sinks and rapidly transmit water to the ground
water reservoir. Most fractures tend to terminate
at depths of about 30 m (Bouwer, 1978~. Thus the
lower parts of the fractures often are filled with
water, and the rock becomes, in a sense, an
aquifer.
Localized areas overlying an aquifer may
contribute much of the recharge to an aquifer.
Such areas may have more favorable conditions for
allowing the relatively rapid movement of water
Greater. In such cases they may
-,
trom the ground surrace co one ground water. These
conditions may be the result of very permeable
.. . .
~ , _ _ _ , ,
soils, solution channels or highly fractured rock,
~ ~ ~ ~ ' areas are often
, . .
termed recharge areas even though recharge may also
be occurring at slower rates over other parts of
the aquifer. Disturbances of these recharge areas
have a great potential for having an impact on the
actual recharge rate of an aquifer.
Streams, lakes, and ponds may be sources of
recharge for some aquifers. In humid regions, the
water table often slopes downward toward surface
water bodies. In such instances the surface water
is being augmented by subsurface or ground water
flow. Under semiarid and arid conditions, the
and adequate orecloltatlon. buck
OCR for page 38
-38-
slope of the ground water surface is often away
from the surface body of water, indicating that the
surface water in a recharge source for the Ground
T.~= t='
~ . screams cnar concr~Dure water co ground
water are known as losing streams, while streams
that receive water from ground water are known as
gaining streams. Any particular stream may be a
gaining stream over a part of its length and a
losing stream over another part of its length. A
stream may be a gaining stream part of the time at
a particular location and a losing stream at the
same location at another time. The factor
determining whether a surface water body is gaining
or losing is the relative elevation of the surface
water and the ground water.
Aquifer characteristics may limit water recharge
in instances where the potential recharge rate
exceeds the rate at which the water is transmitted
away from the recharge area, resulting in the
buildup of a ground water mound. This mound would
continue to build until the hydraulic gradients in
the aquifer were sufficient to cause lateral flows
in the aquifer equal to the recharge rate or until
the mound limited the recharge rate itself.
Sometimes recharge rates are controlled by perched
mounds on restricting layers in the vadose zone.
FRACTURED ROCK HYDROLOGY
Flow systems in fracture zones are very difficult
to quantify. The controlling factors are the
extent, size, distribution, and degree of
interconnection of the fractures. A highly
fractured material may allow rapid transmission of
water and thus promote recharge of ground water.
If fractures are not interconnected, they cannot
serve as conduits for water movement. Slightly
fractured systems are thus not likely to allow
significant movement of water, whereas highly
fractured systems may serve as major conduits.
Fracturing of rock is brought about by stresses
applied to and released from rock formations.
OCR for page 39
-39-
Stress-relief fractures are common in the
Appalachian area where overlying soil material has
gradually eroded away, removing part of the lateral
compression load on the exposed rock walls. As
this load is relieved, the rocks tend to expand and
fracture vertically. Fracture zones in the
Appalachian region may be up to 2S m thick and can
provide pathways for significant movement of water.
After water enters a near-surface fracture
system, it tends to continue its downward
movement. Fracture flow may result in ground water
recharge, hillside seepage, or seepage into
tributary streams. - ~ '
For water to enter any nut the
smallest fractures, it must be at or above
atmospheric pressure. Water from saturated
materials can move readily into fracture systems if
the water is at atmospheric pressure or above.
In a conceptualization of flow in eastern
Kentucky, the fracture flow is limited to the near
surface (top 15 to 25 m) of the hillside (Figure
3.6~. Water makes its way downslope relatively
rapidly through the system. For this system major
recharge to the hillside aquifer occurs when
precipitation soaks through the soil and colluvium
covering the ridges and hillsides or when runoff is
directly intercepted by open fissures in rocks
exposed at the surface. Water percolates down
through the fractured sandstones until it ends
above a confining bed. The perched water then
flows laterally out toward the hillsides along
bedding planes until it can move vertically
downward where fractures penetrate the confining
bed. Wet-weather springs form on the hillside
where the confining bed is relatively unfractured,
and ground water is forced out to the surface.
This results in a stair-step pattern of ground
water movement from the ridgetops to the valley
bottom (Kipp and Dinger, 1988~.
OCR for page 40
-40 -
lLlVAt10H
11H F55t]
1 toot_
100—
1000
8~'URA''O
lANO8SO..
C0~1~1~. sell
_ _
,.,..,,
'.AC'UR''
FIGURE 3.6 Conceptual model of fracture flow in a
ground water system.
SOURCE: Kipp and Dinger, 1988.
9
Representative terms from entire chapter:
surface water
