Ground-Water
Flow is just water movement through a porous media. Ground water can flow
between soil particles; and, through crevices, cracks, joints, and fractures
of consolidated rock. Ground-water flow needs to be viewed from both a micro-scale
and a macro-scale; and, also from views in between. Arrows are used in the
figure below to demonstrate ground water flow routes. Ground water flow
may or
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may not predominate in
the larger soil pores, known as the macropores. Navy blue arrows are used
to distinguish ground water flow in the macropores. Individually less flow
occurs through the smaller and smallest soil pores, known as the mesopores
and the micropores, respectively; however, collectively this may not
be so. Obviously more flow of water can occur in a larger soil pore,
but if there are few large-sized soil pores relative to the shear number
of small-sized soil pores in a given soil, then flow will predominate in
the small-sized soil pores. Flow through these smaller soil pores is depicted
by the light blue arrows. Ground water cannot flow, along a straight line,
from left to the right. Individual water molecules must move around soil
particles: up, down, left, and right. These herky-jerky movements are
ground-water flow on a micro-scale. These herky-jerky movements are called
mechanical mixing. This figure depicts overall ground-water flow from left
to right. This is ground-water flow on a macro-scale. Individual water molecules
follow a tortuous path in going from left to right. This constant change
in direction that individual water molecules must undergo in traversing
a distance results in much mixing of the ground water along with any contaminants
that may be dissolved in the water. Along with diffusion, mechanical mixing
is one of two processes responsible for dispersion of contaminants in the
water table.
What is the driving
force pushing water between soil particles; and through crevices, cracks,
joints, and fractures in rock? The answer to this can be illustrated by
the figure below. Water is lost from the overlying stream. This water must
flow between individual soil particles, or more precisely, the water
must flow through the
soil pores. Just to reiterate, the soil pores are analogous to water pipes.
When you want to drain your water pipes in your home, you shut off the water
supply to your house, then open the faucet in your upstairs bathroom and
the boiler drain in your basement; and then, let gravity take over. Then,
the water drains to the basement. Obviously the water that is lost from
the overlying stream bed, drains vertically-downward through the soil to
recharge the water table below. Typically, water flows from a higher elevation
to a lower elevation, because of gravity. If
the water cannot freely drain through the soils, mounding of the water table
will occur in the soil demonstrating an impedance to flow. Since water cannot
typically freely drain or freely flow through the soil, a gradient (analogous
to relief) is established
at the water table. Water will flow from a higher point on the gradient
(the upgradient point) to a lower point on the gradient (the downgradient
point) due to gravity.
This last figure on this
page, depicts three different ground-water flow regimes. In this figure,
shallow ground-water flow from either side is to the stream. Deeper ground-water
flow is not captured by the
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stream, but continues
to flow in a downgradient direction from right to left. Only the shallow
portion of the water table is captured by the stream. Deeper ground-water
flow paths may be influenced, but not captured by the stream. Remember that
this is just an example. In some hydrogeologic settings, even the deeper
ground-water-flow paths may be captured. Why does ground-water flow from
one point to another? The ultimate driving force is gravity, just as it
is your home plumbing pipes. However, this force gets re-defined as the
water moves from one elevation to another elevation and it is known as total
head: composed of elevation head, pressure head, and if the fluid has sufficient
velocity, velocity head. Normally, velocity head in the ground water is
not measurable and can be neglected.
Please note that water
in your plumbing system can also flow upwards just as can ground water.
Water in your plumbing system can flow uphill because of the driving force,
called total head, and the pipes to contain this head. The ground water
is also contained in pipe-like lines that just happen to be somewhat leaky.
These pipe-like lines are variously called soil pores; or cracks, crevices,
joints, and fractures of consolidated rock. |
