Thought of The Day
Thought of The Day
Thought of The Day
Chapter 6:
SUBSURFACE WATER
GROUND WATER
Originally mapped
in Cecil County,
Maryland in 1899, more
than 10 million acres
(40,000 km²) of the Cecil
soil series (Fine,
kaolinitic, thermic Typic
Kanhapludults) are now
mapped in
the Piedmont region of
the southeastern United
States.
Equilibrium Points
Field Capacity – the moisture content of soil
after gravity drainage is complete.
Permeability Intrinsic
Material Porosity, % Specific Meinzer Permeability,
Yield, % Units m3 /day·m2 D
Quartzite,
Granite 1 0.5 0.01 0.0004 0.0005
MOVEMENT OF GROUNDWATER
In 1856, a French
hydraulic engineer
named Henry Darcy
published an equation
that describes the flow of
a fluid through a porous
medium. It is known as
Darcy’s Law.
𝑞 = 𝑇𝐵𝑠
where:
q = the rate of water flow
T = coefficient of transmissibility
of the aquifer
B = width of the aquifer
s = slope of the hydraulic gradient
PERMEABILITY - is a measure of the ability of a
material to transmit fluids.
• METEORIC WATER - the water derived from precipitation
(snow and rain).
𝑑𝑦
𝑞 = 2𝜋𝑥𝑦𝐾
𝑑𝑥
where:
2𝜋𝑥𝑦 = area of the cylinder
𝑑𝑦
= slope of the water table
𝑑𝑥
EQUILIBRIUM HYDRAULICS OF
WELLS
• When several wells are close together,
their cones of depression may overlap,
or interfere. Where the cones of
depression overlap, the drawdown at a
point is the sum of the drawdowns
caused by the individual wells.
METHOD OF IMAGES
It was devised by Lord Kelvin for electrostatic theory
to treat boundary problems. An image well is assumed
to have all the properties of the real well but to be
located on the opposite side of the stream and at the
same distance from the real well. Its cone of depression
is the same as that of the real well but is inverted. The
resultant cone of depression for the real well is found
by subtracting the drawdown caused by the image well
from that caused by the real well. The corrected water
table between the real well and the stream is therefore
higher than without the effect of the stream. At the
stream, the two drawdowns are equal, and the new
drawdown is zero.
AQUIFER ANALYSIS
Techniques of the previous sections are suitable for
analysis of single wells or a small well field, but
study of a large aquifer generally requires more
efficient computational systems.
SLUG TEST
is a variation on the typical aquifer test where an
instantaneous change is made, and the effects are
observed in the same well. This is often used to get a
quick estimate of the aquifer properties immediately
around the well.
SAFE YIELD
Safe yield is generally considered equal to the average
replenishment rate of the aquifer from natural and
artificial recharge. Evaporation, transpiration and
basin outflow are also factored in to replenishment
rates.