Page 73 - Water Engineering Hydraulics, Distribution and Treatment

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3.8 Aquifer Characteristics
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A confined aquifer for which S in Eq. (3.9) is 3 × 10
2
neous throughout their extent, and laboratory measurements
2
will release from 1 mi 64,125 gal (93,711 L/km )bylow-
are not representative of actual “in-place” values. Most sam-
ering the piezometric surface by 1 ft (0.3048 m).
ples of the material are taken in a vertical direction, whereas
A water table aquifer also releases water from storage by
the dominant movement of water in the aquifer is nearly
two processes: (a) dewatering or drainage of material at the
horizontal, and horizontal and vertical permeabilities differ
free surface as it moves downward and (b) elastic response
markedly. Also, some disturbance is inevitable when the sam-
of the material below the free surface. In general, the quan-
ple is removed from its environment. This method cannot,
tity released by elastic response is very small compared to
therefore, be used to give a reliable quantitative measure of
the dewatering of the saturated material at the water table.
hydraulic conductivity.
Thus the storage coefficient is virtually equal to the specific
The measurement of hydraulic conductivity in undis-
yield of the material. In unconfined aquifers, the full comple-
turbed natural materials can be made by measurement of
ment of storage is usually not released instantaneously. The measurements. Aquifers are seldom, if ever, truly homoge-
speed of drainage depends on the types of aquifer materials. hydraulic gradient and determination of the speed of ground-
Thus in water table aquifers, the storage coefficient varies water movement through the use of tracers. A tracer (dye,
with time, increasing at a diminishing rate. Ultimately it is electrolyte, or radioactive substance) is introduced into the
equal to specific yield. Furthermore, since the dewatered por- groundwater through an injection well at an upstream loca-
tion of the aquifer cannot transmit water, transmissivity of tion, and measurements are made of the time taken by the
the aquifer decreases with the lowering of the water table. tracer to appear in one or more downstream wells. Uranin, a
Transmissivity is thus a function of head in an unconfined sodium salt of fluorescein, is an especially useful dye because
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aquifer. The storage coefficient of unconfined aquifers may it remains visible in dilutions of 1:(14 × 10 ) without a flu-
range from 0.01 to 0.3. A water table aquifer with a stor- oroscope and 1:10 10 with one. Tritium has been used as a
2
2
age coefficient of 0.15 will release from a 1 mi (2.59 km ) radioactive tracer.
area with an average decline in head of 1 ft (0.3048 m) The time of arrival is determined by visual observation
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209 × 10 × 0.15 gal = 31.30 MG (118.4ML). or colorimetry when dyes are added, by titration or electrical
Hydraulic diffusivity is the ratio of transmissivity, T,to conductivity when salt solutions are injected, or by a Geiger
storage coefficient, S, or of permeability, K, to unit storage, or scintillation counter when radioactive tracers are used. The
′
S . Where D is hydraulic diffusivity, distance between the wells divided by the time required for
half the recovered substance to appear is the median velocity.
D = T∕S = K∕S ′ (3.10) The observed velocity is the actual average rate of motion
through the interstices of the aquifer material. The face veloc-
In an unconfined aquifer, even if S is assumed constant,
ity can be calculated, if effective porosity is known. The
the diffusivity will vary with transmissivity, which varies
application of Darcy’s law enables the hydraulic conductiv-
with the position of the free surface.
ity to be computed. The problems of direction of motion,
The conductivity, the transmissivity, the storage coeffi-
dispersion and molecular diffusion, and the slow move-
cient, and the specific yield are usually referred to as forma-
ment of groundwater limit the applicability of this method.
tion constants and provide measures of the hydraulic prop-
The method is impractical for a heterogeneous aquifer that
erties of aquifers.
has large variations in horizontal and vertical hydraulic
The capacity of an aquifer to transmit water can be mea-
conductivity.
sured by several methods:
The drop in head between two equipotential lines in an
aquifer divided by the distance traversed by a particle of
1. Laboratory tests of aquifer samples
water moving from a higher to a lower potential determines
2. Tracer techniques the hydraulic gradient. Changes in the hydraulic gradient
3. Analysis of water level maps may arise from either a change in flow rate, Q, hydraulic
4. Aquifer tests conductivity, K, or aquifer thickness, b (Eq. 3.6). If no water
is being added to or lost from an aquifer, the steepening of
Laboratory measurements of hydraulic conductivity are the gradient must be due to lower transmissivity, reflecting
obtained by using samples of aquifer material in either a either a lower permeability, a reduction in thickness, or both
constant-head or a falling-head permeameter. Undisturbed (Eq. 3.8).
core samples are used in the case of well-consolidated mate- Of the currently available methods for the estimation
rials and repacked samples in the case of unconsolidated of formation constants, aquifer tests (also called pumping
materials. Observations are made of the time taken for a tests) are the most reliable. The mechanics of a test involve
known quantity of water under a given head to pass through the pumping of water from a well at a constant discharge
the sample. The application of Darcy’s law enables hydraulic rate and the observation of water levels in observation wells
conductivity to be determined. The main disadvantage of this at various distances from the pumping well at different
method arises from the fact that the values obtained are point time intervals after pumping commences. The analysis of a