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70 Chapter 3 Water Sources: Groundwater

to 0.3. A water table aquifer with a storage coefficient of 0.15 will release from a 1-mi 2
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2
(2.59 km ) area with an average decline in head of 1 ft (0.3048 m), 209 10 0.15 gal
31.30 MG (118.4 ML).
Hydraulic diffusivity is the ratio of transmissivity, T, to storage coefficient, S, or of per-
meability, K, to unit storage, S . Where D is hydraulic diffusivity,

D = T>S = K>S¿ (3.10)
In an unconfined aquifer, even if S is assumed constant, the diffusivity will vary with
transmissivity, which varies with the position of the free surface.
The conductivity, the transmissivity, the storage coefficient, and the specific yield are
usually referred to as formation constants, and provide measures of the hydraulic proper-
ties of aquifers.
The capacity of an aquifer to transmit water can be measured by several methods:
1. Laboratory tests of aquifer samples
2. Tracer techniques
3. Analysis of water level maps
4. Aquifer tests.

Laboratory measurements of hydraulic conductivity are obtained by using samples of
aquifer material in either a constant-head or a falling-head permeameter. Undisturbed
core samples are used in the case of well-consolidated materials, and repacked samples in
the case of unconsolidated materials. Observations are made of the time taken for a
known quantity of water under a given head to pass through the sample. The application
of Darcy’s law enables hydraulic conductivity to be determined. The main disadvantage
of this method arises from the fact that the values obtained are point measurements.
Aquifers are seldom, if ever, truly homogeneous throughout their extent, and laboratory
measurements are not representative of actual “in-place” values. Most samples of the ma-
terial are taken in a vertical direction, whereas the dominant movement of water in the
aquifer is nearly horizontal, and horizontal and vertical permeabilities differ markedly.
Also, some disturbance is inevitable when the sample is removed from its environment.
This method cannot, therefore, be used to give a reliable quantitative measure of hy-
draulic conductivity.
The measurement of hydraulic conductivity in undisturbed natural materials can be
made by measurement of hydraulic gradient and determination of the speed of groundwa-
ter movement through the use of tracers. A tracer (dye, electrolyte, or radioactive sub-
stance) is introduced into the groundwater through an injection well at an upstream loca-
tion, and measurements are made of the time taken by the tracer to appear in one or more
downstream wells. Uranin, a sodium salt of fluorescein, is an especially useful dye because
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it remains visible in dilutions of 1:(14 10 ) without a fluoroscope and 1:10 10 with one.
Tritium has been used as a radioactive tracer.
The time of arrival is determined by visual observation or colorimetry when dyes are
added, by titration or electrical conductivity when salt solutions are injected, or by a
Geiger or scintillation counter when radioactive tracers are used. The distance between
the wells divided by the time required for half the recovered substance to appear is the
median velocity. The observed velocity is the actual average rate of motion through the
interstices of the aquifer material. The face velocity can be calculated, if effective poros-
ity is known. The application of Darcy’s law enables the hydraulic conductivity to be