Page 93 - Materials Chemistry, Second Edition

76 Practical Design Calculations for Groundwater and Soil Remediation

3.2.4 Transmissivity, Specific Yield, and Storativity
Transmissivity (T) is another commonly used term to describe an aqui-
fer’s capacity to transmit water. It represents the amount of water that can
be transmitted horizontally by the entire saturated thickness of the aquifer
under a hydraulic gradient of one. It is equal to the multiplication product of
the aquifer thickness (b) and the hydraulic conductivity (K). Commonly used
units for T are m /day and gpd/ft.
2
T = Kb (3.8)

An aquifer typically serves two functions: (1) a conduit through which
flow occurs and (2) a storage reservoir. This is accomplished by the openings
in the aquifer matrix. If a unit of saturated formation is allowed to drain by
gravity, not all of the water it contains will be released. The ratio of water that
can be drained by gravity to the entire volume of a saturated soil is called
specific yield, while the part retained is the specific retention. Table 3.4 tabulates
typical porosity, specific yield, and specific retention of soil, clay, sand, and
gravel. The sum of the specific yield and the specific retention of a formation
is equal to its porosity.
The specific yield and the specific retention are related to the attraction
between water and the formation materials. Clayey formation usually has a
lower hydraulic conductivity. This often leads to an incorrect idea that clayey
formation has a smaller porosity. As shown in Table 3.4, clay has a much
larger porosity than sand and gravel. The porosity of clay can be as high as
50%, but its specific yield is extremely low at 2%. Porosity determines the
total volume of water that a formation can store, while specific yield defines
the amount that is available to pumping. The low specific yield explains the
difficulty of extracting groundwater from clayey aquifers.
When the head in a saturated aquifer changes, water will be taken into or
released from storage. Storativity or storage coefficient describes the quan-
tity of water taken into or released from storage per unit change in head

TABLE 3.4
Typical Porosity, Specific Yield, and Specific
Retention of Selected Materials
Porosity Specific Yield Specific Retention
(%) (%) (%)
Soil 55 40 15
Clay 50 2 48
Sands 25 22 3
Gravel 20 19 1
Source: [2].