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Groundwater Transmissivity

Compute groundwater transmissivity from hydraulic conductivity and aquifer thickness

Hydraulic conductivity, K: 
Aquifer thickness, h or b: 
Transmissivity, T: 
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Groundwater transmissivity calculator units: cm=centimeter, ft=foot, gpd=U.S. gallon/day, gph=U.S. gallon/hr, gpm=U.S. gallon/min, gal=gallon (U.S.), hr=hour, km=kilometer, m=meter, min=minute, s=second.


Groundwater Drawing and Transmissivity Equation

Groundwater

Unconfined Aquifer: T = K h

Confined Aquifer: T = K b

Groundwater transmissivity is a useful parameter in groundwater flow modeling. Groundwater transmissivity includes the hydraulic conductivity which is a property of the aquifer. Hydraulic conductivity is a function of the fluid moving through the porous medium (i.e. aquifer, formation) and the permeability of the formation. Many experiments over many years have been conducted of water flowing through various porous media. Hydraulic conductivities determined from many experiments have been tabulated. The table below is typical. In viewing the table, it is apparent that hydraulic conductivity values cover a wide range even for a given formation. If field or laboratory test results of hydraulic conductivity are not available for your particular formation, then use of published values like the ones below are helpful guides.

In addition to hydraulic conductivity, groundwater transmissivity is a function of aquifer thickness. In unconfined aquifers, the aquifer thickness is a height h. h is the water depth above an impermeable boundary. If a well is pumped, h can vary with time and location. In confined aquifers, the aquifer thickness b is independent of any effects of pumping since the aquifer is bound between two impermeable layers. Water in a confined aquifer is under pressure between the layers. If the water level drops below the upper impermeable layer, then the aquifer would no longer be confined.


Hydraulic Conductivity Table (Freeze and Cherry, 1979, p. 29)

The values in the drop-down menu in our groundwater transmissivity calculator are typical numbers within the ranges given below for clayey, silty, sandy, and gravelly soil.

Unconsolidated Deposits Hydraulic Conductivity (m/s)
Unweathered Marine Clay 10-12 - 10-9
Glacial Till 10-12 - 10-6
Silt, Loess 10-9 - 10-5
Silty Sand 10-7 - 10-3
Clean Sand 10-6 - 10-2
Gravel 10-3 - 1

Rocks		  
Unfractured Metamorphic and Igneous Rocks 10-14 - 10-10
Shale 10-13 - 10-9
Sandstone 10-10 - 10-6
Limestone and dolomite 10-9 - 10-6
Fractured Igneous and Metamorphic Rocks 10-8 - 10-4
Permeable Basalt 10-7 - 10-2
Karst Limestone 10-6 - 10-2


Notation (SI units are shown. Groundwater transmissivity calculator allows variety of units)
b = Confined aquifer thickness (m).
h = Height of water table in unconfined aquifer (m).
K = Aquifer hydraulic conductivity (m/s).
T = Aquifer tranmssivity (m2/s).


Reference
Freeze, R. A. and Cherry, J. A. 1979. Groundwater. Prentice-Hall, Inc.

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