# How to Calculate Transmissivity

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The transmissivity of an aquifer is a measure of the quantity of water that the aquifer can transmit horizontally and should not be confused with transmittance, a measure used in optics. An aquifer is a layer of rock or unconsolidated sediments that can yield water to a spring or well. Transmissivity is typically used to determine the water that an aquifer can deliver to a pumping well. It can be calculated directly from the aquifer's average horizontal permeability and thickness.

## The Steps

Define hydraulic conductivity as the volume of water that flows through a 1 square foot cross-section of an aquifer under a hydraulic gradient of 1 foot per foot in a given period of time. the hydraulic conductivity is therefore measured in area of water per unit time.

Define transmissivity mathematically. We have T = KhD where T is the transmissivity, Kh is the average horizontal conductivity and D is the aquifer thickness.

Determine the units of measure for transmissivity. The horizontal conductivity is measured in length per unit time and the aquifer thickness is a length. The transmissivity is therefore measured in area per unit time, typically square feet per day.

Expect a low transmissivity for a confined aquifer. These aquifers are generally completely filled with water and retard the movement of water out of the aquifer. Confined aquifers will have a very low transmissivity.

Examine the range of actual transmissivity values. An aquifer from the Cretaceous age can have a transmissivity as low as 1,000 square feet per day, while a limestone aquifer from the Eocene age may have a transmissivity as high as 50,000 square feet per day.

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