Hydrogeologists calculate average linear groundwater flow velocity to determine how fast the water is moving beneath the land surface. Scientists conduct hydrogeologic evaluations to gather the data necessary to perform the calculation. The speed of the groundwater depends on the characteristics of the soil beneath the surface and the water table's gradient. Therefore, the average linear groundwater flow velocity may vary greatly across a region. Knowing the groundwater velocity of an unconfined aquifer in an area helps geologists to design water supply and contaminant remediation systems.
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Determine the gradient (i) of the groundwater table using a potentiometric surface map. Regional and local potentiometric maps are often available from state natural resources or environmental offices. Calculate the gradient by dividing the change in the water table elevation by the change in the distance. For example, if your potentiometric surface map has a low contour interval of 10 feet and a high contour interval of 25 feet and these contours are 150 feet apart, the gradient would be 15 divided by 150, which is a gradient of 0.1 feet per foot.
Multiply the gradient (i) of the groundwater table by the hydraulic conductivity (K) of the aquifer. Hydraulic conductivity is a measure of how quickly a fluid will move through a porous aquifer. You can measure the hydraulic conductivity of a soil or use generic values from reference tables. Hydraulic conductivity data tables are available in hydrogeologic or soil reference books. Hydraulic conductivity values have the units of length over time, such as feet per minute or centimetres per second.
Divide the value calculated in Step 2 by the effective porosity (né) of the soil. Porosity is a measurement of the open space in the soil. The effective porosity is a measurement of the connected spaces in the soil where groundwater can flow. You can measure the porosity of the soil sample or use generic values from reference tables. Porosity data tables are available in hydrogeologic or soil reference books. The result of this calculation is the average linear velocity (V).
Tips and warnings
- The average linear flow velocity equation is V = iK/né, where V is velocity, i is the gradient, K is the hydraulic conductivity and né is the effective porosity.
- Use consistent units of measure for each variable in your calculation.
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