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Analytical Methods

TECH GUIDE

Physical Properties: Hydraulic Conductivity

Water can move through soil as saturated flow, unsaturated flow, or vapor flow. Saturated flow takes place when the soil pores are completely filled (or saturated) with water. Unsaturated flow occurs when the larger pores in the soil are filled with air, leaving only the smaller pores to hold and transmit water. Vapor flow occurs as vapor pressure differences develop in relatively dry soils. Vapor migrates from an area of high vapor pressure to an area of low vapor pressure. Hydraulic conductivity is a soil property that describes the ease with which the soil pores permit water (not vapor) movement. It depends on the type of soil, porosity, and the configuration of the soil pores. In saturated soils, the hydraulic conductivity is represented as Ksat and in unsaturated soils, the hydraulic conductivity is represented as K.

The quantity of water per unit of time, Q, that flows through a column of saturated soil can be expressed by Darcy's Law, as follows:

where Ksat is the saturated hydraulic conductivity, A is the cross-sectional area of the column through which the water flows, DP is the hydrostatic pressure difference from the top of the column to the bottom of the column, and L is the length of the column. Since area A and length L of a given column are fixed, the rate of flow is determined by the hydraulic force DP driving the water through the soil (commonly gravity) and the saturated hydraulic conductivity, Ksat.

Flow through an unsaturated soil is more complicated than flow through continuously saturated pore spaces. Macropores are filled with air, leaving only finer pores to accommodate water movement. The movement of water in unsaturated soils is dictated by differences in matric potential, not gravity. The matric potential gradient is the difference in the matric potential of the moist soil areas (high matric potential) and nearby drier areas (low matric potential) into which the water is moving (Brady and Weil, 1999).

Hydraulic conductivity is an important soil property when determining the potential for widespread groundwater contamination by a contaminating source. Soils with high hydraulic conductivities and large pore spaces are likely candidates for far reaching contamination. The hydraulic conductivity in a saturated soil can be measured by injecting a non-reactive tracer (i.e. bromide) in a monitoring well and measure the time it takes for the tracer to reach a downgradient monitoring well. Remediation of contaminated groundwater can be a likely step in the reclamation process. For more information on groundwater remediation strategies, click here.

For more information on measuring hydraulic conductivity in the field and in the lab, see LaGrega (2001) and Driscoll (1986).

 

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