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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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