Key Features

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Hydraulic Conductivity Measurement

Hydraulic conductivity (K) defines the rate of movement of water through a porous medium such as a soil or aquifer. It is the constant of proportionality in Darcy’s Law and as such is defined as the flow volume per unit cross-sectional area of porous medium under the influence of a unit hydraulic gradient. This translates to SI units of m3/m2/day or m/d, but other measurement units are commonly used (Table 1)

Table 1: Commonly used units for hydraulic conductivity (K)
Description metres/day (m/d) metres/second (m/s) millimetres/day (mm/d) millimetres/hour (mm/hr)
Extremely slow 0.000001 1.5741x10-11 0.001 0.000041667
Very Slow 0.0001 1.5741x10-9 0.1 0.0041667
Slow 0.01 1.5741x10-7 10 0.41667
Moderate 1 1.5741x10-5 1000 41.667
Fast 10 1.5741x10-4 10000 416.667
Very Fast 100 1.5741x10-3 100000 4166.667

Measurement of hydraulic conductivity is problematic, considering the parameter can differ over several orders of magnitude across the spectrum of sediments and rock types, as indicated in Table 2. The parameter can also vary markedly in space, even with apparently minor changes in sediment characteristics. Hydraulic conductivity is influenced by the properties of the fluid being transmitted (such as viscosity) as well as the porous medium. Hydraulic conductivity is also scale dependent, so that measurements taken at the core sample level may not be directly extrapolated to the aquifer scale. It is also direction dependent, so that hydraulic conductivity can be markedly different in the vertical from the horizontal. Hydraulic conductivity cannot be directly measured but inferred from field, laboratory or modelled data.

Table 2: Indicative hydraulic conductivities of some rock types (Brassington, 1988)
Rock Type Grain size (mm) Hydraulic Conductivity K (m/d)
Clay 0.0005-0.002 10-8-10-2
Silt 0.002-0.06 10-2 - 1
Fine Sand 0.06 -0.25 1-5
Medium Sand 0.25-0.50 5-20
Coarse Sand 0.50-2 20-100
Gravel 2-64 100-1000
Shale small 5x10-8 - 5x10-6
Sandstone medium 10-3-1
Limestone variable 10-5-1
Basalt small 0.0003-3
Granite large 0.0003-0.03
Slate small 10-8-10-5
Schist medium 10-7-10-4

Different approaches have been taken to estimate hydraulic conductivity, including:

  1. Using seepage meters which directly measure the flux (Q) at the interface between the surface water feature and the aquifer. The basic method is to isolate part of the sediment-water interface with a chamber open at the base (with surface area A) and measure the change in water volume contained in a bag attached to the chamber over a measured time period. When combined with head gradient measurements (dh/dl) between the sediment bed and the surface water body from minipiezometers (Lee & Cherry, 1978), the vertical hydraulic conductivity can be derived from Darcy’s Law:

    (Equation 1)

    Mathematical equation
  2. Infiltration Tests, where infiltrometers (also known as permeameters) are used to measure the rate that water can infiltrate downwards through the sediment/soil profile which is a function of vertical hydraulic conductivity. Two basic methods can be employed. In falling-head tests water is added to reach a target level in the infiltrometer, after which the subsequent decline in water level is recorded as infiltration occurs. In constant-head tests, the target level is maintained during the test duration by adding increments of water of known volume. These tests are commonly used for unsaturated soils, but can also be carried out in calm, shallow water bodies (McMahon et al, 1995; Duwelius 1996; Lindgren & Landon, 2000; Rosenberry 2000). Various instruments are available that are based on measuring infiltration, including well, disc and ring (double and single) permeameters (ANCID, 2000). Similar tests using constant-head or falling-head configurations can be undertaken in the laboratory on core samples taken from the field site.
  3. Pump Tests, involving pumping groundwater from the piezometer and monitoring the pumping rate, as well as the groundwater level in the piezometer or in nearby piezometers. The pump test (also called aquifer test) indicates how the aquifer responds to groundwater withdrawals, with the data used to estimate aquifer characteristics such as hydraulic conductivity. A wide variety of formulas are available for the analysis of pump test data, based on differences in aquifer type and geometry, boundary conditions, and underlying assumptions. Slug Tests are particular tests where the rate of groundwater recovery is measured after a small volume (slug) of water is suddenly displaced (Duwelius 1996; Cey et al, 1998; Springer et al; 1999).

  4. Grainsize analysis, involving determining the distribution of grainsizes within the sediment using standard sieves. Empirical relationships are used to estimate hydraulic conductivity from standard grainsize parameters (Vukovic & Soro, 1992). The grainsize diameter at which 10% of the sediment is finer (d10) is applied in a commonly used empirical formula, initially developed by Hazen (1893):

    (Equation 2)

    Mathematical equation

    where AH is a dimension coefficient (= 1.0 for m/d), C is an empirical constant (=860) and T is a temperature correction factor (=1 at 10° C). Another empirical relationship developed by Alayamani & Sen (1993) uses the slope and intercept (I0) of the grainsize distribution curve between d10and the median grainsize (d50):

    (Equation 3)

    Mathematical equation

References