Representative Values of Hydraulic Properties

by Glenn M. Duffield, President, HydroSOLVE, Inc.

Aquifer tests (pumping tests, slug tests and constant-head tests) are performed to estimate the hydraulic properties of aquifers and aquitards including horizontal and vertical hydraulic conductivity, storativity, specific yield and porosity. The following sections present representative hydraulic property values reported in the literature.

hydraulic conductivity | anisotropy | storativity | specific yield | porosity

Hydraulic Conductivity (K)

Hydraulic conductivity is a measure of a material's capacity to transmit water. It is defined as a constant of proportionality relating the specific discharge of a porous medium under a unit hydraulic gradient in Darcy's law:

ν = -Ki

where ν is specific discharge [L/T], K is hydraulic conductivity [L/T] and i is hydraulic gradient [dimensionless]. Coefficient of permeability is another term for hydraulic conductivity.

Note that hydraulic conductivity, which is a function of water viscosity and density, is in a strict sense a function of water temperature; however, given the small range of temperature variation encountered in most groundwater systems, the temperature dependence of hydraulic conductivity is often neglected.

The transmissivity of an aquifer is related to hydraulic conductivity as follows:

T = Kb

where T is transmissivity [L²/T] and b is aquifer thickness [L].

The following tables show representative values of hydraulic conductivity for various unconsolidated sedimentary materials, sedimentary rocks and crystalline rocks (from Domenico and Schwartz 1990):

Hydraulic Conductivity Anisotropy Ratio (Kz/Kr)

An anisotropy ratio relates hydraulic conductivities in different directions. For example, vertical-to-horizontal hydraulic conductivity anisotropy ratio is given by K_z/K_r where K_z is vertical hydraulic conductivity and K_r is radial (horizontal) hydraulic conductivity. Anisotropy in a horizontal plane is given by K_y/K_x where K_x and K_y are horizontal hydraulic conductivities in the x and y directions, respectively.

Todd (1980) reports values of K_z/K_r ranging between 0.1 and 0.5 for alluvium and possibly as low as 0.01 when clay layers are present.

The following table shows representative values of horizontal and vertical hydraulic conductivities for selected rock types (from Domenico and Schwartz 1990):

Storativity (S)

The storativity of a confined aquifer (or aquitard) is defined as the volume of water released from storage per unit surface area of a confined aquifer (or aquitard) per unit decline in hydraulic head. Storativity is also known by the terms coefficient of storage and storage coefficient.

In a confined aquifer (or aquitard), storativity is defined as

S = S_sb

where S is storativity [dimensionless], S_s is specific storage [L^-1] and b is aquifer (or aquitard) thickness [L]. Specific storage is the volume of water that a unit volume of aquifer (or aquitard) releases from storage under a unit decline in head by the expansion of water and compression of the soil or rock skeleton.

Specific storage is related to the compressibilities of the aquifer (or aquitard) and water as follows:

S_s = ρg(α + n_eβ)

where ρ is mass density of water [M/L³], g is gravitational acceleration (= 9.8 m/sec²) [L/T²], α is aquifer (or aquitard) compressibility [T²L/M], n_e is effective porosity [dimensionless], and β is compressibility of water (= 4.4x10^-10 m sec²/kg or Pa^-1) [T²L/M].

In an unconfined aquifer (or aquitard), storativity is given by

S = S_y + S_sb

where S_y is specific yield. Because S_sb is typically small in comparison to S_y, storativity in an unconfined aquifer is often simply equated with specific yield.

The storativity of a confined aquifer, which varies with specific storage and aquifer thickness, typically ranges from 5x10^-5 to 5x10^-3 (Todd 1980); in unconfined aquifers, storativity ranges from 0.1 to 0.3 (Lohman 1972).

The following table provides representative values of specific storage for various geologic materials (Domenico and Mifflin 1965 as reported in Batu 1998):

Freeze and Cherry (1979) provided the following compressibility values for various aquifer materials:

Pa^-1 = m²/N = m sec²/kg

Specific Yield (Sy)

Specific yield is defined as the volume of water released from storage by an unconfined aquifer per unit surface area of aquifer per unit decline of the water table.

Bear (1979) relates specific yield to total porosity as follows:

n = S_y + S_r

where n is total porosity [dimensionless], S_y is specific yield [dimensionless] and S_r is specific retention [dimensionless], the amount of water retained by capillary forces during gravity drainage of an unconfined aquifer. Thus, specific yield, which is sometimes called effective porosity, is less than the total porosity of an unconfined aquifer (Bear 1979).

Heath (1983) reports the following values (in percent by volume) for porosity, specific yield and specific retention:

The following table shows representative values of specific yield for various geologic materials (from Morris and Johnson 1967):

Porosity (n)

Porosity is defined as the void space of a rock or unconsolidated material:

n = V_v/V_T

where n is porosity [dimensionless], V_v is void volume [L³] and V_T is total volume [L³].

The following tables show representative porosity values for various unconsolidated sedimentary materials, sedimentary rocks and crystalline rocks (from Morris and Johnson 1967):

See also: Argonne National Laboratory

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