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 site-specific values for the hydraulic properties of aquifers and aquitards. Under certain circumstances, however, site-specific hydraulic property data may not be available when needed. For example, reconnaissance studies or scoping calculations may require hydraulic property values before on-site investigations are performed.
The following sections present representative hydraulic property values reported in the literature for horizontal and vertical hydraulic conductivity, storativity, specific yield and porosity. Refer to these values if site-specific data are unavailable for your study or to check the results of field and laboratory tests conducted at an investigation site.
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:
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.
Transmissivity is the rate of flow under a unit hydraulic gradient through a unit width of aquifer of given saturated thickness. The transmissivity of an aquifer is related to its hydraulic conductivity as follows:
The following tables show representative values of hydraulic conductivity for various unconsolidated sedimentary materials, sedimentary rocks and crystalline rocks (from Domenico and Schwartz 1990):
|Unconsolidated Sedimentary Materials|
|Gravel||3×10-4 to 3×10-2|
|Coarse sand||9×10-7 to 6×10-3|
|Medium sand||9×10-7 to 5×10-4|
|Fine sand||2×10-7 to 2×10-4|
|Silt, loess||1×10-9 to 2×10-5|
|Till||1×10-12 to 2×10-6|
|Clay||1×10-11 to 4.7×10-9|
|Unweathered marine clay||8×10-13 to 2×10-9|
|Rock Type||Hydraulic Conductivity
|Karst and reef limestone||1×10-6 to 2×10-2|
|Limestone, dolomite||1×10-9 to 6×10-6|
|Sandstone||3×10-10 to 6×10-6|
|Siltstone||1×10-11 to 1.4×10-8|
|Salt||1×10-12 to 1×10-10|
|Anhydrite||4×10-13 to 2×10-8|
|Shale||1×10-13 to 2×10-9|
|Permeable basalt||4×10-7 to 2×10-2|
|Fractured igneous and metamorphic rock||8×10-9 to 3×10-4|
|Weathered granite||3.3×10-6 to 5.2×10-5|
|Weathered gabbro||5.5×10-7 to 3.8×10-6|
|Basalt||2×10-11 to 4.2×10-7|
|Unfractured igneous and metamorphic rock||3×10-14 to 2×10-10|
|To Convert||Multiply By||To Obtain|
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 where is vertical hydraulic conductivity [L/T] and is radial (horizontal) hydraulic conductivity [L/T]. Anisotropy in a horizontal plane is given by where and are horizontal hydraulic conductivities in the and directions, respectively [L/T].
Todd (1980) reports values of 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):
|Material||Horizontal Hydraulic Conductivity
|Vertical Hydraulic Conductivity
|Anhydrite||10-14 to 10-12||10-15 to 10-13|
|Chalk||10-10 to 10-8||5×10-11 to 5×10-9|
|10-9 to 10-7||5×10-10 to 5×10-8|
|Sandstone||5×10-13 to 10-10||2.5×10-13 to 5×10-11|
|Shale||10-14 to 10-12||10-15 to 10-13|
The storativity of a confined aquifer (or aquitard) is defined as the volume of water released from storage per unit surface area of the aquifer or aquitard per unit decline in hydraulic head. Storativity is also known by the terms coefficient of storage and storage coefficient.
Pumping a well in a confined aquifer releases water from aquifer storage by two mechanisms: compression of the aquifer and expansion of water.
In a confined aquifer (or aquitard), storativity is defined as
The typical storativity of a confined aquifer, which varies with specific storage and aquifer thickness, ranges from 5×10-5 to 5×10-3 (Todd 1980).
Specific storage is the volume of water that a unit volume of aquifer (or aquitard) releases from storage under a unit decline in head. Specific storage is related to the compressibilities of water and the aquifer (or aquitard) as follows:
where is mass density of water [M/L3], is gravitational acceleration (= 9.8 m/sec2) [L/T2], is aquifer (or aquitard) compressibility [T2L/M], is effective porosity [dimensionless], and is compressibility of water (= 4.4×10-10 m sec2/kg or Pa-1) [T2L/M].
The storativity of an unconfined aquifer includes its specific yield or drainable porosity:
where is specific yield [dimensionless].
Lowering of the water table in an unconfined aquifer leads to the release of water stored in interstitial openings by gravity drainage.
Compared to gravity drainage, aquifer compression and water expansion in a water-table aquifer yield relatively little water from storage; hence, and in unconfined aquifers.
Storativity in unconfined aquifers typically ranges from 0.1 to 0.3 (Lohman 1972).
|Plastic clay||7.8×10-4 to 6.2×10-3|
|Stiff clay||3.9×10-4 to 7.8×10-4|
|Medium hard clay||2.8×10-4 to 3.9×10-4|
|Loose sand||1.5×10-4 to 3.1×10-4|
|Dense sand||3.9×10-5 to 6.2×10-5|
|Dense sandy gravel||1.5×10-5 to 3.1×10-5|
|Rock, fissured||1×10-6 to 2.1×10-5|
|Rock, sound||< 1×10-6|
|To Convert||Divide By||To Obtain|
Freeze and Cherry (1979) provided the following compressibility values for various aquifer materials:
|Material||Compressibility, α (m2/N or Pa-1)|
|Clay||10-8 to 10-6|
|Sand||10-9 to 10-7|
|Gravel||10-10 to 10-8|
|Jointed rock||10-10 to 10-8|
|Sound rock||10-11 to 10-9|
Pa-1 = m2/N = m sec2/kg
- Use compressibility data to estimate the storativity of a 35-ft thick confined sand aquifer (assume = 1000 kg/m3 and = 0.3).
= (1000 kg/m3)(9.8 m/sec2) [10-8 m2/N + (0.3) (4.4×10-10 m2/N)](35 ft)(0.3048 m/ft) = 1.1×10-3
How much does the expansion of water contribute to the total storativity in this example?
= (1000 kg/m3)(9.8 m/sec2)(0.3) (4.4×10-10 m2/N)(35 ft)(0.3048 m/ft) = 1.4×10-5
- Use specific storage data to estimate storativity for the same confined sand aquifer given in the preceding example.
= (5×10-5 ft-1)(35 ft) = 1.8×10-3
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.
where is total porosity [dimensionless], is specific yield [dimensionless] and 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):
|Material||Specific Yield (%)|
|Sandstone, fine grained||21|
|Sandstone, medium grained||27|
|Till, predominantly silt||6|
|Till, predominantly sand||16|
|Till, predominantly gravel||16|
Porosity is defined as the void space of a rock or unconsolidated material:
where is porosity [dimensionless], is void volume [L3] and 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):
|Unconsolidated Sedimentary Materials|
|Gravel, coarse||24 - 37|
|Gravel, medium||24 - 44|
|Gravel, fine||25 - 39|
|Sand, coarse||31 - 46|
|Sand, medium||29 - 49|
|Sand, fine||26 - 53|
|Silt||34 - 61|
|Clay||34 - 57|
|Rock Type||Porosity (%)|
|Sandstone||14 - 49|
|Siltstone||21 - 41|
|Claystone||41 - 45|
|Shale||1 - 10|
|Limestone||7 - 56|
|Dolomite||19 - 33|
|Rock Type||Porosity (%)|
|Basalt||3 - 35|
|Weathered granite||34 - 57|
|Weathered gabbro||42 - 45|