Characterizing seasonal and temporal variations in ground water flow is important for

site investigations involving aquifer tests. Methods are available for correcting most seasonal

and temporal effects, and they should be considered when designing aquifer tests and

interpreting the results. When determining hydraulic conductivities and other aquifer

parameters using aquifer tests, piezometers/wells should be installed and continuously

monitored during the test outside of the stressed aquifer zone to document and allow

correction for any changes in the potentiometric surface or water table that are not related to

the aquifer test.

4.3.4

Determining Hydraulic Conductivity

Hydraulic conductivity is a measure of a material's ability to transmit water.

Generally, poorly sorted silty or clayey materials have low hydraulic conductivities, whereas

well sorted sands and gravels have high hydraulic conductivities. An aquifer may be

classified as either homogeneous or heterogeneous and either isotropic or anisotropic

according to the way its hydraulic conductivity varies in space. An aquifer is homogeneous if

the hydraulic conductivity is independent of location within the aquifer; it is heterogeneous if

hydraulic conductivities are dependent on location within the aquifer. If the hydraulic

conductivity is independent of the direction of measurement at a point in a geologic

formation, the formation is isotropic at that point. If the hydraulic conductivity varies with

the direction of measurement at a point, the formation is anisotropic at that point.

In heterogeneous aquifers, owners and operators should determine horizontal hydraulic

conductivity as a function of vertical position in the aquifer. Knowledge of the variation in

hydraulic conductivity as a function of vertical position in the subsurface is essential to

understanding the potential migration of contaminants. Molz et al. (1989) explain that the

common practice of averaging hydraulic conductivity over a vertical interval can mislead

investigators about the dispersive properties of an aquifer. Impeller flowmeters, multilevel

slug tests, or tracer tests may be used to determine hydraulic conductivity with vertical

position in an aquifer (Molz et al., 1990; Molz et al., 1989).

Determining values for hydraulic conductivity as a function of direction of

measurement within an anisotropic saturated zone also is important in evaluating ground 

water flow and contaminant migration. Anisotropy within an aquifer is typically the result of

small scale stratification (bedding) of sedimentary deposits and/or fractures (Hsieh and

Neuman, 1985; McWhorter and Sunada, 1977). In bedded deposits, hydraulic conductivity in

the direction parallel to bedding is typically (1) the maximum hydraulic conductivity, and (2)

the same magnitude in all directions within planes parallel to the bedding. The magnitude of

hydraulic conductivity is typically smallest in the direction perpendicular to bedding

(McWhorter and Sunada, 1977). Therefore, for the purpose of understanding ground water

flow and contaminant migration in stratified aquifers, investigators are typically concerned

with determining the ratio of the horizontal component of hydraulic conductivity (Kh) and the

vertical component of hydraulic conductivity (Kv), or Kh:Kv ratio. Way and McKee (1982)

November 1992

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