4.3.4.2

Determining Hydraulic Conductivity Using Laboratory Methods

It may be beneficial to use laboratory measurements of hydraulic conductivity to

augment results of field tests; however, field methods provide the best estimate of hydraulic

conductivity in most cases. Because of the limited sample size, laboratory tests can miss

secondary porosity features such as fractures and joints, and hence, can greatly underestimate

overall aquifer hydraulic conductivities. Laboratory tests may provide valuable information

about the vertical component of hydraulic conductivity of aquifer materials. However,

laboratory test results always should be confirmed by field measurements, which sample a

much larger portion of the aquifer. In addition, laboratory test results can be profoundly

affected by the test method selected and by the manner in which the tests are carried out

(e.g., the extent to which sample collection and preparation have changed the in situ hydraulic

properties of the tested material). Special attention should be given to the selection of the

appropriate test method and test conditions, and to quality control of laboratory results.

McWhorter and Sunada (1977), Freeze and Cherry (1979), and Sevee (1991) discuss

determining hydraulic conductivity in the laboratory. Laboratory tests may provide the best

estimates of hydraulic conductivity for materials in the unsaturated zone, but are likely to be

less accurate than field methods for materials in the saturated zone (Cantor et al., 1987).

4.3.4.3

Data Evaluation

For comparisons of hydraulic conductivity measurements, the following criteria should

be used to determine the accuracy or completeness of information:

Use of a single well test will necessitate that more individual tests be

conducted at different locations to define sufficiently the variation in hydraulic

conductivity across the site.

Field hydraulic conductivity measurements generally provide average values for

the entire area across a well screen. Short well screens are necessary to

measure the hydraulic conductivity of discrete stratigraphic intervals. On the

other hand, in situations where well screens only partially penetrate an aquifer,

it is difficult, if not impossible, to correct mathematical equations for the

resultant distortion in flow patterns created during the pump test (Driscoll,

1986; Fetter, 1980). If the average hydraulic conductivity for a formation is

required, entire formations may be screened, or data may be combined from

overlapping clusters.

It is important that measurements define both the vertical and horizontal components

of hydraulic conductivity across a site. Laboratory tests on cores collected during the boring

program may be helpful in ascertaining vertical hydraulic conductivity in saturated strata. In

assessing the accuracy of hydraulic conductivity measurements at a site, results from the

boring program used to characterize the site geology should be considered. Zones of high

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