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
November 1992
4 51






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