PUBLICATIONS--A new field technique for determining air permeability hydraulic conductivity in the unsaturated zone.



                           UNITED STATES
                    DEPARTMENT OF THE INTERIOR
                         GEOLOGICAL SURVEY
                      RESTON, VIRGINIA  22092

                                                      GW Branch

                                              February 13, 1978
                                                 Code 4351 5016

GROUND WATER BRANCH TECHNICAL MEMORANDUM NO. 78.04

Subject:   PUBLICATIONS--A new field technique for determining air
                         permeability hydraulic conductivity in
                         the unsaturated zone.

A report describing a new method to determine the permeability of
layered material in the unsaturated zone has recently been
approved for publication as U.S. Geological Survey Professional
Paper 1051, "Field determination of vertical permeability to air
in the unsaturated zone", by E. P. Weeks.  Experience to date
indicates that the method should be useful to evaluate the
hydraulic conductivity of potential perching zones occurring at
depth in the unsaturated zone.  Estimates of hydraulic
conductivity are frequently needed for evaluation of artificial-
recharge potential or for evaluation of ground-water pollution
hazards due to surface or near-surface sources of contamination.
Persons involved in such studies should consider application of
the method.

In principle, the method utilizes the fact that movement of air
into or from the unsaturated zone in response to barometric
changes at land surface is slowed by the finite permeability of
the material and by its capacity to store or release air during
the change in pressure.  Instrumentation for the method includes a
piezometer nest in the layered unsaturated zone, with one
piezometer in the bottom layer and additional piezometers at each
layer boundary.  These piezometers are connected through a
manifold to an inclined manometer.  Data, including barometric
pressure recorded by a microbarograph and periodic differential
manometer readings, are obtained during a period in which the
atmospheric pressure is changing, typically the normal afternoon
diurnal barometric decline.  In addition to barometric pressure
and manometer data, data are needed on unsaturated zone thickness,
screen depths, and on the air-filled porosity or specific yield
for each layer comprising the unsaturated zone.  The data are
analyzed by numerical simulation to determine the permeability to
air, and subsequently, the hydraulic conductivity.

The method is relatively quick and inexpensive and has a number of
advantages over existing methods for determining unsaturated zone
properties.  For example, laboratory data from cores frequently
are unreliable because of small sample size and because of
disturbance or compaction of material during the coring operation.
Moreover, the cores are of little use in evaluating fracture
permeability.  Double-ring infiltrometer or auger-hole methods are
useful only to evaluate near-surface materials, sample only a
small amount of material, or measure predominantly the horizontal
component of the permeability tensor.  Large scale ponded
infiltration experiments, on the other hand, are costly and time
consuming.

The main disadvantage of the air-permeability method is that the
intrinsic permeability of the materials may be altered by
structural changes in the medium during wetting, particularly if
swelling clays are present.  Because of these changes, the
permeability of the material to air, as determined by the method,
may differ significantly from its permeability to water.
Experience with the method indicates that this problem is
particularly severe for near-surface materials.  However, the
problem was less severe for near-surface materials.  However, the
problem was less severe than anticipated for layers at depth.
Another disadvantage of the method is that the materials must be
well drained, a situation not always encountered in areas which
have a humid climate.  Also, the method is not sensitive enough to
measure hydraulic conductivities of several meters per day if the
unsaturated zone is no more than 10-20 meters thick.

These advantages and disadvantages should be considered in any
evaluation of the method for potential use.

Prior to publication of the Professional Paper, expected sometime
in the summer, 1978, open file copies of the report may be
obtained by contacting the author:

                                   Mr. Edwin P. Weeks
                                   U.S. Geological Survey
                                   Water Resources Division
                                   P. O. Box 3355
                                   Lubbock, Texas  79410



                                   (s) Gerald Meyer
                                   Chief, Ground Water Branch

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