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 WRD DISTRIBUTION: A, B, S, FO, PO