Standard Guide for Comparison of Field Methods for Determining Hydraulic Conductivity in Vadose Zone

摘要

1.1This guide covers a review of the test methods for determining hydraulic conductivity in unsaturated soils and sediments. Test methods for determining both field-saturated and unsaturated hydraulic conductivity are described.1.2Measurement of hydraulic conductivity in the field is used for estimating the rate of water movement through clay liners to determine if they are a barrier to water flux, for characterizing water movement below waste disposal sites to predict contaminant movement, and to measure infiltration and drainage in soils and sediment for a variety of applications. Test methods are needed for measuring hydraulic conductivity ranging from 1 × 10−2to 1 × 10−8cm/s, for both surface and subsurface layers, and for both field-saturated and unsaturated flow.1.3For these field test methods a distinction is made between “saturated” (Ks) and “field-saturated” (Kfs) hydraulic conductivity. True saturated conditions seldom occur in the vadose zone except where impermeable layers result in the presence of perched water tables. During infiltration events or in the event of a leak from a lined pond, a “field-saturated” condition develops. True saturation does not occur due to entrapped air(1).2The entrapped air prevents water from moving in air-filled pores that, in turn, may reduce the hydraulic conductivity measured in the field by as much as a factor of two compared to conditions when trapped air is not present(2). Field test methods should simulate the “field-saturated” condition.1.4Field test methods commonly used to determine field-saturated hydraulic conductivity include various double-ring infiltrometer test methods, air-entry permeameter test methods, and borehole permeameter tests. Many empirical test methods are used for calculating hydraulic conductivity from data obtained with each test method. A general description of each test method and special characteristics affecting applicability is provided.1.5Field test methods used to determine unsaturated hydraulic conductivity in the field include direct measurement techniques and various estimation methods. Direct measurement techniques for determining unsaturated hydraulic conductivity include the instantaneous profile (IP) test method and the gypsum crust method. Estimation techniques have been developed using borehole permeameter data and using data obtained from desorption curves (a curve relating water content to matric potential).1.6The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.1.7All observed and calculated values shall conform to the guidelines for significant digits and rounding established in PracticeD6026.1.7.1The method used to specify how data are collected, calculated, or recorded in this standard is not directly related to the accuracy to which the data can be applied in design or other uses, or both. How one applies the results obtained using this standard is beyond its scope.1.8This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.1.9This guide offers an organized collection of information or a series of options and does not recommend a specific course of action. This document cannot replace education or experience and should be used in conjunction with professional judgment. Not all aspects of this guide may be applicable in all circumstances. This ASTM standard is not intended to represent or replace the standard of care by which the adequacy of a given professional service must be judged, nor should this document be applied without consideration of a project's many unique aspects. The word “Standard” in the title of this document means only that the document has been approved through the ASTM consensus process. ====== Significance And Use ======5.1Saturated hydraulic conductivity measurements are made for a variety of purposes varying from design of landfills and construction of clay liners to assessment of irrigation systems. Infiltrometers are commonly used where infiltration or percolation rates through a surface or subsurface layer are desired. Evaluation of the rate of water movement through a pond liner is one example of this kind of measurement. Penetration of the liner by a borehole would invalidate the measurement of liner permeability. It has been noted that small-ring infiltrometers are subject to error due to lateral divergence of flow. Therefore, techniques using very large (1 to 2-m diameter) infiltration basins have been recommended for measuring the very slow percolation rates typically needed for clay liners. The air-entry permeameter can be used instead of infiltrometer tests to avoid lateral divergence of flow. However, because a cylinder must be driven into the media tested, the actual soil column tested may be disrupted by introduction of the cylinder, especially in structured soils.5.2Borehole tests for determining saturated hydraulic conductivity are applicable for evaluating the rate of water movement through subsurface layers. For slowly permeable layers, an accurate method of measuring the rate of water movement into the borehole will need to be developed. Use of a flexible bag as a reservoir that can be periodically weighed is advisable for these conditions. A number of mathematical solutions for borehole outflow data are available (Stephens et al.(17), Reynolds et al.(18), and Philip(19)).5.3Information on unsaturated flow rates is needed to design hazardous waste landfills and impoundments where prevention of flow of contaminants into groundwater is needed. Of the test methods available, the primary differences are cost and resultant bias and precision. The instantaneous profile test method appears to provide very reliable data because it uses a large volume of soil (several cubic metres) and is performed on undisturbed soils in the field. However, a single test can cost several thousand dollars. The gypsum crust test method, although more rapid than the instantaneous profile test method, sacrifices precision of results due to the smaller spatial extent of the tested area. Methods for estimating unsaturated hydraulic conductivity from fundamental soil hydraulic functions like the desorption curves may deviate from true values by an order of magnitude, but may be of use where relative differences in permeability between materials or across water content ranges is of interest.Note 1:The quality of the result produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used. Agencies that meet the criteria of PracticeD3740are generally considered capable of competent and objective testing/sampling/inspection/etc. Users of this standard are cautioned in that compliance with PracticeD3740does not in itself assure reliable results. Reliable results depends on many factors; PracticeD3740provides a means of evaluating some of those factors.