1、Designation: D3404 91 (Reapproved 2013)D3404 15Standard Guide forMeasuring Matric Potential in Vadose Zone UsingTensiometers1This standard is issued under the fixed designation D3404; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision,
2、 the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide covers the measurement of matric potential in the vadose zone using tensiometers. The theoreti
3、cal and practicalconsiderations pertaining to successful onsite use of commercial and fabricated tensiometers are described. Measurement theoryand onsite objectives are used to develop guidelines for tensiometer selection, installation, and operation.1.2 UnitsThe values stated in SI units are to be
4、regarded as the standard. The inch-pound units given in parentheses are forinformation only.1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety and health pract
5、ices and determine the applicability of regulatorylimitations prior to use. The use of a mercury manometer has inherent safety concerns regarding the handling of and potentialexposure to mercury. Mercury metal vapor poisoning has long been recognized as a hazard. When using equipment containing orre
6、quiring the use of mercury, take all precautions and care to avoid the escape of mercury vapor or the spillage of mercury.Maximum limits for mercury concentrations in industrial atmospheres are set by governmental agencies. These limits are usuallybased upon recommendations made by the American Conf
7、erence of Governmental Industrial Hygienists*. It is possible for theconcentration of mercury vapors accompanying spills from broken thermometers, barometers, and other instruments usingmercury to exceed these limits. Mercury, being a heavy liquid with high surface tension, readily disperses into sm
8、all droplets afterspills, lodging in cracks and crevices. Resultant increased surface area of the mercury due to this dispersion promotes highermercury concentrations in the surrounding air. Mercury vapor concentrations are readily measured using commercially availableinstrumentation. To monitor env
9、ironmental hazards it is advisable to make periodic checks for mercury content at locations wheremercury is exposed to the atmosphere. Use a spill kit for clean-up whenever spillage occurs. After spills and clean-up, makethorough checks for mercury vapor concentrations in the atmosphere. *In 1993, t
10、his Conference had headquarters located inBuilding D-7 at 6500 Glenway Drive, Cincinnati, Ohio 45211.1.4 This guide offers an organized collection of information or a series of options and does not recommend a specific courseof action. This document cannot replace education or experience and should
11、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 replacethe standard of care by which the adequacy of a given professional service must be judged, nor should this document be app
12、liedwithout consideration of a projects many unique aspects. The word“ Standard” in the title of this document means only that thedocument has been approved through the ASTM consensus process.2. Referenced Documents2.1 ASTM Standards:2D653 Terminology Relating to Soil, Rock, and Contained Fluids3. T
13、erminology3.1 For definitions of common technical terms in this standard, refer to Terminology D653.3.2 Definitions of Terms Specific to This Standard:3.2.1 accuracy of measurementthe difference between the value of the measurement and the true value.1 This guide is under the jurisdiction of ASTM Co
14、mmittee D18 on Soil and Rock and is the direct responsibility of Subcommittee D18.21 on Groundwater and VadoseZone Investigations.Current edition approved June 15, 2013April 15, 2015. Published June 2013May 2015. Originally approved in 1991. Last previous edition approved in 20042013 asD3404 91 (200
15、4).(2013). DOI: 10.1520/D3404-91R13.10.1520/D3404-15.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.This doc
16、ument is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as
17、 appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.2.2 hysteresisthat part of inaccuracy attributable to
18、the tendency of a measurement device to lag in its response toenvironmental changes. Parameterschanges; parameters affecting pressure-sensor hysteresis are temperature and measuredpressure.3.2.3 precision (repeatability)the variability among numerous measurements of the same quantity.3.2.4 resolutio
19、nthe smallest division of the scale used for a measurement, and it is a factor in determining precision andaccuracy.4. Summary of Guide4.1 The measurement of matric potential in the vadose zone can be accomplished using tensiometers that create a saturatedhydraulic link between the soil water and a
20、pressure sensor. A variety of commercial and fabricated tensiometers are commonlyused. A saturated porous ceramic material that forms an interface between the soil water and bulk water inside the instrument isavailable in many shapes, sizes, and pore diameters. A gage,gauge, manometer, or electronic
21、 pressure transducer is connected tothe porous material with small- or large-diameter tubing. Selection of these components allows the user to optimize one or morecharacteristics, such as accuracy, versatility, response time, durability, maintenance, extent of data collection, and cost.5. Significan
22、ce and Use5.1 Movement of water in the unsaturated zone is of considerable interest in studies of hazardous-waste sites (1, 2, 3, 4)3;recharge studies (5, 6); irrigation management (7, 8, 9); and civil-engineering projects (10, 11). Matric-potential data alone can beused to determine direction of fl
23、ow (11) and, in some cases, quantity of water flux can be determined using multiple tensiometerinstallations. In theory, this technique can be applied to almost any unsaturated-flow situation whether it is recharge, discharge,lateral flow, or combinations of these situations.5.2 If the moisture-char
24、acteristic curve is known for a soil, matric-potential data can be used to determine the approximate watercontent of the soil (10). The standard tensiometer is used to measure matric potential between the values of 0 and -867 cm of water;this range includes most values of saturation for many soils (
25、12).5.3 Tensiometers directly and effectively measure soil-water tension, but they require care and attention to detail. In particular,installation needs to establish a continuous hydraulic connection between the porous material and soil, and minimal disturbanceof the natural infiltration pattern ar
26、e necessary for successful installation. Avoidance of errors caused by air invasion,nonequilibrium of the instrument, or pressure-sensor inaccuracy will produce reliable values of matric potential.5.4 Special tensiometer designs have extended the normal capabilities of tensiometers, allowing measure
27、ment in cold or remoteareas, measurement of matric potential as low as -153 m of water (-15 bars), measurement at depths as deep as 6 m (recorded atland surface), and surface) for conventional tensiometers, depths up to 200 m and greater with advanced and portable versions(13,14), and automatic meas
28、urement using as many as 22 tensiometers connected to a single pressure transducer, but these requirea substantial investment of effort and money.5.5 Pressure sensors commonly used in tensiometers include vacuum gages,gauges, mercury manometers, and pressuretransducers. Only tensiometers equipped wi
29、th pressure transducers allow for the automated collection of large quantities of data.However, the user needs to be aware of the pressure-transducer specifications, particularly temperature sensitivity and long-termdrift. Onsite measurement of known zero and “full-scale” readings probably is the be
30、st calibration procedure; however, onsitetemperature measurement or periodic recalibration in the laboratory may be sufficient.6. Measurement Theory6.1 In the absence of osmotic effects, unsaturated flow obeys the same laws that govern saturated flow: Darcys Law and theEquation of Continuity, that w
31、ere combined as the Richards Equation (1315). Baver et al. (1416) presents Darcys Law forunsaturated flow as follows:q 52Kpi1Z! (1)where:q = the specific flow, FLTG,K =the unsaturated hydraulic conductivity, FLTG, = the matric potential of the soil water at a point, L,Z = the elevation at the same p
32、oint, relative to some datum, L, andpi = the gradient operator, L1.The sum of + Z commonly is referred to as the hydraulic head.3 The boldface numbers in parentheses refer to a list of references at the end of this standard.D3404 1526.2 Unsaturated hydraulic conductivity, K, can be expressed as a fu
33、nction of either matric potential, , or water content, L3of water/L3 of soil, although both functions are affected by hysteresis (5). If the wetting and drying limbs of the K() functionare known for a soil, time series of onsite matric-potential profiles can be used to determine which limb is more a
34、ppropriate todescribe the onsite K(), the corresponding values of the hydraulic-head gradient, and an estimate of flux using Darcys Law. If,instead, K is known as a function of , onsite moisture-content profiles (obtained, for example, from neutron-scattering methods)can be used to estimated K, and
35、combined with matric-potential data to estimate flux. In either case, the accuracy of the fluxestimate needs to be assessed carefully. For many porous media, dKd and dKd are large, within certain ranges of or , makingestimates of K particularly sensitive to onsite-measurement errors of or . (Onsite-
36、measurement errors of also have direct effecton pi( + Z) in Darcys Law). Other sources of error in flux estimates can result from inaccurate data used to establish the K()or K() functions (accurate measurement of very small permeability values is particularly difficult) (1517); use of an analyticale
37、xpression for K() or K() that facilitates computer simulation, but only approximates the measured data; an insufficient densityof onsite measurements to define adequately the or profile, which can be markedly nonlinear; onsite soil parameters that aredifferent from those used to establish K() or K()
38、; and invalid assumptions about the state of onsite hysteresis. Despite thepossibility of large errors, certain flow situations occur where these errors are minimized and fairly accurate estimates of flux canbe obtained (6, 1618) . The method has a sound theoretical basis and refinement of the theor
39、y to match measured data markedlywould improve reliability of the estimates.6.3 The concept of fluid tension refers to the difference between standard atmospheric pressure and the absolute fluid pressure.Values of tension and pressure are related as follows:TF 5PAT2PF (2)where:TF = the tension of an
40、 elemental volume of fluid, F MLT2G,PAT = the absolute pressure of the standard atmosphere, F MLT2G, andPF = the absolute pressure of the same elemental volume or fluid F MLT2G.Soil-water tension (or soil-moisture tension) similarly is equal to the difference between soil-gas pressure and soil-water
41、pressure. Thus:TW1PG 5PW (3)where:TW = the tension of an elemental volume of soil water,F MLT2G,PG = the absolute pressure of the surrounding soil gas,F MLT2G, andPW = the absolute pressure of the same elemental volume of soil water, F MLT2G.In this guide, for simplicity, soil-gas pressure is assume
42、d to be equal to 1 atm, except as noted. Various units are used to expresstension or pressure of soil water, and are related to each other by the equation:1.000 bar5100.0 kPa50.9869 atm5 (4)1020 cm of water at 4C51020gpercm 2 in astandardgravitational field.A standard gravitational field is assumed
43、in this guide; thus, centimetres of water at 4C are used interchangeably with gramsper square centimetre.D3404 1536.4 The negative of soil-water tension is known formally as matric potential. The matric potential of water in an unsaturatedsoil arises from the attraction of the soil-particle surfaces
44、 for water molecules (adhesion), the attraction of water molecules for eachother (cohesion), and the unbalanced forces across the air-water interface. The unbalanced forces result in the concave water filmstypically found in the interstices between soil particles. Baver et al. (1416) present a thoro
45、ugh discussion of matric potential andthe forces involved.6.5 The tensiometer, formally named by Richards and Gardner (1719), has undergone many modifications for use in specificproblems (1, 11, 18-20-3032). However, the basic components have remained unchanged. A tensiometer comprises a poroussurfa
46、ce (usually a ceramic cup) connected to a pressure sensor by a water-filled conduit. The porous cup, buried in a soil, transmitsthe soil-water pressure to a manometer, a vacuum gage,gauge, or an electronic-pressure transducer (referred to in this guide as apressure transducer). During normal operati
47、on, the saturated pores of the cup prevent bulk movement of soil gas into the cup.6.6 An expanded cross-sectional view of the interface between a porous cup and soil is shown in Fig. 1. Water held by the soilparticles is under tension; absolute pressure of the soil water, PW, is less than atmospheri
48、c. This pressure is transmitted throughthe saturated pores of the cup to the water inside the cup. Conventional fluid statics relates the pressure in the cup to the readingobtained at the manometer, vacuum gage,gauge, or pressure transducer.6.6.1 In the case of a mercury manometer (see Fig. 2(a):TW
49、5PA 2PW 5Hg2H2O!r 2H2Oh1d! (5)where:TW = the soil-water tension relative to atmospheric pressure, in centimetres of water at 4C,PA = the atmospheric pressure, in centimetres of water at 4C,PW = the average pressure in the porous cup and soil, in centimetres of water at 4C,Hg = the average density of the mercury column, in grams per cubic centimetre,H2O = the average density of the water column, in grams per cubic centimetre,water = the average density of the water column, in grams per cubic centimetre,r = the readi
