ASTM D6642-01(2006)

The determination of the soil-moisture flux is one of the fundamental needs in the soil physics and hydrology disciplines. The need arises from requirements for defining recharge rates to groundwater for water supply predictions, for contaminant transport estimates, for performance/risk assessment studies, and for infiltration testing purposes. The techniques outlined in this guide provide a number of alternatives for quantifying soil-moisture flux and/or the recharge rate for various purposes and conditions. This guide is not intended to be a comprehensive guide to techniques available for quantifying soil-moisture flux, but rather a “state-of-the-practice” summary. Likewise, this guide is not intended to be used as a comprehensive guide to performance of these methods, those detailed methods may come at a later time. Techniques that might be useful for the implementation of these methods, for example, sampling network design, are not part of this guide, but may come at a later time.

All of the techniques discussed in this guide have merit when it comes to quantification of the soil-moisture flux. Factors influencing the choice of methods include: need/objectives; cost; time scale of test; and defensibility/reproducibility/reduction in uncertainty. If the need for soil-moisture flux information is crucial in the decision making process for a give site or study, the application of multiple techniques is recommended. Most of the techniques identified above have independent assumptions associated with their use/application. Therefore, the application of two or more techniques at a given site may help to bound the results, or corroborate data distributions. The uncertainties involved in these analyses are sometimes quite large, and therefore the prospect of acquiring independent data sets is quite attractive.

As stated above, each of these techniques for quantification of soil-moisture flux has assumptions and limitations associated with it. The user is cautioned to be cognizant of those limitations/assumptions in applying these techniques at a given site so as not to violate any conditions and thereby invalidate the data.

In general, the tracer techniques for quantifying soil-moisture flux will have less uncertainty associated with them than do the soil-physics based modeling approaches because they are based on direct measures of transport phenomena, rather than indirect measures of soil characteristic data/parameters. However, the forward problem of predicting future soil-water movement rates or transient behavior is best served by the modeling applications. The tracer methods may be used to calibrate, or supply boundary condition data to, the modeling techniques.

Published reviews of these methods are also available in the literature (1, 2, 3).

1.1 This guide describes techniques that may be used to quantify the soil-water (or soil-moisture) flux, the soil-water movement rate, and/or the recharge rate within the vadose zone. This guide is not intended to be all-inclusive with regard to available methods. However, the techniques described do represent the most widely used methods currently available.

1.2 This guide was written to detail the techniques available for quantifying soil-moisture flux in the vadose zone. These data are commonly required in studies of contaminant movement and in estimating the amount of water replenishing a renewable groundwater resource, that is, an aquifer. State and federal regulatory guidelines typically require this information in defining contaminant travel times, in performance assessment, and in risk assessment. Both unsaturated and saturated flow modelers benefit from these data in establishing boundary conditions and for use in calibrations of their computer simulations.

1.3 This guide is one of a series of standards on vadose zone characterization methods. Other standards have been prepared on vadose zone characterization techniques.

1.4 This 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.

1.5 This 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 requirements prior to use.