Significance and Use
4.1 General—In the past ten (plus) years, the Interstate Technology Regulatory Council (ITRC) has provided several technical and regulatory documents on the use of passive groundwater sampling methods (1, 4-6). Collectively, these documents have provided information and references on the technical basis for their use, comparison of sampling results with more traditional sampling methods, descriptions of their proper use, limitations, and a survey of their acceptance and use by responding state regulators.
4.1.1 Because of the large number of passive samplers that have been developed over the past fifteen years for various types of environmental sampling, it is beyond the scope of this standard to discuss separately each of the methods that could or can be used to sample groundwater. Extensive literature reviews on diffusion- and accumulation-passive samplers can be found in the scientific literature (that is, 3, 7-13). These reviews provide information on a wide variety of passive sampling devices for use in air, soil vapor, and water. A review paper on the use of diffusion and accumulation-type passive samplers specifically for sampling volatile organic compounds (VOCs) in groundwater (14) includes information on other passive samplers that are not included in the ITRC documents (1, 6) and discusses their use with respect to measuring mass flux.
4.2 Use—Passive samplers are deployed at a pre-determined depth, or depths, within a well for a pre-determined period of time and should remain submerged for their entire deployment time. All of the passive technologies described in this document rely on the sampling device being exposed to the groundwater during deployment and the continuous flushing of the open or screened interval of the well by ambient groundwater flow (15) to produce water quality conditions in the well bore that effectively mimic those conditions in the aquifer adjacent to the screen or open interval. For samplers that require the establishment of equilibrium, it is important that the equilibration period be long enough to allow the well to recover from any disturbance caused by placing the sampler in the well and to prevent, or reduce, losses of analytes from the water sample by sampler materials due to sorption. For kinetic accumulation samplers (used as kinetic samplers), it is important that the deployment time is long enough that quantitative uptake can occur but not so long that uptake is no longer in the linear portion of the uptake curve (that is, has become curvilinear).
4.2.1 As with all types of groundwater sampling methods, the appropriate use of passive methods assumes that the well has been properly located (laterally and vertically), designed, constructed, and was adequately developed (as described in Guide D5521) and maintained (as described in Practices D5092 and D6725, or Guide D6724). These measures are necessary so that the well is in hydraulic communication with the aquifer.
4.2.2 Each type of passive sampler has its own attributes and limitations, and thus data-quality objectives (DQOs) for the site should be reviewed prior to selecting a device. For wells in low-permeability formations, diffusive flux may become more important than advective flow in maintaining aquifer-quality water in the well.
4.3 Advantages—While passive methods are not expected to replace conventional pumped sampling in all situations, they often offer an easier-to-use (requiring only minimal operator training), alternative “tool” for sampling groundwater monitoring wells when their use meets DQOs and regulatory requirements. Other advantages include that these samplers can be used in most wells and typically have no depth limitation. These samplers are either disposable or dedicated to a well, and this eliminates or reduces the need for decontamination. Passive samplers typically reduce the logistics associated with sampling and are especially useful at sites where it is difficult to bring larger equipment (such as pumps and compressors) onto the site.
4.3.1 Passive groundwater sampling techniques typically provide a much lower “per-sample” cost than conventional pumped sampling methods (16-25). This is primarily because the labor associated with collecting a sample is substantially reduced.
4.3.2 If there is interest in identifying contaminant stratification within the well, multiple passive samplers can be used to characterize vertical contaminant distribution with depth. Baffles or packers can be used to segregate the sampling zones and often provide better characterization of each zone. (In cases where turbidity is a concern, it is important to deploy the sampling devices as gently as possible.) Profiling contamination with depth in a well can be especially useful when trying to decide where to place a passive sampler within the well screen; placing a sampler at the mid-point of the screen may not yield a sample with the highest contaminant concentrations or one that agrees best with previous low-flow concentrations (for example, 25).
4.4 Disadvantages—As with any groundwater sampling method, rapid or rigorous deployment of the sampler(s) (or pumps in the case of active sampling methods) can increase turbidity in the well. For passive groundwater samplers, this can be reduced or eliminated if the equilibration time is long enough to allow the return of the natural ambient turbidity in the well. In many cases, passive samplers are deployed at the end of a sampling event and left in the well until the next scheduled sampling event; this practice provides more than enough time for equilibration to occur.
4.4.1 It is also possible that some wells where only passive sampling methods are used may require more frequent maintenance than wells that are routinely pumped. However in other instances, sampling methods that utilize pumping can bring fines into the well and the well may need more maintenance than if a passive method is used.
4.5 Limitations—There are three primary limitations with passive samplers: analyte capability, sample volume, and size (that is, with respect to well diameter). For the diffusion and accumulation samplers, the membrane and sorbent (for accumulation samplers) determine the specificity of the sampler. Although, two or more individual types of samplers can be used simultaneously to sample for a broader spectrum of analyte types. In contrast, passive-grab samplers collect whole water samples and can be used for most analytes.
4.5.1 With respect to volume limitations, the passive-grab and passive-diffusion samplers collect a finite sample volume. This volume may not be sufficient when there are several types of analytes to be analyzed or when several water-quality measurements (such as dissolved oxygen (DO), pH, etc.) need to be conducted (that is, surface measurements versus in-situ measurements). Additional samplers or larger volume samplers may be available and can be used to meet the volume requirements. Alternatively, because laboratories typically use only a small portion of the sample collected, it may be possible to provide the laboratory with a smaller sample volume. Table X1.1 provides suggested minimum volumes for several analyte classes. However prior to sampling, the total volume of sample needed to run all of the chemical analyses should be confirmed (for each sampling point) with the laboratory.
4.5.2 Finally, the diameter of the sampler or combination of samplers must be able to fit in the well or multi-level sampler.
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 Practice D3740 are generally considered capable of competent and objective testing. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.
Scope
1.1 This standard provides guidance and information on passive sampling techniques for collecting groundwater from monitoring wells. Passive groundwater samplers are able to acquire a sample at a discrete depth or interval in a well, without the active transport associated with a pump or purge technique (1).2 Passive groundwater sampling is a type of no-purge groundwater sampling method where the samplers are left in the well for a predetermined period of time prior to collecting the sample.
1.2 Methods for sampling monitoring wells include low-flow purging and sampling methods, traditional well-volume purging and sampling methods, post-purge grab sampling methods (for example, using a bailer), passive no-purge sampling methods, and active no-purge sampling methods such as using a bailer to collect a sample without purging the well. This guide focuses on passive no-purge sampling methodologies for collecting groundwater samples. These methodologies include the use of diffusion samplers, accumulation samplers, and passive-grab samplers. This guide provides information on the use, advantages, disadvantages, and limitations of each of these passive sampling technologies.
1.3 ASTM Standard D653 provides standard terminology relevant to soil, rock, and fluids contained in them. ASTM Standard D4448 provides a standard guide to sampling groundwater wells, and ASTM Standards D5903 and D6089 provide guides for planning and documenting a sampling event. Groundwater samples may require preservation (Guide D6517), filtration (Guide D6564), and measures to pack and ship samples (Guide D6911). Standard D7069 provides guidance on the quality control and quality assurance of sampling events. ASTM Standard D5092 provides standard practice for the design and installation of groundwater monitoring wells, ASTM Standard D5521 provides a standard guide for developing groundwater monitoring wells in granular aquifers, and D6452 provides a standard guide for purging methods used in groundwater quality investigations. Consult ASTM Standard D6724 for a guide on the installation of direct-push groundwater monitoring wells and ASTM Standard D6725 for a guide on the installation of direct-push groundwater monitoring wells with pre-pack screens.
1.4 The values stated in SI Units are to be regarded as the standard. Values in inches (such as with well diameters) are given in parentheses, and are provided for information. Use of units other than SI shall not be regarded as nonconforming with this standard.
1.5 This guide provides information on passive groundwater sampling in general and also provides a series of considerations when selecting a passive groundwater sampling method. However, it does not recommend a specific course of action, and not all aspects of this guide may be applicable in all field situations. This document cannot replace education or experience and should be used in conjunction with professional judgment. 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.
1.6 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 limitations prior to use.
