Standard Guide for Selection of Passive Techniques for Sampling Groundwater Monitoring Wells

Importancia y uso:

4.1 General—Passive groundwater sampling has increased use since the polyethylene diffusion bag sampler was first introduced (5). As defined above, different types of passive samplers are now available with different functions and usages. The Interstate Technology Regulatory Council (ITRC) has provided several technical and regulatory documents on the use of passive groundwater sampling methods (1, 5-7). 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. However, the ITRC documents are older and do not include more recent assessments and publications. This Standard seeks to provide newer information on current practice and implementation of passive groundwater sampling techniques.

4.1.1 Because of the large number of passive samplers that have been developed over the 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, 8-14). 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 (15) includes information on other passive samplers that are not included in the ITRC documents (1, 7) 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 minimum or pre-determined period of time. They should remain submerged at the target depth for their entire deployment period. 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 ((4), (5-7), 16) 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/D5521M) and maintained (as described in Practices D5092/D5092M and D6725/D6725M, or Guide D6724/D6724M). 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 a faster alternative “tool” for sampling groundwater monitoring wells because purging is eliminated from the pre-sampling procedure. 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. 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) to the well location.

4.3.1 Passive groundwater sampling techniques typically provide a lower “per-sample” cost than conventional pumped sampling methods (17-26). This is primarily because the labor associated with collecting a sample is substantially reduced and waste handling and disposal is substantially reduced. Eliminating handling and disposal of purge water is an environmental benefit and advantage.

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. Profiling contamination with depth in a well can be informative when trying to decide where to place a single passive sampler within the well screen for long-term monitoring; 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, 26).

4.4 Disadvantages—As with any groundwater sampling method, rapid or rigorous deployment of the sampler(s) 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. Some methods require dedicated equipment purchase which may increase the cost for the initial sampling event in order to obtain the overall cost advantage.

4.5 Limitations—There are three primary limitations with passive samplers: analyte capability, sample volume, and physical size with respect to well diameter. For the diffusion and accumulation samplers, the membrane and or sorbent, respectively, determine the analyte capability of the sampler. In contrast, passive-grab samplers collect whole water samples and can be used for any analyte, subject to sample volume and physical size limitations.

4.5.1 Analyte capability is often unique to individual passive samplers. In the case of diffusion-based passive samplers, the user should verify that the membrane is suitable for the analytes to be tested. ITRC (5-7) describes the analyte capability of diffusion-bases passive samplers. Two or more individual types of passive samplers can be used simultaneously to sample for a broader spectrum of analyte types.

4.5.2 Passive-grab and passive-diffusion samplers collect a finite sample volume. Total sampler volume may limit the number and type of analytes that can be practically collected. Additional samplers or larger volume samplers may be available and can be used to meet the volume requirements. Also, 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. The laboratory should be consulted to confirm adequate sample volume during the method selection process.

4.5.3 Regarding physical sizes of the sampler(s), 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.

Subcomité:

D18.21

Referida por:

D6517-18R23, D6699-24, D7069-24, D6771-21, D6044-21, D6725_D6725M-16R24E01, D6232-21, D6724_D6724M-16R24E01, D5092_D5092M-16R24

Volúmen:

04.09

Número ICS:

13.060.10 (Water of natural resources)

Palabras clave:

accumulation sampler; diffusion sampler; equilibrated-grab sampler; equilibrium sampler; groundwater sampling; integrative sampler; kinetic sampler; passive diffusion sampler; passive-grab sampler; passive groundwater sample; passive groundwater sampling; passive sampler; sorptive sampler; time-integrated passive sampler;