The Diffusion/Passive Sampler Team of the Interstate Technology & Regulatory Council has published “Protocol for Use of Five Passive Samplers to Sample for a Variety of Contaminants in Groundwater.” This article is comprised of excerpts from that document.
A passive ground water sampler is able to acquire a sample from a discrete position in a well without active media transport induced by pumping or purge techniques. The passive technologies described in this document rely on the sampling device being exposed to media in ambient equilibrium during the designated sampler deployment period. In wells, the water is expected to be in natural exchange with the formation water. The devices provide a sample from a discrete interval within the open borehole or screened interval of a well.
The five passive sampler technologies addressed here fall into three categories on the basis of sampler mechanism and nature of the collected sample:
- Devices that rely on diffusion and sorption to accumulate analytes in the sampler. Samples are a time-integrated representation of conditions at the sampling point over the entire deployment period. The accumulated mass and duration of deployment are used to calculate analyte concentrations in the sampled medium. This describes the GORE Module.
- Devices that recover a grab well water sample. Samples are an instantaneous representation of conditions at the sampling point at the moment of sample collection. These include the HydraSleeve and the Snap Sampler.
- Devices that rely on diffusion of analytes across the sampler membrane to reach and maintain equilibrium with the sampled medium. Samples are time-weighted toward conditions at the sampling point during the latter portion of the deployment period.
The Diffusion/Passive Sampler Team consensus is that these samplers have been validated through laboratory and field testing. When deployed appropriately, they produce reliable and accurate data.
Passive sampler technologies have advantages specific to the nature of each technology. When they are selected appropriately and operated in accordingly, users can realize resource savings and accurate results from most ground water sampling programs.
Passive sampler advantages:
- are relatively easy to use;
- can be deployed in most wells;
- are practical for use where access is difficult or where discretion is desirable;
- can sample discrete intervals in a well;
- can be deployed in series to provide a vertical contaminant profile;
- have no depth limit;
- reduce field-sampling variability, resulting in highly reproducible data;
- allow rapid field sample collection;
- decrease field labor and project management costs for long-term monitoring; and
- eliminate purge-water production and thus all or most disposal cost.
Passive sampler limitations:
- must be submerged in the screened interval during deployment;
- require the aquifer to be in hydraulic communication with the screened portion of the well;
- require special consideration in wells having a layer of free product;
- may have volume/analyte limitations; and
- require consideration of contaminant stratification.
Ground water sampling is performed to collect a sample of formation-quality water from the screened or open portion of a well. Research shows that many – if not most – wells exhibit ambient flow-through under natural ground water gradients. The screened sections of these wells may be considered in equilibrium with the formation water without pumping. Ongoing research suggests that natural ambient flow-through, temperature inversions, and density effects can induce mixing within wells, resulting in a flow-weighted averaging effect in many wells without purging. Though not all wells are thoroughly mixed, many wells show relatively narrow ranges of vertical concentrations when vertically profiled. Deployment of multiple passive samplers within a well may be advised to characterize vertical contaminant distribution. A single passive sampler may be used for long-term monitoring, depending on data quality objectives (DQOs).
Data Quality ObjectivesWhen using passive samplers, the user must consider DQOs, target analytes, and hydrogeologic concerns. Each sampling technique characterizes contamination in the ground water differently. Differences may occur when comparing well volume purge, low flow, or passive sampling techniques. It is important to understand the conceptual basis of any sampling technology because results from the methods may differ. These differences do not necessarily indicate inaccuracies, but reflect the nature of the sampling methods. These differences should be considered when comparing and interpreting sampling results.
It is highly recommended that all parties involved in the implementation of new monitoring programs identify and agree on the site-specific DQOs and data evaluation techniques prior to implementation. As with any sampling technique, site-specific DQOs guide the design of sampling programs, including the selection of sampling devices.
Deployment IssuesIn addition to DQOs, there are certain deployment considerations for passive samplers.
- As with all ground water sampling, adequate information should be available on well
- construction (diameter, screen interval, etc.), water level, type and concentration of
- contaminants, and hydraulic properties of the formation. The sampling device must be suitable for collecting the analytes of interest and required sample volume.
Passive samplers must allow formation water and well water to restabilize after sampler deployment. Additionally, membrane samplers (RPP, dialysis, GORE Module) must be submerged in a well for a prescribed length of time, based on the permeability of the membrane and the constituents of interest.
Hydrological ConsiderationsPassive sampling relies on flow through the well screen to provide formation-quality water from the adjacent aquifer. In interpreting sampling results, it may be important to know whether there is contaminant stratification in the well, and to what extent vertical and horizontal flows within the well affect sample collection.
Studies have shown that, with sufficient aquifer flow conditions, ground water will continually flow through a properly constructed well. Borehole dilution tests can be used to determine whether water is freely exchanged between the aquifer and the well screen. Under these conditions, ground water in the screened interval may be replaced in as little as 24 hours. For water in the well to be formation-quality water from the aquifer, the rate of solute contribution from the aquifer to the well must equal or exceed the rate of in-well contaminant loss, such as through volatilization or convection. This condition may not occur where ground water velocities are very low or the well has a low yield, which is commonly a result of a very low gradient or a very low hydraulic conductivity. It is difficult to collect a formation-quality water sample from low-yield wells due to possible dewatering, aeration and increased turbidity associated with purging. Passive samplers may be a preferred alternative if considerations are made for restabilization (the period of time well water requires to reach its ambient state following physical agitation) and equilibration (the period of time required for well water and/or sampler material to reach chemical equilibrium with the formation water). In limited cases, water in a well screened in an anaerobic aquifer may be affected if oxygenated water at the air-water interface is disturbed.
Vertical flow is common in longer-screened wells and fractured bedrock. If vertical flow is suspected and discrete interval sampling is required by the DQOs of the project, vertical-flow profiling should be conducted. Vertical-flow profiling can be conducted with a borehole flow meter or a short interval packer/pump located in the wellbore to determine the depth of the primary inflow and outflow of ground water from the open interval of a well.
The screened interval of monitoring wells often contains zones of different contaminant concentrations. For instance, stratification of trichloroethene (TCE) has been observed over vertical distances of as little as three feet. A single passive sampler represents a discrete interval within the well; therefore, if stratified contaminant concentrations are migrating through the aquifer above or below the depth where the sampler is positioned, a single passive sampler may not represent the higher concentration intervals. In this case, it is recommended that the well be vertically profiled using multiple passive samplers to describe the vertical variation in contaminant concentration through the screened interval, and to document the most appropriate depth interval for a single passive sampler deployment. As discrete interval samplers, passive samplers depend on a clear understanding of contaminant stratification for proper interpretation of the data. A refinement of knowledge of contaminant stratification can allow refinement of the site conceptual model, and potentially optimize any remediation system.
If contaminant stratification is found or suspected, vertical chemical profiling can be done by suspending multiple samplers – in series – at discrete intervals within the screened water columns or open interval. This approach will locate zone(s) of higher and lower contaminant concentrations in the open interval of a well. It has been recommended that screens or open intervals greater than five feet should be vertically profiled to detect contaminant stratification. However, longer or shorter intervals may be profiled based on site-specific data requirements. Vertical profiling inform tion can be used to select the optimal vertical location for a single sampler deployment. To lower the cost of multiple vertical profile samples, samples can be analyzed with field analytical screening tools or by a certified laboratory for appropriate indicator parameters.
Deployment DepthThe depth at which a passive sampler is deployed should not be arbitrary. The decision must be made based on knowledge of the aquifer, vertical contaminant distribution, well construction, and flow within the well, as well as on historical sampling results. After the user has an adequate understanding of the hydrogeologic environment and contaminant distribution in a given monitoring well, there remains the question of the depth at which a passive sampler should be deployed to collect samples. That decision must be made in accordance with site-specific and even well-specific sampling objectives.
If previous vertical profiling of a known or suspected stratified well has been conducted, a selected single deployment depth may be chosen based on the sampling objective. For example, previous data may conclude that the bottom three feet of a well historically have contained the highest contaminant concentration; deployment at this depth could be selected based on an objective to sample the highest known concentration within stratified wells. Alternatively, if a well is not stratified, a midscreen deployment may be appropriate. When performing ongoing sampling events, it is critical to place the sampler in the same location or depth for sample consistency and data comparability over time. Sampling at a consistent deployment depth in a well with vertical contaminant stratification improves data reproducibility.
As mentioned previously, a passive sampler must be fully submerged. Ground water levels should be monitored to ensure the sampler re-mains submerged during the deployment period. This consideration is particularly important where long deployment times are required or where water levels fluctuate (e.g., tidal, temporal, adjacent pumping).
Volume of SamplesPassive samplers collect limited sample volumes. With the exception of the GORE Module, the volume needed to fill all bottles for the chosen analyses must be calculated, and a safety factor included to make sure enough water volume is collected to complete the analysis and any quality assurance/quality control that might be required. However, laboratories using the new technologies, such as large volume injectors, do not require the standard sample volumes of many volatile and semi-volatile analytes. For example, samples that required 1,000 milliliters for standard analysis (for low detection limits) may be reduced to as little as 100 milliliters when using the large volume injector analysis. Consult your laboratory prior to collecting samples.