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What is a Lesson Learned?
A lesson learned is a good work practice, innovative approach, or experience that is captured and shared to improve the intended application or raise awareness. A lesson learned may also be an adverse work practice or experience that is captured and shared to avoid recurrence. The ITRC Diffusion Sampler Team is dedicated to capturing relevant PDB sampler application lessons learned and sharing them with interested parties. This section is a “work in progress” and will be updated as the scale of Passive Diffusion Bag Sampler deployment increases and new information becomes available.Contaminant Stratification
Contaminant stratification in monitoring wells appears to be more common than previously thought. Field studies conducted on wells with ten foot screens at the North Island Naval Air Station (San Diego, CA) have shown concentration changes in the hundreds to thousands of parts per billion within a vertical distance of three feet. Other field studies included in Part II of the USGS User’s Guide have reported similar results. This emphasizes the need to properly evaluate the potential for contaminant stratification to be present when deploying PDB in wells with greater than five feet of screen or open hole. PDB samplers do not transmit alkalinity
Therefore, in highly alkaline environments, where sample preservation by acidification of VOA vials may result in foaming and loss of VOCs from the sample, the PDB samplers allow collection of ambient VOC concentrations in a matrix that can be safely preserved by adding acid. Add sufficient weight
Because there is some amount of buoyancy associated with PDB samplers, care should be taken to add sufficient weight to the samplers to ensure they remain suspended at the intended target zone within the well. Different results do not mean PDB's are wrong
Differences between results from PDB sampling and other methods do not necessarily mean that the diffusion sampler results are wrong. Differences may result from a variety of factors. The PDB samplers represent approximate point samples (over about 1.5 ft). Low-flow samples represent an approximate point sample in some wells and an integration of water over larger areas in other wells. The area integrated and mixed in low-flow sampling may vary from well to well, and may include the entire screened interval in some wells. Sampling by purging 3 or more casing volumes prior to sample collection mixes and integrates water over an even larger area, sometimes moving water to the well screen from areas not adjacent to the screen. The differences between sampling methods are most probable to be seen in wells containing hydrogeologic or chemical heterogeneity. Combining multiple PDB samplers and bore-flowmeter
The combination of multiple PDB samplers and borehole-flowmeter testing can be effective in determining contaminant inflow points and stratification in wells. PDB samplers do not transmit water
It should be noted that PDB samplers do not transmit water through the membrane. If there is an observed loss of water/sample volume in the bag, you have a leak somewhere and need to check the bag for leakage. An early bag design allowed leakage around the tip insert portion of the bag. The design has since been modified to take care of the problem. Share your lesson learned
Do you have a lesson learned to share with us? Please e-mail George Nicholas at gnichola@dep.state.nj.us
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A brief description of passive sampling devices.
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Name/Device |
Acronym |
Description |
| Polyethylene Diffusion Bag sampler |
PDB sampler
[PDB sampling] |
A typical PDB sampler consists of a low-density polyethylene, lay-flat tube closed at both ends and filled with deionized water prior to deployment. The sampler is positioned at the target horizon of the well by attachment to a weighted line or fixed pipe. The sampler is used to obtain concentrations of volatile organic compounds in ground water from wells or at interfaces of groundwater and surface water. |
| Passive Vapor Diffusion sampler |
PVD sampler
[PVD sampling] |
A PVD sampler is typically used to detect the vapor phase of volatile organic compounds discharging from ground water to surface water. It consists of an empty glass vial enclosed in two low-density polyethylene (LDPE) bags. The sampler is buried in the bottom sediment of surface water bodies, allowed to equilibrate, and then recovered by placing an open-hole septated cap onto the vial and inner LDPE bag. The enclosed vapor in the sampler is recovered by piercing the septated cap with a syringe. |
| Screen sampler |
-- |
These typically are water-filled, rigid containers with the opening(s) covered by screen(s). Water is held within the sampler by surface tension against the screen and, in some cases, by the pull of a vacuum against the weight of the water. Screens are constructed using various mesh sizes and materials, including nylon and types of filter material. Typical applications include determining concentrations of dissolved and suspended constituents in aqueous environments, including surface water, marsh sediments, and groundwater. |
| Dialysis samplers |
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These samplers are constructed of dialysis membranes and rely on diffusion (the movement of solutes across the membrane) and osmosis (the movement of water across the membrane) to equilibrate with ambient fluids. The membranes may be constructed of a variety of materials, including regenerated cellulose, polysulfone, polyvinylidene fluoride, polycarbonate, cellulose ester or acetate, and polyester. The samplers are used to obtain concentrations of dissolved organic and inorganic constituents in water. |
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If you want a PDF version of the FAQ, you can view or download the PDF here (right-click the link and choose Save Target As to save a copy to your computer).
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Passive diffusion bag (PDB) sampling is a cost-effective alternative to standard (or low-flow) purge and sample techniques for collecting concentrations of a variety of volatile organic compounds (VOCs) in groundwater at monitoring wells. Using PDB samplers can result in significant cost savings at sites where VOCs are the constituents of concern and particularly where long-term monitoring of groundwater is required. PDB sampling can also provide inexpensive and precise vertical contaminant concentration profiles that can be used to optimize remedial systems. The Interstate Technology and Regulatory Council (ITRC) Diffusion Sampler Team has gathered the following information about this technology and its uses.
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 A PDB sampler is a low-density polyethylene bag filled with deionized water, which acts as a semipermeable membrane and is suspended in a well to passively collect groundwater samples. PDB samplers rely on the free movement of groundwater from the aquifer or water-bearing zone through the well screen. VOCs in groundwater will diffuse across the bag material until constituent concentrations within the bag reach equilibrium with concentrations in the surrounding groundwater.
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PDB samplers are inexpensive and have the potential to eliminate or substantially reduce the amount of purge water associated with sampling. The samplers are easy to deploy and recover. Because PDB samplers are disposable, there is no down-hole equipment to be decontaminated between wells. PDB samplers require a minimal amount of field equipment. Sampler recovery is rapid. Because of the small amount of time and equipment required for the sampling event, the method is practical for use where access is a problem or where discretion is desirable (for example, residential communities, business districts, or busy streets where traffic control is a concern). Multiple PDB samplers distributed vertically along the screened or open interval may be used in conjunction with bore-hole flow meter testing to gain insight on the movement of contaminants into and out of the well screen or open interval or to locate the zone of highest concentration in the well. As the pore size of low-density polyethylene is only 10 angstroms or less, sediment does not pass through the membrane into the bag. Thus, PDB samplers are not subject to interference from turbidity. Finally, because alkalinity-contributing solutes do not pass through the membrane, the samplers allow collection of VOCs in a nonalkaline matrix, even if the well is in a limestone aquifer. This feature eliminates the VOC losses seen when highly alkaline water 'foams' upon attempting to preserve samples by acidification.
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PDB samplers are not appropriate for all compounds. PDB samplers also integrate concentrations over time, which may be a limitation if the goal of sampling is to collect a representative sample at a point of time in an aquifer where VOC concentrations substantially change more rapidly than the samplers equilibrate. PDB samplers rely on the free movement of water through the well screen. In situations where groundwater flows horizontally through the well screen, the VOC concentrations in the open interval of the well are probably representative of the aquifer water in the adjacent formation. However, if the well screen is less permeable than the aquifer or the sandpack than under nonpumping conditions, flow lines may be diverted around the screen. In this case, the VOC concentrations in the PDB samplers may not represent concentrations in the formation water because of inadequate exchange across the well screen.
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 Field and lab testing have shown that PDB samplers are effective for a number of VOCs, including chlorinated aliphatic compounds and petroleum hydrocarbons. A list of the VOCs evaluated is included in the accompanying table. The majority of VOCs evaluated were shown to readily diffuse into the sample bag.
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PDB samplers should not be used for the compounds listed at the bottom of the table shown above (those showing poor correlation). They are also unsuitable for inorganic ions and have a limited applicability for non-VOCs and for some VOCs. For example, although methyl-tert-butyl ether, acetone, and most semivolatile compounds are transmitted through the polyethylene bag, laboratory tests have shown that the resulting concentrations are lower than in ambient water. The samplers should not be used to sample for phthalates because of the potential for the low-density polyethylene to contribute phthalates to the water sample. They are not useful for inorganic polar molecules because the membrane is nonpolar organic. PDB samplers are also inappropriate for ionic natural attenuation parameters, e.g., nitrates and sulfates. They are effective for the nonionic natural attenuation parameter methane; however, if natural attenuation monitoring is required, a combination of sampling techniques could be considered. For example, annual monitoring of natural attenuation parameters can be performed using a traditional sampling method, while quarterly monitoring of VOCs can be accomplished using diffusion sampling technology.
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PDB samplers can be used in most formations, but they are not currently recommended for use in low-permeability formations because the flow of water through the well screen is restricted if the rate of VOC change in the well (by volatilization) is less than the rate that the VOCs are replaced by movement into the well screen. In this case, the VOC concentrations in the well water may not be representative of the VOC concentrations in the formation. PDB sampling relies on self-purging of wells to maintain concentrations within a well that are representative of the surrounding aquifer conditions. However, it should be noted that sampling of low-yielding wells is problematic for any currently available groundwater sampling method. PDB samplers work on the principle of diffusion. In low-permeability formations, diffusion is the major mechanism for contaminant transport. Under these conditions, the PDB samplers may indeed represent a viable sampling approach compared to other more conventional methods, but not enough data are currently available to support their use in low-permeability formations. Wells often do not adequately self-purge within a two-week period in low-permeability aquifers to allow diffusion sampling to be effective. As a general rule, diffusion sampling should not be used in water-bearing zones with a hydraulic conductivity of less than 1 x 10-6 cm/s.
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PDB samplers can come prefilled or can be filled in the field with laboratory-grade deionized water immediately prior to deployment. Appropriate weight, connectors, and line are attached to the sampler, and it is set at the desired depth in the screened interval. Licensed diffusion bag suppliers can provide more specific installation and recovery instructions.
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It takes about 10 minutes to collect a groundwater sample from a diffusion bag and about the same amount of time to install a new bag. If quarterly sampling is being done, another bag can be installed immediately after the sample is collected. If annual or semiannual sampling is being done, another bag can be installed immediately after the sample is collected in some situations. PDB samplers have successfully been left in place for a year with no obvious loss of bag integrity. The longevity of PDB samplers in a variety of potentially hostile groundwater environments has not yet been tested, however, so the user should be alert for potential losses of bag integrity during recovery. If a single event is being done, a separate trip will be required to install the bags at least two weeks prior to sample collection.
PDB samplers should be left in place for a minimum of two weeks prior to sample collection to ensure that the deionized water within the diffusion bag has reached equilibrium with the surrounding groundwater. Under low-flow conditions, the PDB should be left in place longer prior to sample collection.
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Cost information is available from vendors. There currently are two authorized distributors from whom passive diffusion bags can be obtained. EON Products (www.eonpro.com, 1-800-474-2490) are filled in the field. Columbia Analytical Services (www.caslab.com, 1-800-695-7222) samplers come prefilled with deionized water. PDB samplers employ patented technology (U.S. patent number 5,804,743) and therefore require that the user purchase commercially produced samplers from a licensed manufacturer or negotiate a nonexclusive license for sampler construction from the United States Geological Survey (USGS).
 The cost savings resulting from the decreased time required to collect diffusion samples (relative to standard purge or low-flow purge and sample techniques) often more than offset the cost of the passive diffusion sampler and replacement bags. Prices vary based on volume, hardware, and optional accessories, but the overall cost of a diffusion sampler generally ranges $16 to $35 per bag plus reusable hardware (approximately $20 to $25).
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A validation study typically includes conducting a side-by-side test with both PDB samples and conventional sampling (typically low-flow sampling). In wells showing little temporal variability in concentrations, a validation study may not be needed. Diffusion sampling results can be compared to historical data from samples collected using other techniques, but the results are less conclusive than those of a side-by-side test. The biggest uncertainties associated with diffusion sampling are the presence of vertical stratification within the well and vertical flow within the well itself. Stratification can produce PDB sampler results that do not always agree with pumped results because PDB samplers represent approximate points (about 1 foot long) and tend to show the stratification with high precision, while the pumped sample often is a mixture of waters from different parts of the stratification. In this situation, it is advisable initially to delineate the stratification with diffusion samplers to decide on the optimum deployment depth. Vertical flow within the well means that the water may be coming from a horizon not adjacent to the PDB sampler and, therefore, not representative of the formation immediately adjacent to the PDB sampler. In this situation, a validation study should be completed to determine whether diffusion sampling is appropriate for the site conditions.
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It is practically impossible not to introduce statistical variability when you switch sampling methods. For sites that are in compliance monitoring and using statistics to demonstrate a decreasing trend, PDB samplers may not be appropriate. However, if there is already a strong decreasing trend established in the data, switching methodologies would be less critical because there is good evidence that the plume concentrations are decreasing. If good correlation with existing results is obtained, the decision to switch methodologies would not be scientifically complex. In a more complicated case where data do not correlate well, additional work would be required to document why there is poor correlation (e.g., vertical profiling, bore-hole flow testing, review of hydrogeology and well construction information). It should be noted that some of the questions regarding PDB sample data correlation and use, such as vertical placement of the sampler, also can apply to low-flow samples. PDB samplers have the potential to generate detailed data that may not always agree with the existing database of samples that have undergone some degree of mixing during pumping. The significance of these new data, in terms of contaminant fate, transport, and site management, is a site-specific consideration.
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Yes. In collaboration with the USGS, U.S. Air Force, U.S. Naval Facilities Engineering Command, U.S. Environmental Protection Agency (U.S. EPA), Federal Remediation Technologies Roundtable, Defense Logistics Agency, and U.S. Army Corps of Engineers, ITRC has published The User's Guide for Polyethylene Based Passive Diffusion Bag Samplers to Obtain Volatile Organic Concentrations in Wells (Vroblesky, D. A. March 2001). The document can be obtained in our Key Documents section and at other Web sites. The purposes of this document are to present methods for PDB sampler deployment and recovery, discuss approaches for determining the applicability of passive diffusion samplers, and discuss various factors influencing interpretation of the data. The intended audience for the methodology sections of the report includes managers and field personnel involved in using PDB samplers. The discussion of PDB sampler applicability and interpretation of the data is suited for project managers, technical personnel, and the regulatory community. Part 2 of the document presents case studies of PDB sampler field applications.
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In collaboration with the U.S EPA/Technology Innovation Office (TIO) and USGS, ITRC has developed Internet-based training on the User's Guide. See our Training section for details. In conjunction with U.S. EPA/TIO, ITRC hosts these two-hour training courses via the Internet to reach a geographically diverse audience of regulators, consultants, and other members of the environmental community. The sessions are also archived on the Web.
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ITRC developed the Diffusion Sampler Information Center Web site which you are now viewing. This site contains information on diffusion samplers, recent news, deployments, cost and performance data, technical documents, lessons learned, technology advances, available training, frequently asked questions, and contacts. The Web site also has links to other sources of information about PDB samplers. There is a growing body of data from numerous facilities comparing the results of diffusion sampling with other techniques such as bailing and low-flow sampling. This information is summarized in numerous technical reports that are being published or produced. The ITRC Web site will track and summarize this technical information on diffusion sampler deployments nationwide.
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Please see our Contacts page.
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DISCLAIMER
The ITRC does not endorse the use nor does it attempt to determine the merits of any specific technology or technology provider through the publication of any ITRC document or fact sheet; nor does it assume any liabilities with respect to the use of, or for damages resulting from the use of, any information, apparatus, method or process discussed in this brochure. Mention of trade names or commercial products does not constitute endorsement or recommendation of use. This FAQ sheet is intended to provide basic information on the technology and to provide other sources of information. | |
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