Diffusion samplers (also called equilibrium samplers) are devices that rely on the analytes to reach equilibrium between the sampler and the groundwater via diffusion. Samples are time-weighted toward conditions at the sampling point during the latter portion of the deployment period. The degree of weighting depends on analyte and device-specific diffusion rates. Typically, conditions during only the last few days of sampler deployment are represented (ITRC 2006). Depending upon the contaminant of concern, equilibration times range from a few days to several weeks (ITRC 2006).

As a time and cost saving approach, passive diffusion bag samplers constructed of low density polyethylene (LDPE) have been left in wells for quarters at a time. After the first collection event, a new set of samplers is deployed. These samplers are collected and replaced at the next scheduled collection time (Vroblesky 2001). Regenerated-cellulose dialysis membrane samplers, however, should be collected within three to four weeks of deployment, as they are prone to biological attack and damage (ITRC 2006).

In some wells, vertical uniformity in VOC concentrations is observed in the screened interval under field conditions. The lack of stratification may be due to in-well diffusive mixing (Britt and Tunks 2003), vertical flow in the well, or thermally induced convection cells in the well (Vroblesky and others, 2009). In these wells, passive samplers likely will show relatively uniform concentrations across the screened interval. In other wells, however, stratification in well screens is observed under natural conditions. The source of this stratification may include such factors as vertical differences in contaminant concentrations outside the well screen, vertical flow through a portion of the well screen, density contrasts, and, in wells screened at the water table, volatilization loss at the air/water interface. Multiple passive samplers deployed along the saturated screened interval or open borehole of these wells can provide information on the vertical distribution of contaminants within the well. When the stratification reflects aquifer stratification or zones of inflowing contamination, the information can assist in locating the zones of highest concentrations.

The interval that is investigated depends upon the length of the sampler, which can vary from 2-3 inches (e.g., nylon screen sampler and passive vapor diffusion sampler) to feet (e.g., passive diffusion bag sampler and regenerated-cellulose dialysis membrane sampler). Vroblesky (2001) recommends that the vertical distribution of contaminants be determined in wells having 10-ft-long well screens, and that both the vertical distribution of contaminants and the potential for intra-borehole flow be determined in wells having screens longer than 10 ft.

The decision to deploy diffusion samplers should be made on a well-by-well basis. Sites where demonstrations have been held are given at the end of each sampler discussion.

Depending upon sampler design, diffusion samplers also have been used to sample VOCs, SVOCs, and inorganic contaminants in groundwater discharging to surface water (Lyford 2000, Zimmerman 2005, Church 2002). The point where a contaminant plume discharges in a surface water's sediments is not always apparent, and these samplers can aid in determining the location and potential for ecological damage. More information on sediment sampling can be found in the Sediments focus area.

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Nylon-Screen Passive Diffusion Samplers (NSPDS)

System Components and Operation

Nylon screen passive diffusion samplers are constructed using polypropylene wide-mouth bottles, a ring style cap, and an appropriate size square of nylon mesh screen. The bottles are immersed in deionized water (or organic-free deionized water if organics are target compounds), a sheet of nylon is placed over the mouth, and the cap is screwed on. They can be deployed singly or stacked in a polyethylene mesh bag.

Nylon Screen Secured on (A) Open-Top Jar, and (B) Three Jars in Low-Density Polyethylene Mesh (Source: Vroblesky 2002)

Vroblesky et al. (2002) found that nylon screen samplers (mesh 125 and 250µ) reached equilibrium with cations and anions in water within 20.5 hours. Mesh size might affect the equilibration of some cations and anions in the samplers (Vroblesky et al. 2002 and 2003). The direction the bottles are facing can also affect their function. Work by Webster et al (1998) as cited in Vroblesky et al. (2002) indicated that samplers facing down in water having an high ionic strength failed to equilibrate due to density differences in the sampler and ambient water. An orientation with the sampler membrane facing the well screen was preferred. For 2-inch wells where the horizontal deployment is not possible and the water is not strongly ionic, Vroblesky et al. (2002) orients the bottles downward. The stated purpose of this orientation was to minimize mixing of water in the samplers with shallower well water during sampler recovery.

Target Analytes

While much of the published work with this sampler has examined cations, anions, and general groundwater parameters (e.g., dissolved oxygen), it should be suitable for most contaminants of concern. As cited in ITRC (2006), equilibration studies conducted in 2003 of nylon screen samplers collecting VOCs (benzene; tetrachloroethene, trichloroethene, and 1,4-dioxane) and inorganic constituents (perchlorate, chloride, arsenic, and iron) indicated excellent diffusion from the test jars into the sampler water, with equilibration generally achieved in 24 hours. Note that a literature search did not find any citations that reference a nylon screen sampler being used for SVOC collection.

Advantages

• Probably good for most analytes.
• Does not require bringing gas (e.g., nitrogen, air) or electricity into the field.
• Does not mix water from different intervals.
• Small sampling interval provides good profile location.
• Can be stacked to profile screen length.
• Small sampling interval provides a good screened interval profile for identifying contaminant stratification.

Limitations

• Limited commercial availability (but easy to construct).
• Small water volume can limit the number of chemical classes that can be analyzed while still achieving project-required detection limits.
• Sampling for reduction-oxidation (redox)-sensitive metals, such as lead, iron, and manganese, in an open borehole with NSPDS (or other passive in-well methods) is subject to a number of uncertainties and should be approached with caution (ITRC 2006).
• In a laboratory experiment, silver recoveries were low for the nylon screen sampler as compared with good recoveries for other metals (Zimmerman et al. 2005).
• Smaller volume jars used for 2-inch wells have shown inconsistent results, which suggests the use of these devices is better suited to larger wells, where the larger volume samplers can be used (ITRC 2006).

Costs

Estimated commercial cost is approximately $40 to $50 each (ITRC 2006).

Demonstration Sites

Evaluation of Passive Diffusion Bag Samplers, Dialysis Samplers, and Nylon-Screen Samplers in Selected Wells at Andersen Air Force Base, Guam, March-April 2002
Vroblesky, Don, Manish Joshi, Jeffrey Morrell, and J E. Peterson
USGS, Water-Resources Investigations Report 03-4157, 36 pp, 2003

Field Tests of Diffusion Samplers for Inorganic Constituents in Wells and at a Ground-Water-Discharge Zone
Vroblesky, Don A., Matthew D. Petkewich, and Ted R. Campbell
USGS, Water-Resources Investigations Report 02-4031, 31 pp, 2002

Field Tests of Nylon-Screen Diffusion Samplers and Pushpoint Samplers for Detection of Metals in Sediment Pore Water, Ashland and Clinton, Massachusetts, 2003
Zimmerman, Marc, Don A. Vroblesky, Kimberly W. Campo, Andrew J. Massey, and Walter Scheible
USGS, Scientific Investigations Report 2005-5155, 56 pp, 2005

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Passive Vapor Diffusion Samplers

System Components and Operation

Passive Vapor Diffusion Samplers (PVDs) can be constructed or purchased commercially. They consist of an uncapped volatile organic analysis (VOA) vial (generally 40 ml) surrounded by a 2 to 3 mil, sealed, low density polyethylene (LDPE) bag drawn tightly across the mouth of the vial. The vial and bag are placed into a second sealed LDPE bag, which prevents the inner bag from coming in contact with sediments (Church et al. 2002).

PVDs are primarily used as a reconnaissance tool for detecting volatile organic compounds (VOCs) in groundwater discharge to surface water bodies. In shallow water they are typically placed by shovel or auger 0.5 to 1.5 feet below the bottom sediment of the surface water. In deeper water, a drive point approach can be used (Church et al. 2002). Ideally, the samplers should be buried at the bottom of the transition zone from surface water to groundwater to ensure that the sample collected represents VOCs in groundwater (Church et al. 2002).

Another issue in placing the samplers is to ensure they are in a part of the surface water that is a gaining reach. Church et al. (2002) provides a description of the various methods that can be used to determine what reaches of a stream are gaining.

Source: Church et al. 2002

After samplers are buried in VOC-contaminated pore water in the bottom sediment of surface-water bodies, an equilibrium begins to develop between VOC concentrations in water and the air in the vial. Equilibrium times, which are dependent on many factors such as hydraulic conductivity of the sediment and temperature of the pore water, generally range from one to three weeks (Church et al. 2002).

After retrieval, the air in the vial is tested by GC or GC/MS. Vapor-phase concentrations in the vial are typically reported in parts per billion by volume (ppbv). In theory, these vapor concentrations can be converted to concentrations in water through Henry's Law and Henry's Law constants for specific chemicals. In practice, however, uncertainties about Henry's Law constants, pore water temperatures, equilibration times for various types of sediments, and analytical precision limit this application (Church et al. 2002).

The results of a PVD survey provide insights about contaminant distributions and groundwater-flow patterns in discharge areas, and can be used to guide the design of more expensive and precise focused characterization activities.

Target Analytes

The PVD sampler can be used for most VOCs. Table 1 presents some detection limits obtained from field deployments in New England.

Volatile Organic Compound	Range of minimum reporting limits, in parts per billion by volume
Benzene	6 to 25
Ethylbenzene	40 to 90
Meta/para Xylene	40 to 90
Ortho-Xylene	60 to 100
Toluene	20 to 40
Tetrachloroethene	5 to 25
Trichloroethene	5 to 25
Chlorobenzene	40 to 70
Cis-Dichloroethene	25 (a target compound at only one site)
1,1,1-Trichloroethane	8 (a target compound at only one site)
Source: Church et al. 2002

Since the diffusion media is the same, PVD samplers should behave similarly to LDPE diffusion bag samplers. Research done on the bag samplers indicates that they are most useful for sampling non-polar organic compounds (ITRC 2006). Vroblesky (2001) reports that LDPE bags are not effective for acetone, methyl tert-butyl ether, and styrene.

Advantages (after Church et al. 2002)

• The method has been effective in delineating VOC-contamination-discharge areas beneath surface water bodies.
• PVD samplers can be areally and vertically distributed to gain information on contaminant discharge heterogeneity.
• The samplers are inexpensive; a low-cost sampler can be made from grocery-store sandwich bags and empty glass vials.
• In many situations, the samplers are easy to deploy and recover. Sampler recovery is rapid. The data can be analyzed on site with field gas chromatography, or the capped samples can be stored for later analysis.
• PVDs produce no investigation derived waste other than the bags and vials, which are disposed of following analysis.

Limitations (after Church et al. 2002)

• Because the change in VOC concentrations within PVD samplers in response to changes in ambient concentrations typically takes 24 hours or longer, VOC concentrations within the samplers represent an integration of concentrations from the most recent part of the deployment period until the samplers attain equilibrium.
• The PVD samplers are appropriate only for volatile compounds.
• Analysis of the samples requires a gas chromatograph.
• Deployment of the samplers in deep waters might require the services of SCUBA divers or other installation methods. Further work might be needed to determine source of detected VOCs in streams where complex hydraulics and sediment heterogeneity lead to unusual contaminant-discharge distributions.
• The samplers must be deployed in an area where groundwater is discharging to surface water to adequately reflect groundwater concentrations.
• Transient flow of groundwater, such as bank storage after a flood wave or tidal cycle, might cause temporary or cyclic changes in concentrations of VOCs that could affect interpretations about the extent and concentration level of VOCs in groundwater.
• Follow-up studies that use other sampling methods are needed to determine actual concentrations of VOCs in water.

Costs

PVD samplers can be purchased for less than $10.00 (ITRC 2006).

Demonstration Sites

Delineation of Discharge Areas of Two Contaminant Plumes by Use of Diffusion Samplers, Johns Pond, Cape Cod, Massachusetts, 1998
Savoie, J., D.LeBlanc, D.Blackwood, T. McCobb, R. Rendigs, and S. Clifford
USGS, Water Resources Investigations Report 00-4017, 33 pp, 2000

Field Tests of Polyethylene-Membrane Diffusion Sampler for Characterizing Volatile Organic Compounds in Stream-Bottom Sediments, Nyanza Chemical Waste Dump Superfund Site, Ashland, Massachusetts
Lyford, F. P.; R. Willey, and S. Clifford
USGS, Water-Resources Investigations Report 00-4108, 24 pp 2000.

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Peepers

System Components and Operation

Conventional peepers are small chambers with membrane or mesh walls containing either distilled water or clean water of the appropriate salinity and hardness. Samples are collected by burying the devices in sediments and allowing surrounding interstitial waters to infiltrate. In principle, dissolved solutes will diffuse through the porous wall into the peeper and the contained water will reach equilibrium with the ambient interstitial water (EPA 2001). Note: if the water to be sampled is anoxic the sampler water should be too.

The sampler can be either pushed into the sediment or, if in a cylindrical configuration, driven. If an objective of the investigation is to sample groundwater, a check should be made to ensure the sample area is in a gaining reach of the surface water. Also, several of the peepers must be placed below the groundwater to surface water transition zone to accurately represent groundwater quality. The number and closely stacked nature of the peepers will provide a profile of water quality from the groundwater through the transition zone to the surface water.

Peeper Plate Device (Source: EPA 2001)

Peeper Cylindrical Probe Device

Following initial placement, the equilibration time for peepers may range from hours to a month, but a deployment period of one to two weeks is most common. Equilibration time is a function of sediment type, study objectives, contaminants of concern, and temperature. Membrane pore size also affects equilibration time, with larger pore sizes being used to achieve reduced equilibration times (EPA 2001). Always ensure that the membrane material is compatible with the expected chemical conditions of the water.

Peeper samplers can be constructed of lexan, acrylic, teflon, stainless steel or any millable material. Material selection is a function of required depth and analytes of interest. Peeper rack sizes can vary from 5 to 100 cm in length and approximately 1 to 3 cm in depth. Peeper samplers can also be constructed as "cylinders" that have outer diameters ranging from 1 cm to 7 cm and range in length up to 4 meters (adapted from ITRC 2006).

Target Analytes

Theoretically, these samplers should be capable of monitoring most compounds (inorganic and organic) present in dissolved phases (ITRC 2006).

Advantages (Adapted from EPA 2001)

• Provides high resolution of pore water changes in the sediments.
• Can monitor most analytes including dissolved gases.
• Can preserve inorganic speciation under anaerobic conditions (Carr and Nipper 2001).
• Inexpensive and easy to construct.
• Some selectivity possible depending on nature of sample via specific membranes.
• Wide range of membrane/mesh pore sizes.
• Useful in determining contaminant availability.

Limitations (Adapted from EPA 2001)

• Cell sample volume may limit types of analysis that can be performed.
• Requires special transport and handling when sampling potentially anoxic pore water.
• Requires hours to weeks for equilibration (varies with site and chamber).
• Some membranes such as dialysis/cellulose are subject to biofouling and over time dialysis/cellulose is subject to microbial attack and destruction.
• Some construction materials may yield chemical artifacts.
• Must deoxygenate chamber and materials to prevent oxidation effects when sampling in anoxic environments.
• A high degree of technical competence and effort is required for proper use (Carr and Nipper 2001).

Costs

A commercially available peeper plate sampler is approximately $312.00 per sampler (April 2010), which consists of both the skeleton and membrane (ITRC 2006).

Monitoring Well Sampling

A demonstration of peeper technology for sampling monitoring wells took place at the former McClellan Air Force Base in 2005. Samplers were constructed using PVC well casing of 2-inch outside diameter (OD) by 2-inch-long blank (108 ml volume). The casing openings were covered with polysulfone®, a membrane that can be used in peepers, held in place by sliding a PVC coupling over the casing. The samplers were filled with purified water and tethered to a line with the plane of the membranes positioned orthogonally to horizontal groundwater flow. The samplers were deployed in 4-inch OD wells for approximately three weeks. Analysis of the sampler water indicated equivalence to low-flow sampling results for 1,4-dioxane and anions, but gave higher values for volatile organics and lower values for metals (Parsons 2005; ITRC 2006).

The 2005 report indicated that potential advantages or disadvantages of these samplers have not been quantified due to a lack of field- or bench-scale testing. A search of the literature found no other examples of this sampler construction used in groundwater monitoring wells. These samplers are easily constructed but are not commercially available.

Demonstration Sites

Natural Attenuation of Chlorinated Volatile Organic Compounds in a Freshwater Tidal Wetland, Aberdeen Proving Ground, Maryland
Lorah, M., L. Olsen, B. Smith, M. Johnson, and W. Fleck
USGS, Water-Resources Investigations Report 97-4171, 108 pp, 1997

Natural Attenuation of Chlorinated Solvent Ground-Water Plumes Discharging Into Wetlands
Lorah, M., D. Burris, and L. Dyer
USGS, Scientific Investigations Report 2004-5220, 190 pp, 2005

Results Report for the Demonstration of No-Purge Groundwater Sampling Devices at Former McClellan Air Force Base, California
Parsons Inc.
USACE (U.S. Army Corps of Engineers), 79 pp, 2005

Water-Quality and Water-Level Data for a Freshwater Tidal Wetland, West Branch Canal Creek, Aberdeen Proving Ground, Maryland, October 1998-September 1999
Spencer, T., L. Olsen, M. Lorah, and M. Mount
USGS, Open-File Report 00-282, 190 pp, 2000

Additional Resources

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Polyethylene Diffusion Bag Samplers (PDBs)

System Components and Operation

A typical PDB sampler consists of a 1- to 2-foot long low density polyethylene (LDPE) tube closed at both ends and containing laboratory grade organic-free deionized water. The typical diameter for PDB samplers used in a 2-inch well is 1.2 to 1.75 inches (Vroblesky 2001 and ITRC 2006). These dimensions provide 200 to 350 ml of sample for multiple VOA samples and duplicates (ITRC 2006). The samplers are buoyant, so they must be weighted for deployment. They can be lowered into a well using polyester rope, stainless steel or Teflon coated stainless steel wire. For data interpretation purposes the exact location of the sampler should be known.

The effectiveness of the use of a single PDB sampler in a well is dependent on the assumption that there is horizontal flow through the well screen and that the quality of the water is representative of the ground water in the aquifer directly adjacent to the screen. If there are vertical components of intra-borehole flow, multiple intervals of the formation contributing to flow, or varying concentrations of volatile organic compounds (VOC) vertically within the screened or open interval, then a multiple deployment of PDB samplers within a well might be more appropriate for sampling the well (Vroblesky 2001).

It is important that the vertical distribution of contaminants be determined in wells having 10-ft-long well screens, and that both the vertical distribution of contaminants and the potential for intra-borehole flow be determined in wells having screens longer than 10 ft (Vroblesky 2001).

After deployment, VOCs will diffuse through the polyethylene until an equilibrium is established. The amount of time that the sampler should remain in the well prior to recovery depends on the time required by the PDB sampler to equilibrate with ambient water and the time required for the environmental disturbance caused by sampler deployment to return to ambient conditions (Vroblesky 2001).This usually occurs within two weeks; however, diffusion times are chemical and temperature specific. If published data are not available for the chemical and expected site groundwater temperatures, a laboratory test is recommended to determine equilibration times (Vroblesky 2001). Also, low-yielding wells require more time to equilibrate than high-yielding wells. While an equilibration time is required before retrieving the sample, there is no set time on how long it can be left in the well. Since. Diffusion is a dynamic process and diffusion rates vary by compound; therefore, the sample in the PDB typically represents a discrete time-weighted concentration of the last several days prior to removal (ITRC 2006).

Leaving the sampler in the well between sampling times also has the potential for cost savings in long term monitoring as a sampling team need only return to the wells to collect samples at the regularly scheduled sampling times (e.g., quarterly). There is no need to place the samplers and then collect them two to three weeks later.

The suggested application of the method is for long-term monitoring of VOCs in groundwater wells at well characterized sites (Vroblesky 2001). The decision to use PDBs at a site should be done on a well-by-well basis.

PDBs also have been used to sample VOCs in groundwater discharging to surface water (Lyford 2000). Care must be taken to ensure they are placed in a gaining reach of the surface water sediment. Placement of PDBs in sediments is similar to that of passive vapor diffusion samplers. See the passive vapor diffusion samplers section for a more detailed discussion.

Target Analytes

PDBs are generally only used for nonpolar VOCs. The samplers are not appropriate for hydrophilic polar molecules, such as inorganic ions (Vroblesky 2001).

Benzene Bromodichloromethane Bromoform Chlorobenzene Carbon tetrachloride Chloroethane Chloroform Chloromethane	2 Chlorovinyl ether Dibromochloromethane Dibromomethane 1,2-Dichlorobenzene 1,3-Dichlorobenzene 1,4-Dichlorobenzene Dichlorodifluoromethane 1,2-Dichloroethane 1,1-Dichloroethene	cis-1,2-Dichloroethene trans-1,2-Dichloroethene 1,2-Dichloropropane cis-Dichloropropene 1,2-Dibromoethane trans-1,3-Dichloropropene Ethyl benzene Naphthalene Toluene	1,1,1-Trichloroethane 1,1,2-Trichloroethane Trichloroethene Trichlorofluoromethane 1,2,3-Trichloropropane 1,1,2,2-Tetrachloroethane Tetrachloroethene Vinyl chloride Total xylenes
Source: Vroblesky 2001a

Acetone, methyl-tertbutyl ether, and styrene did not show good laboratory correlation (Vroblesky 2001).

Advantages (Adapted from Vroblesky 2001)

• PDB samplers have the potential to eliminate or substantially reduce the amount of purge water associated with sampling.
• PDB samplers are inexpensive.
• The samplers are easy to deploy and recover.
• Because PDB samplers are disposable, there is no downhole equipment to be decontaminated between wells.
• A minimal amount of field equipment is required.
• Multiple PDB samplers, distributed vertically along the screened or open interval can be used in conjunction with borehole 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.
• Because the pore size of LDPE is only about 10 angstroms or less, sediment does not pass through the membrane into the bag. Thus, PDB samplers are not subject to interferences from turbidity.

Limitations (Adapted from Vroblesky 2001)

• Not effective for inorganics, SVOCs and some VOCs.
• PDB samplers integrate concentrations over time. This can be a limitation if the goal of sampling is to collect a representative sample at a point in time in an aquifer where VOC-concentrations substantially change more rapidly than the samplers equilibrate.
• VOC concentrations in PDB samplers represent groundwater concentrations in the vicinity of the screened or open well interval that move to the sampler under ambient flow conditions. This is a limitation if the groundwater contamination lies above or below the well screen or open interval, and requires the operation of a pump to conduct contaminants into the well for sampling.
• If the well screen is less permeable than the aquifer or the sandpack, then under ambient conditions, flow lines might be diverted around the screen. Such a situation might arise from inadequate well development or from iron bacterial fouling of the well screen. In this case, the VOC concentrations in the PDB samplers might not represent concentrations in the formation water because of inadequate exchange across the well screen.
• In wells with screens or open intervals with stratified chemical concentrations, the use of a single PDB sampler set at an arbitrary (by convention) depth might not provide accurate concentration values for the most contaminated zone.
• Regulatory agencies may ask for a show of equivalency before approving the use of PDB samplers.

Costs

Sampler bag ~$15.00 (April 2010)

Demonstration Sites

Comparison of Diffusion- and Pumped-Sampling Methods to Monitor Volatile Organic Compounds in Ground Water, Massachusetts Military Reservation, Cape Cod, Massachusetts, July 1999-December 2002
Archfield, Stacey A. and Denis R. LeBlanc
USGS, Scientific Investigations Report 2005-5010, 60 pp, 2005

Diffusion Sampler Testing at Naval Air Station North Island, San Diego County, California, November 1999 to January 2000
Vroblesky, D. and B. Peters
USGS, Water-Resources Investigation 00-4182, 34 pp, 2000

Evaluation of Passive Diffusion Bag Samplers, Dialysis Samplers, and Nylon-Screen Samplers in Selected Wells at Andersen Air Force Base, Guam, March-April 2002
Vroblesky, Don, Manish Joshi, Jeffrey Morrell, and J E. Peterson
USGS, Water-Resources Investigations Report 03-4157, 36 pp, 2003

Evaluation of Passive Diffusion Bag Samplers in Selected Wells at the Naval Surface Warfare Center, Louisville, Kentucky, July 1999 to January 2000
Vroblesky, D., M. Petkewich, and C. Casey
USGS, Water Resources Investigations Report 01-4163, 22 pp, 2001a

Field Tests of Polyethylene-Membrane Diffusion Samplers for Characterizing Volatile Organic Compounds in Stream-Bottom Sediments, Nyanza Chemical Waste Dump Superfund Site, Ashland, Massachusetts
Lyford, F., R.Willey, and S. Clifford
USGS, Natural Resources Investigations Report 00-4108, 24 pp, 2000

Flow-Meter and Passive Diffusion Bag Tests and Potential Influences on the Vertical Distribution of Contaminants in Wells at Galena Airport, Galena, Alaska, August to October 2002
Vroblesky, Don A. and J E. Peterson
USGS Open-File Report 2004-1241, 44 pp, 2004

Results Report for the Demonstration of No-Purge Groundwater Sampling Devices at Former McClellan Air Force Base, California
Parsons Inc.
USACE (U.S. Army Corps of Engineers), 79 pp, 2005

User's Guide for Polyethylene-Based Passive Diffusion Bag Samplers to Obtain Volatile Organic Compound Concentrations in Wells. Part 2: Field Tests
D.A. Vroblesky (ed.).
U.S. Geological Survey, Water-Resources Investigations Report 01-4061, 102 pp, 2001b

Additional Resources

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Regenerated-Cellulose Dialysis Membrane Samplers

System Components and Operation

Partially Assembled Regenerated-Cellulose Dialysis Membrane Samplers (Source: Imbrigiotta 2007)

The dialysis sampler consists of a deionized water-filled tube of high-grade regenerated-cellulose dialysis membrane inside an outer protective layer of low density polyethylene (LDPE) mesh. The sampler can have PVC pipes external to the dialysis membrane in low-ionic strength waters or an internal perforated PVC pipe to support the membrane in high ionic strength waters. The sampler can have a stopcock at one end to facilitate sample transfer. Each dialysis sampler has an attached weight to overcome its buoyancy and is suspended in a well by means of a dedicated or disposable wire or polyethylene rope. The regenerated cellulose diffusion membrane has a pore size of 18 Angstroms and a molecular weight cut-off (MWCO) of 8,000 Daltons. The sampler can be constructed using either 31.8 mm (1.25 inches) or 63.7 mm (2.5 inches) diameter membranes. The sampler volume depends on the diameter and length of the dialysis bag. The 31.8-mm (1.25- inch) diameter dialysis membrane contains a volume of 5.1 mL/cm. The 63.7-mm (2.5-inches) diameter membrane contains 31.8 mL/cm.

Regenerated Cellulose Dialysis Membrane Diffusion Sampler with Perforated PVC Pipe Support Inside the Membrane (Source: Imbrigiotta 2007) So, for example, dialysis bags 30.5 cm (12 inches) long will contain volumes of 155 mL and 969 mL for the narrow-diameter and wide-diameter membranes, respectively. Larger sample volumes can be collected using longer bags (taken directly from ITRC 2006). It is important that the vertical distribution of contaminants be determined in wells having 10-ft-long well screens, and that both the vertical distribution of contaminants and the potential for intra-borehole flow be determined in wells having screens longer than 10 ft (Vroblesky 2001).

An assembled regenerated-cellulose dialysis membrane sampler is deployed by lowering it down a well to its target screen depth. The amount of time that the sampler should be left in the well prior to recovery depends on the time required by the sampler to equilibrate with ambient water and conditions in the well to recover from any disturbance caused by sampler deployment. Laboratory tests have shown that this usually occurs within two weeks (Imbrigiotta 2007). Unlike PDB samplers, dialysis samplers should not be left in a well for an extended period and generally should be retrieved within three to four weeks, at most. Samplers left longer than this can be compromised by biological activity.

Target Analytes (from Imbrigiotta 2007)

Laboratory equilibration testing has shown that dialysis samplers equilibrate within:

• 1-3 days for anions, silica, methane, dissolved organic carbon, and all VOCs on the EPA 8260B list (including MTBE).
• 3-7 days for most cations and trace elements.
• 7-14 days for most explosive compounds.

Table of water quality parameters tested in the laboratory (Imbrigiotta 2007).

Advantages (from Imbrigiotta 2007)

• No purge water is produced to drum, transport, or treat. Sample filtration is not necessary, as particulates cannot pass through the membrane.
• Samplers are disposable, so no decontamination is needed between wells.
• Time in field is minimized for field personnel.
• Dialysis membrane is fairly inexpensive; the cost is slightly more than LDPE, but far less than a pump setup.
• Can be used to sample for both inorganic and organic dissolved chemical species.

Limitations (from Imbrigiotta 2007)

• Pre-cleaned dialysis membrane must be kept wet in preservative solution prior to use.
• Dialysis samplers lose water with time due to the nature of the dialysis process.
• Dialysis membranes are subject to attack by bacteria and fungi and hence should not be left in a well for an extended period (over 3-4 weeks).
• Sample volume is finite.
• Requires two trips to the field—one to deploy and one to retrieve sampler (ITRC 2007).

Costs (from Imbrigiotta 2007)

Total materials cost per sampler is approximately $21. Material costs include: a 2.5-in diameter by 2-ft long dialysis bag, PVC supports, outer protective mesh, stopcock, clamp, weights, and suspension rope.

Note: This constructs a 2.5-inch diameter by 2-ft long sampler suitable for use in a 4-inch diameter well. Smaller dialysis membrane (1.25-inch diameter) can be purchased to make samplers that can be used in a 2-inch diameter well. The smaller diameter dialysis membrane costs essentially the same per unit length as the larger size but holds less volume.

Demonstration Sites

Evaluation of Passive Diffusion Bag and Dialysis Samplers in Selected Wells at Hickam Air Force Base, Hawaii, July 2001
Vroblesky, D. and Tasha Pravecek
USGS, Water-Resources Investigations Report 02-4059, 34 pp 2002

Evaluation of Passive Diffusion Bag Samplers, Dialysis Samplers, and Nylon-Screen Samplers in Selected Wells at Andersen Air Force Base, Guam, March-April 2002
Vroblesky, Don, Manish Joshi, Jeffrey Morrell, and J E. Peterson
USGS, Water-Resources Investigations Report 03-4157, 36 pp, 2003

Field Tests of Diffusion Samplers for Inorganic Constituents in Wells and at a Ground-Water-Discharge Zone
Vroblesky, Don A., Matthew D. Petkewich, and Ted R. Campbell
USGS, Water-Resources Investigations Report 02-4031, 31 pp, 2002

Results Report for the Demonstration of No-Purge Groundwater Sampling Devices at Former McClellan Air Force Base, California
Parsons, Inc.
USACE (U.S. Army Corps of Engineers), 79 pp, 2005

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Rigid Porous Polyethylene Samplers (RPPS)

System Components and Operation

Source: ITRC 2007

The RPPS consists of a 1.5-inch OD, six to seven-inch-long, rigid polyethylene tube with caps on both ends. The tube is constructed from thin sheets of foam-like porous polyethylene with pore sizes of 6 to 15 microns. The sampler is filled with water free of the target analytes, capped at both ends, and placed inside a mesh liner (ITRC 2006). More recent designs use a Delrin plug for the bottom cap to reduce leakage at recovery (ITRC 2007).

Tests performed to date indicate that the maximum feasible sampler length is approximately 7.5 inches. Use of a longer sampler results in leakage of sampled water out of the sampler walls due to the higher head pressure present in the sampler (ITRC 2006). Commercially available samplers are 1.5 inches in diameter, 6 inches in length, and have a volume of approximately 100 mL.

The RPP sampler is attached to a weighted line and deployed with the plug end down so that it hangs at the desired depth of the screened interval of the well. More than one sampler can be deployed at the same well screen interval (for wells greater than 3 inches ID) or in tandem if vertical profiling is of interest (ITRC 2007). Before deployment, it is important that the vertical distribution of contaminants be determined in wells having 10-ft-long well screens, and that both the vertical distribution of contaminants and the potential for intra-borehole flow be determined in wells having screens longer than 10 ft (Vroblesky 2001).

Source: ITRC 2007

Commercial samplers are currently supplied field-ready. They are shipped in a sealed polyethylene sleeve filled with the appropriate grade water to ensure that air does not fill the pores of the RPP (ITRC 2007). The porous polyethylene sampler pores tend to retain air even when submerged. Therefore, the air entrained in the pore space must be removed by flushing with water prior to deployment if the sampler is to be used for nonvolatile solutes. This step is not needed when sampling for volatile organic compounds (VOC) only (ITRC 2006).

The RPP should be deployed down the well as soon as possible after opening and discarding the protective sleeve to minimize exposure to the air (ITRC 2007).

Target Analytes

Testing by Columbia Analytical Services (2006) indicates that RPP samplers can be used for most metals (copper and silver appear to be exceptions); 1,4-dioxane; wet chemistry analytes; most VOCs; methane, ethane, and ethene, and some semi-volatile organic compounds (see Table for results).

Advantages (ITRC 2007)

• Can be used to collect most inorganic and many organic analytes.
• Are easily deployed and retrieved.
• Significantly reduce field sampling costs.
• Can be supplied field-ready, enhancing field QC control.

Limitations (ITRC 2007)

• May require additional equilibrium time for less-water-soluble VOCs and SVOCs.
• Must be stored and shipped fully immersed in deionized organic free water.
• Are not suitable for wells smaller than 2 inches in diameter.
• Have not been tested for all analytes.
• Is not effective for collection of polycyclic aromatic hydrocarbons.
• May collect insufficient sample volume for multiple analyses and/or QC.

Costs

The sampler (6 inch length, 1.5 inch diameter, and about 100ml volume) is available commercially for $65 (April 2010).

Demonstration Sites

Comparison of Pumped and Diffusion Sampling Methods to Monitor Concentrations of Perchlorate and Explosive Compounds in Ground Water, Camp Edwards, Cape Cod, Massachusetts, 2004-05
LeBlanc, D.R., and D.A. Vroblesky USGS (U.S. Geological Survey), Scientific Investigations Report 2008-5109, 16 pp, 2008

Results Report for the Demonstration of No-Purge Groundwater Sampling Devices at Former McClellan Air Force Base, California
Parsons Inc.
USACE (U.S. Army Corps of Engineers), 79 pp, 2005

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Sampler Comparisons

Comparison of No-Purge and Pumped Sampling Methods for Monitoring Concentrations of Ordnance-Related Compounds in Groundwater, Camp Edwards, Massachusetts Military Reservation, Cape Cod, Massachusetts, 2009-2010
Savoie, J.G. and D.R. LeBlanc.
U.S. Geological Survey, Scientific Investigations Report 2012-5084, 36 pp, 2012

Field tests to determine the utility of no-purge groundwater sampling for monitoring concentrations of RDX, HMX, and perchlorate included (1) a diffusion sampler constructed of rigid porous polyethylene, (2) a diffusion sampler constructed of regenerated-cellulose membrane, and (3) a tubular grab sampler (bailer) constructed of polyethylene film. Overall, diffusion sampling with the rigid porous polyethylene and regenerated-cellulose membranes and grab sampling with the polyethylene-film samplers provided data on the concentrations of munitions constituents in groundwater at the test site that was comparable to data obtained by low-flow pumped sampling with dedicated bladder pumps.

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