USER’S GUIDE FOR POLYETHYLENE-BASED PASSIVE 

DIFFUSION BAG SAMPLERS TO OBTAIN VOLATILE

ORGANIC COMPOUND CONCENTRATIONS IN WELLS

PART 1:

DEPLOYMENT, RECOVERY, DATA INTERPETATION,

AND QUALITY CONTROL AND ASSURANCE

Water-Resources Investigations Report 01-4060

Prepared in cooperation with the

U.S. AIR FORCE 

U.S. NAVAL FACILITIES ENGINEERING COMMAND 

U.S. ENVIRONMENTAL PROTECTION AGENCY 

FEDERAL REMEDIATION TECHNOLOGIES ROUNDTABLE 

DEFENSE LOGISTICS AGENCY

U.S. ARMY CORPS OF ENGINEERS and

INTERSTATE TECHNOLOGY REGULATORY COOPERATION WORK GROUP
Acknowledgments

Technical Advisory Board

U.S. Air Force Base Conversion Agency (AFBCA) 

U.S. Air Force Center For Environmental Excellence (AFCEE)

Maj. Jeff Cornell (AFCEE)

Mario Ierardi (AFBCA)

Dr. Javier Santillan (AFCEE)

Defense Logistics Agency (DLA) 

Lt. Col. Daniel L. Welch

U.S. Environmental Protection Agency (EPA) 

Steve Schmelling, Office of Research and Development (ORD)

Dick Willey, EPA Region 1

Kathy Davies, EPA Region 3

Richard Steimle, Technology Innovation Office (TIO)

Interstate Technology Regulatory Cooperative Work Group (ITRC)

George H. Nicholas (New Jersey) Team Lead

Paul M. Bergstrand (South Carolina)

Chris A. Guerre (California) 

David Randolph (Tennessee)

Federal Remediation Technologies Roundtable 

U.S. Naval Facilities Engineering Command (NAVFAC)

U.S. Naval Facilities Engineering Service Center (NFESC)

Cliff C. Casey (Southern Division)

Dennis Howe (NFESC)

Richard G. Mach, Jr. (Southwest Division)

Nick Ta (NFESC)

U.S. Army Corp of Engineers (USACE) 

Dave Becker

Johnette Shockley

Funding for this Guide was provided by the U.S. AIR FORCE and NAVFAC (Southern and Southwest D

Additionally, the following persons are recognized for their leadership and support to this project: Marty 

Dunkle, Kay Wishkaemper, Vince Malott, and the Passive Diffusion Bag Sampler (PDBS) Work Group.
User’s Guide for Polyethylene-Based Passive 

Diffusion Bag Samplers to Obtain Volatile 

Organic Compound Concentrations in Wells 

Part 1: Deployment, Recovery, Data Interpretation, and 

Quality Control and Assurance

By Don A. Vroblesky

U.S. Geological Survey

Water-Resources Investigations Report 01–4060

Prepared in cooperation with the

U.S. AIR FORCE

U.S. NAVAL FACILITIES ENGINEERING COMMAND 

U.S. ENVIRONMENTAL PROTECTION AGENCY

FEDERAL REMEDIATION TECHNOLOGIES ROUNDTABLE

DEFENSE LOGISTICS AGENCY

U.S. ARMY CORPS OF ENGINEERS and 

INTERSTATE TECHNOLOGY REGULATORY COOPERATION WORK GROUP

Columbia, South Carolina

2001
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Use of trade, product, or firm names in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Geological Survey.

Copies of this report can be For additional information,

obtained from: write to:

U.S. Environmental Protection Agency District Chief

(USEPA)/National Service Center for U.S. Geological Survey

Environmental Publications (NSCEP) Stephenson Center–Suite 129

Box 42419 720 Gracern Road

Cincinnati, OH 45242-0419 Columbia, SC 29210-7651

and

U.S. Geological Survey Additional information about water

Branch of Information Services resources in South Carolina is

Box 25286 available on the World Wide Web

Denver, CO 80225 at http://sc.water.usgs.gov

Phone: 888-ASK-USGS

Report can be downloaded from http://www.itrcweb.org and http://www.frtr.gov
Contents III

CONTENTS

Executive Summary ................................................................................................................................................................. 1

Introduction .............................................................................................................................................................................. 3

Summary of Passive Diffusion Bag Sampler Advantages and Limitations .....................................................................5

Advantages .............................................................................................................................................................5

Limitations..............................................................................................................................................................5

Passive Diffusion Bag Sampler Deployment .............................................................................................................................6

Passive Diffusion Bag Sampler and Sample Recovery..............................................................................................................9

Determining Applicability of Passive Diffusion Bag Samplers and Interpretation of Data ....................................................11

Influences of Hydraulic and Chemical Heterogeneity on Sample Quality in Long-Screened Wells.............................12

Comparison of Passive Diffusion Bag Sampling Methodology to Conventional Methodologies.................................12

Quality Control and Assurance ................................................................................................................................................14

Summary ..................................................................................................................................................................................14

References ................................................................................................................................................................................16

Figures

1. Photo showing typical water-filled passive diffusion bag samplers used in wells, including diffusion 

bag with polyethylene mesh, diffusion bag without mesh, and bag and mesh attached to bailer bottom ........................4

2. Photo showing example of multiple passive diffusion bag samplers prepared for deployment ...........................................8

Table

1. Compounds tested under laboratory conditions for use with passive diffusion bag samplers..............................................4
IV Contents

Conversion Factors, Vertical Datum, Acronyms, and Abbreviations

Temperature is given in degrees Celsius (°C ), which can converted to degrees Fahrenheit (°F) by the following equation: °F = 9/5 (°C) + 32

Sea level refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)—a geodetic datum derived from a general adjustment of 

the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929.

Chemical concentration in water is expressed in metric units as milligrams per liter (mg/L) or micrograms per liter (μg/L).

Multiply By To obtain

Length

inch (in.) 25.4 millimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

square mile (mi2

) 2.590 square kilometer

Flow

foot per day (ft/d) 0.3048 meter per day

foot squared per day (ft2

/d) 0.09294 meter squared per day

gallon per minute (gal/min) 0.06308 liter per second

gallon per day (gal/d) 0.003785 cubic meter per day

inch per year (in/yr) 25.4 millimeter per year

Volume

gallon (gal) 3.785 liter

Additional Abbreviations

EDB 1,2-Dibromomethane

AFCEE Air Force Center for Environmental Excellence

cDCE cis-1,2-Dibromoethene

ft3

/d cubic feet per day

ft3/mg cubic feet per milligram

°C degrees Celsius

g gram

ITRC Interstate Technology Regulatory Cooperation

LDPE low-density polyethylene

L liter

μg microgram

μm micrometer

μL microliter

mg milligram

mL milliliter

mL/min milliliter per minute

MTBE Methyl-tert-butyl ether

NAVFAC Naval Facilities Engineering Command

NAPL non-aqueous phase liquid

PDB passive diffusion bag

PCE Tetrachloroethene

TCE Trichloroethene

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VOA Volatile organic analysis

VOC Volatile organic compound
Executive Summary 1

User’s Guide for Polyethylene-Based Passive Diffusion 

Bag Samplers to Obtain Volatile Organic Compound 

Concentrations in Wells 

Part 1: Deployment, Recovery, Data Interpretation, and 

Quality Control and Assurance

By Don A. Vroblesky

EXECUTIVE SUMMARY

Water-filled passive diffusion bag (PDB) samplers 

described in this report are suitable for obtaining concentrations of a variety of volatile organic compounds 

(VOCs) in ground water at monitoring wells. The suggested application of the method is for long-term monitoring of VOCs in ground-water wells at wellcharacterized sites. 

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 intrabore-hole flow, multiple intervals of the formation 

contributing to flow, or varying concentrations of 

VOCs vertically within the screened or open interval, 

then a multiple deployment of PDB samplers within a 

well may be more appropriate for sampling the well. 

A typical PDB sampler consists of a low-density 

polyethylene (LDPE) lay-flat tube closed at both ends 

and containing deionized water. The sampler is positioned at the target horizon of the well by attachment to 

a weighted line or fixed pipe. 

The amount of time that the sampler should be 

left 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. The rate that the water within the 

PDB sampler equilibrates with ambient water depends 

on multiple factors, including the type of compound 

being sampled and the water temperature. The 

concentrations of benzene, cis-1,2-dichloroethene, 

tetrachlorethene, trichloroethene, toluene, naphthalene, 

1,2-dibromoethane, and total xylenes within the PDB 

samplers equilibrated with the concentrations in an 

aqueous mixture of those compounds surrounding 

the samplers under laboratory conditions within 

approximately 48 hours at 21 degrees Celsius (°C). 

A subsequent laboratory study of mixed VOCs at 10 °C 

showed that tetrachloroethene and trichloroethene were 

equilibrated by about 52 hours, but other compounds 

required longer equilabration times. Chloroethane, 

cis-1,2-dichloroethene, trans-1,2-dichloroethene, and 

1,1-dichloroethene were not equilibrated at 52 hours, 

but appeared to be equilibrated by the next sampling 

point at 93 hours. Vinyl chloride, 1,1,1-trichloroethane, 

1,2-dichloroethane, and 1,1-dichloroethane were not 

equilibrated at 93 hours, but were equilibrated by the 

next sampling point at 166 hours. Different equilibration times may exist for other compounds. Differences 

in equilibration times, if any, between single-solute or 

mixed VOC solutions have not yet been thoroughly 

examined. 

The samplers should be left in place long enough 

for the well water, contaminant distribution, and flow 

dynamics to restabilize following sampler deployment. 

Laboratory and field data suggest that 2 weeks of equilibration probably is adequate for many applications; 

therefore, a minimum equilibration time of 2 weeks is 

suggested. In less permeable formations, longer equilibration times may be required. When applying PDB 

samplers in waters colder than previously tested 

(10 °C) or for compounds without sufficient corroborating data, a side-by-side comparison with conventional methodology is advisable to justify the field 

equilibration time. 
2 User’s Guide for Polyethylene-Based Passive Diffusion Bag Samplers to Obtain Volatile Organic Compound 

Concentrations in Wells—Part 1: Deployment, Recovery, Data Interpretation, and Quality Control and Assurance

Following the initial equilibration period, the 

samplers maintain equilibrium concentrations with the 

ambient water until recovery. Thus, there is no specified 

time for sampler recovery after initial equilibration. 

PDB samplers routinely have been left in ground waters 

having concentrations of greater than 500 parts per 

million (ppm) of trichloroethene for 3 months at a time 

with no loss of bag integrity, and at one site, the PDB 

samplers have been left in place in VOC-contaminated 

ground water for 1 year with no reported loss of sampler 

integrity. The effects of long-term (greater than 1 month) 

PDB-sampler deployment on sampler and sample integrity have not yet been thoroughly tested for a broad 

range of compounds and concentrations, however. 

Moreover, in some environments, development of a 

biofilm on the polyethylene may be a consequence of 

long-term deployment. Investigations of semipermeable 

membrane devices (SPMDs) have shown that the transfer of some compounds across a heavily biofouled polyethylene membrane may be reduced, but not stopped. 

If a heavy organic coating is observed on a PDB 

sampler, it is advisable to determine the integrity of the 

sample by comparison to a conventional sampling 

method before continuing to use PDB samplers for 

long-term deployment in that well. 

Recovery consists of removing the samplers 

from the well and immediately transferring the 

enclosed water to 40-milliliter sampling vials for analysis. The resulting concentrations represent an integration of chemical changes over the most recent portion 

of the equilibration period (approximately 48 to 166 

hours, depending on the water temperature and the 

type of compound). 

The method has both advantages and limitations 

when compared to other sampling methods. Advantages include the potential for PDB samplers to eliminate or substantially reduce the amount of purge water 

associated with sampling. The samplers are relatively 

inexpensive and easy to deploy and recover. Because 

PDB samplers are disposable, there is no downhole 

equipment to be decontaminated between wells, and 

there is a minimum amount of field equipment 

required. The samplers also have the potential to 

delineate contaminant stratification in the formation 

across the open or screened intervals of monitoring 

wells where vertical hydraulic gradients are not 

present. In addition, the samplers integrate concentrations over time, which may range between about 

48 to 166 hours depending on the compound of 

interest. 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. In addition, 

none of the data collected suggest that VOCs leach 

from the LDPE material, or that there is a detrimental 

effect on the VOC sample from the PDB material. 

Water-filled polyethylene PDB samplers are not 

appropriate for all compounds. The samplers are not 

suitable for inorganic ions and have a limited applicability for non-VOCs and for some VOCs. For example, 

although methyl-tert-butyl ether and acetone and most 

semivolatile compounds are transmitted through the 

polyethylene bag, laboratory tests have shown that the 

resulting concentrations were lower than in ambient 

water. A variety of factors influence the ability of 

compounds to diffuse through the polyethylene. These 

factors include the molecular size and shape and the 

hydrophobic nature of the compound. Unpublished laboratory test data of semivolatile compounds in contact 

with PDB samplers showed a higher concentration of 

phthalates inside the PDB sampler than outside the 

PDB sampler, suggesting that the polyethylene may 

contribute phthalates to the enclosed water. Thus, the 

samplers should not be used to sample for phthalates.

VOC concentrations in PDB samplers represent 

concentrations in the vicinity of the sampler within the 

well screen or open interval. This may be a limitation 

for PDB samplers and some other types of sampling, 

such as low-flow sampling, if the ground-water 

contamination is above or below the screen or not in 

the sample intervals providing water movement to the 

PDB samplers. If there is a vertical hydraulic gradient 

in the well, then the concentrations in the sampler may 

represent the concentrations in the water flowing vertically past the sampler rather than in the formation 

directly adjacent to the sampler. Vertically spaced 

multiple PDB samplers may be needed in chemically 

stratified wells or where flow patterns through the 

screen change as a result of ground-water pumping or 

seasonal water-level fluctuations. 

The purposes of this document are to present 

methods for PDB sampler deployment, and recovery; 

to discuss approaches to determine the applicability of 

passive diffusion samplers; and to discuss various 

factors influencing interpretation of the data. The 

intended audience for the methodology sections of this 

report is managers and field personnel involved in using 

PDB samplers. The discussion of passive diffusion 

sampler applicability and interpretation of the data is 
Introduction 3

suited for project managers, technical personnel, and the 

regulatory community. Part 2 of this report presents case 

studies of PDB sampler field applications.

INTRODUCTION

The use of PDB samplers for collecting groundwater samples from wells offers a cost-effective 

approach to long-term monitoring of VOCs at wellcharacterized sites (Vroblesky and Hyde, 1997; Gefell 

and others, 1999). 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 

VOCs vertically within the screened or open interval, 

then deployment of multiple PDB samplers within a 

well may be more appropriate for sampling the well. 

The samplers consist of deionized water 

enclosed in a LDPE sleeve (fig. 1) and are deployed 

adjacent to a target horizon within a screened or open 

interval of a well. The suggested application is for 

long-term monitoring of VOCs in ground-water wells. 

Where the screened interval is greater than 10 feet (ft), 

the potential for contaminant stratification and/or intraborehole flow within the screened interval is greater 

than in screened intervals shorter than 10 ft. 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. For many 

VOCs of environmental interest (table 1), the VOC 

concentration in water within the sampler approaches 

the VOC concentration in water outside of the PDB 

sampler over an equilibration period. The resulting 

concentrations represent an integration of chemical 

changes over the most recent part of the equilibration 

period (approximately 48 to 166 hours, depending on 

the water temperature and the type of compound being 

sampled). The approach is inexpensive and has the 

potential to eliminate or substantially reduce the 

amount of purge water removed from the well. 

A variety of PDB samplers have been utilized in 

well applications (fig. 1). Although the samplers vary 

in specific construction details, a typical PDB sampler 

consists of a 1- to 2-ft-long LDPE tube closed at both 

ends and containing laboratory-grade deionized water 

(fig. 1). The typical diameter for PDB samplers used in 

a 2-inch-diameter well is approximately 1.2 inches; 

however, other dimensions may be used to match the 

well diameter. Equilibration times may be longer for 

larger diameter PDB samplers. On the outside of the 

PDB sampler, a low-density polyethylene-mesh sometimes is used for protection against abrasion in open 

boreholes and as a means of attachment at the prescribed depth. The PDB sampler can be positioned at 

the target horizon by attachment to a weighted line or 

by attachment to a fixed pipe. 

PDB samplers for use in wells are available 

commercially. Authorized distributors as of March 

2001 are Columbia Analytical Services (800-695-7222) 

and Eon Products (800-474-2490). A current list of 

vendors and PDB-sampler construction details can be 

obtained from the U.S. Geological Survey Technology 

Transfer Enterprise Office, Mail Stop 211, National 

Center, 12201 Sunrise Valley Drive, Reston, Virginia 

20192 (telephone 703-648-4344; fax 703-648-4408). 

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 purchase a nonexclusive license for sampler construction from the U.S. 

Geological Survey Technology Enterprise Office at the 

above address.

The purposes of this document are to present 

methods for PDB sampler deployment, and recovery; 

to discuss approaches for determining the applicability 

of passive diffusion samplers; and to discuss various 

factors influencing interpretation of the data. The 

intended audience for the methodology sections of this 

report is 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 this report presents case 

studies of PDB-sampler field applications. 
4 User’s Guide for Polyethylene-Based Passive Diffusion Bag Samplers to Obtain Volatile Organic Compound Concentrations in

Wells—Part 1: Deployment, Recovery, Data Interpretation, and Quality Control and Assurance

Figure 1. Typical water-filled passive

diffusion bag samplers used in wells,

including (A) diffusion bag with

polyethylene mesh, (B) diffusion bag 

without mesh, and (C) bag and mesh

attached to bailer bottom.

   

 

	 
	 



 

 
 

	 

 	  	



	  	

[

Table 1. Compounds tested under laboratory conditions for use with passive diffusion bag samplers

From Vroblesky and Campbell, 2001]

Tested compounds showing good correlation (average differences in concentration of 11 percent or less

between diffusion-sampler water and test-vessel water) in laboratory tests

Benzene 2 Chlorovinyl ether cis-1,2-Dichloroethene 1,1,1-Trichloroethane

Bromodichloromethane Dibromochloromethane trans-1,2-Dichloroethene 1,1,2-Trichloroethane

Bromoform Dibromomethane 1,2-Dichloropropane Trichloroethene

Chlorobenzene 1,2-Dichlorobenzene cis-Dichloropropene Trichlorofluoromethane

Carbon tetrachloride 1,3-Dichlorobenzene 1,2-Dibromoethane 1,2,3-Trichloropropane

Chloroethane 1,4-Dichlorobenzene trans-1,3-Dichloropropene 1,1,2,2-Tetrachloroethane

Chloroform Dichlorodifluoromethane Ethyl benzene Tetrachloroethene

Chloromethane 1,2-Dichloroethane Naphthalene Vinyl chloride

1,1-Dichloroethene Toluene Total xylenes

Tested compounds showing poor correlation (average differences in concentration greater than 20 percent 

between diffusion-sampler water and test-vessel water) in laboratory tests

Acetone* Methyl-tert-butyl ether Styrene

*T.M Sivavec and S.S. Baghel, General Electric Company, written commun., 2000
Introduction 5

Summary of Passive Diffusion Bag Sampler 

Advantages and Limitations

Advantages

1. PDB samplers have the potential to eliminate 

or substantially reduce the amount of purge water associated with sampling.

2. PDB samplers are inexpensive.

3. The samplers are easy to deploy and recover.

4. Because PDB samplers are disposable, there is no 

downhole equipment to be decontaminated between wells.

5. A minimal amount of field equipment is required.

6. 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 (that 

is, residential communities, business districts, or busy 

streets where vehicle traffic control is a concern). 

7. Multiple PDB samplers, distributed vertically 

along the screened or open interval, may 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. Analytical costs when 

using multiple PDB samplers sometimes can be reduced 

by selecting a limited number of the samplers for laboratory analysis based on screening by using field gas chromatography at the time of sample collection. 

8. 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. In addition, none 

of the data collected suggest that VOCs leach from the 

LDPE material or that there is a detrimental effect from 

the PDB material on the VOC sample. 

Limitations

1. PDB samplers integrate concentrations over 

time. This may 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. Laboratory 

results obtained indicate that a variety of compounds 

equilibrated within 48 hours at 21 °C (Vroblesky and 

Campbell, 2001). Vinyl chloride, 1,1,1-trichloroethane, 

1,2-dichloroethane, and 1,1-dichloroethane may require 

between 93 and 166 hours to equilibrate at 10 °C 

(T.M. Sivavec and S.S. Baghel, General Electric 

Company, written commun., 2000). The initial equilibration under field conditions may be longer to allow 

well water, contaminant distribution, and flow dynamics 

to restabilize following sampler deployment. 

2. Water-filled polyethylene PDB samplers are 

not appropriate for all compounds. For example, 

although methyl-tert-butyl ether and acetone 

(Vroblesky, 2000; Paul Hare, General Electric 

Company, oral commun., 2000) and most semivolatile 

compounds are transmitted through the polyethylene 

bag, laboratory tests have shown that the resulting 

concentrations were lower than in ambient water. 

A variety of factors influence the ability of compounds 

to diffuse through the polyethylene membrane. These 

factors include the molecular size and shape and the 

hydrophobic nature of the compound. Compounds 

having a cross-sectional diameter of about 10 

angstroms or larger (such as humic acids) do not pass 

through the polyethylene because the largest (transient) 

pores in polyethylene do not exceed about 10 angstroms 

in diameter (Flynn and Yalkowsky, 1972; Hwang and 

Kammermeyer, 1975; Comyn, 1985). The samplers are 

not appropriate for hydrophilic polar molecules, such as 

inorganic ions. A detailed discussion of the relation 

between hydrophobicity and compound transport 

through polyethylene can be found in Gale (1998). 

Unpublished laboratory test data (D.A. Vroblesky, U.S. 

Geological Survey, written commun., 1998) of semivolatile compounds in contact with PDB samplers 

showed a higher concentration of phthalates inside the 

PDB sampler than outside the PDB sampler, suggesting 

that the polyethylene may contribute phthalates to the 

enclosed water. Thus, the samplers should not be used 

to sample for phthalates.

3. PDB samplers rely on the free movement of 

water through the well screen. In situations where 

ground water flows horizontally through the well screen, 

the VOC concentrations in the open interval of the well 

probably are representative of the aquifer water in the 

adjacent formation (Gillham and others, 1985; Robin 

and Gillham, 1987; Kearl and others, 1992; Powell and 

Puls, 1993; Vroblesky and Hyde, 1997). In these situations, the VOC concentration of the water in contact 

with the PDB samplers, and therefore, the water within 

the diffusion samplers, probably represents local conditions in the adjacent aquifer. However, if the well screen 

is less permeable than the aquifer or the sandpack, then 

under ambient conditions, flowlines may be diverted 

around the screen. Such a situation may arise from inadequate well development or from iron bacterial fouling 

of the well screen. In this case, the VOC concentrations 

in the PDB samplers may not represent concentrations in 
6 User’s Guide for Polyethylene-Based Passive Diffusion Bag Samplers to Obtain Volatile Organic Compound 

Concentrations in Wells—Part 1: Deployment, Recovery, Data Interpretation, and Quality Control and Assurance

the formation water because of inadequate exchange 

across the well screen. PDB samplers have not yet been 

adequately tested to determine their response under 

such conditions.

4. VOC concentrations in PDB samplers represent 

ground-water 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 

ground-water 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.

5. In cases where the well screen or open interval transects zones of differing hydraulic head and 

variable contaminant concentrations, VOC concentrations obtained using a PDB sampler may not reflect 

the concentrations in the aquifer directly adjacent to 

the sampler because of vertical transport in the well. 

However, a vertical array of PDB samplers, used in 

conjunction with borehole flow meter testing, can 

provide insight on the movement of contaminants into 

or out of the well. This information then can be used to 

help determine if the use of PDB samplers is appropriate for the well, and to select the optimal vertical 

location(s) for the sampler deployment. 

6. 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 

may not provide accurate concentration values for the 

most contaminated zone. However, multiple PDB 

samplers distributed vertically along the screened or 

open interval, in conjunction with pump sampling 

(as appropriate), can be used to locate zone(s) of highest concentration in the well. Multiple PDB samplers 

also may be needed to track the zone of maximum 

concentration in wells where flow patterns through the 

screened interval change as a result of ground-water 

pumping or seasonal water-table fluctuations.

PASSIVE DIFFUSION BAG SAMPLER 

DEPLOYMENT

A variety of approaches can be used to deploy 

the PDB samplers in wells. A typical deployment 

approach, described in this section, is to attach the 

PDB samplers to a weighted line. It also is acceptable 

to attach the weights directly to the PDB sampler if the 

attachment point is of sufficient strength to support the 

weight. The weights attached to the bottom of the 

line are stainless steel and can be reused, but must be 

thoroughly decontaminated with a detergent before the 

first use or before using in a different well. Rope, such 

as 90 pound, 3/16 inch braided polyester, can be used 

as the line for single-use applications if it is of sufficient strength to support the weight and sampler, is 

nonbuoyant, and is subject to minimal stretch; however, the rope should not be reused because of the high 

potential for cross contamination. Stainless-steel or 

Teflon-coated stainless-steel wire is preferable. The 

weighted lines should not be reused in different wells 

to prevent carryover of contaminants. A possible 

exception is coated stainless-steel wire, which can be 

reused after sufficient decontamination. An alternative 

deployment approach, not discussed in this section, is 

to attach the PDB samplers to a fixed pipe in the well 

(Vroblesky and Peters, 2000, p. 3; also included in Part 2 

of this publication). The PDB samplers should not contact non-aqueous phase liquid (NAPL) during deployment or retrieval to prevent cross contamination. An 

approach that can be utilized to deploy diffusion samplers through a layer of floating NAPL is described in 

the field test at Naval Station North Island, California 

(Vroblesky and Peters, 2000, p. 3-4; also included in 

Part 2 of this publication).

If the PDB sampler is to be compared with a 

conventional pumping approach to sampling, then it is 

suggested that both the pump and the PDB sampler be 

deployed at the same time, with the sampler attached 

near (such as directly below) the pump inlet. This 

approach eliminates potential concentration differences 

between the two methods that may result from well 

disturbance during equipment removal and deployment at the time of sampling. An alternative method is 

to deploy the PDB samplers independently of the 

pumps and recover the samplers immediately prior to 

placing the pump down the well.

PDB samplers are available either prefilled with 

laboratory-grade deionized water or unfilled. The 

unfilled samplers are equipped with a plug and funnel 

to allow for field filling and sample recovery. To fill 

these samplers, remove the plug from the sampler bottom, insert the short funnel into the sampler, and pour 

laboratory-grade deionized water into the sampler. The 

sampler should be filled until water rises and stands at 

least half way into the funnel. Remove excess bubbles 

from the sampler. Remove the funnel and reattach 

the plug. A small air bubble from the plug is of no 

concern.
Passive Diffusion Bag Sampler Deployment 7

The following steps should be used for deploying 

PDB samplers in wells:

1. Measure the well depth and compare the 

measured depth with the reported depth to the bottom 

of the well screen from well-construction records. This 

is to check on whether sediment has accumulated in the 

bottom of the well, whether there is a nonscreened 

section of pipe (sediment sump) below the well screen, 

and on the accuracy of well-construction records. If 

there is an uncertainty regarding length or placement of 

the well screen, then an independent method, such as 

video imaging of the well bore, is strongly suggested.

2. Attach a stainless-steel weight to the end of the 

line. Sufficient weight should be added to counterbalance the buoyancy of the PDB samplers. This is 

particularly important when multiple PDB samplers are 

deployed. One approach, discussed in the following 

paragraphs, is to have the weight resting on the bottom 

of the well, with the line taut above the weight. Alternatively, the PDB sampler and weight may be suspended 

above the bottom, but caution should be exercised to 

ensure that the sampler does not shift location. Such 

shifting can result from stretching or slipping of the line 

or, if multiple samplers are attached end-to-end rather 

than to a weighted line, stretching of the samplers.

3. Calculate the distance from the bottom of the 

well, or top of the sediment in the well, up to the point 

where the PDB sampler is to be placed. A variety of 

approaches can be used to attach the PDB sampler to 

the weight or weighted line at the target horizon. The 

field-fillable type of PDB sampler is equipped with a 

hanger assembly and weight that can be slid over the 

sampler body until it rests securely near the bottom of 

the sampler. When this approach is used with multiple 

PDB samplers down the same borehole, the weight 

should only be attached to the lowermost sampler. 

An additional option is to use coated stainless-steel 

wire as a weighted line, making loops at appropriate 

points to attach the upper and lower ends of PDB 

samplers. Where the PDB sampler position varies 

between sampling events, movable clamps with rings 

can be used. When using rope as a weighted line, a 

simple approach is to tie knots or attach clasps at the 

appropriate depths. Nylon cable ties or stainless-steel 

clips inserted through the knots can be used to attach 

the PDB samplers. An approach using rope as a 

weighted line with knots tied at the appropriate 

sampler-attachment points is discussed below. 

(a) For 5-ft-long or shorter well screens, the 

center point of the PDB sampler should be the 

vertical midpoint of the saturated well-screen 

length. For example, if the well screen is at a 

depth of 55 to 60 ft below the top of casing, and 

the measured depth of the well is 59 ft, then the 

bottom of the well probably has filled with sediment. In this case, the midpoint of the sampler 

between the attachment points on the line will be 

midway between 55 and 59 ft, or at 57 ft. Thus, 

for a 1.5-ft-long sampler, the attachment points 

on a weighted line should be tied at distances of 

1.25 ft (2 ft – 0.75 ft) and 2.75 ft (2 ft + 0.75 ft) 

from the top of the sediment in the well, or the 

bottom of the well, making adjustments for the 

length of the attached weight. When the PDB 

sampler is attached to the line and installed in the 

well, the center of the sampler will be at 57-ft 

depth. If, however, independent evidence is 

available showing that the highest concentration 

of contaminants enters the well from a specific 

zone within the screened interval, then the PDB 

sampler should be positioned at that interval.

(b) For 5- to 10-ft-long well screens, it is 

advisable to utilize multiple PDB samplers vertically along the length of the well screen for at 

least the initial sampling (fig. 2). The purposes of 

the multiple PDB samplers are to determine 

whether contaminant stratification is present and 

to locate the zone of highest concentration. The 

midpoint of each sampler should be positioned at 

the midpoint of the interval to be sampled. For 

1.5-ft-long samplers, at each sampling depth in 

the screened interval, make two attachment 

points on the weighted line at a distance of about 

1.5 ft apart. The attachment points should be 

positioned along the weighted line at a distance 

from the bottom end of the weight such that the 

midpoint between the knots will be at the desired 

sampling depth along the well screen. Sampler 

intervals are variable, but a simple approach is to 

use the top knot/loop of one sampler interval as 

the bottom knot/loop for the overlying sampler 

interval.
8 User’s Guide for Polyethylene-Based Passive Diffusion Bag Samplers to Obtain Volatile Organic Compound 

Concentrations in Wells—Part 1: Deployment, Recovery, Data Interpretation, and Quality Control and Assurance

(c) PDB samplers should not be used in wells 

having screened or open intervals longer than 

10 ft unless used in conjunction with borehole 

flow meters or other techniques to characterize 

vertical variability in hydraulic conductivity and 

contaminant distribution or used strictly for 

qualitative reconnaissance purposes. This is 

because of the increased potential for cross contamination of water-bearing zones and hydraulically driven mixing effects that may cause the 

contaminant stratification in the well to differ 

from the contaminant stratification in the adjacent aquifer material. If it is necessary to sample 

such wells, then multiple PDB samplers should 

be installed vertically across the screened or 

open interval to determine the zone of highest 

concentration and whether contaminant stratification is present. 

4. The samplers should be attached to the 

weights or weighted line at the time of deployment. 

For samplers utilizing the hanger and weight assembly, 

the line can be attached directly to the top of the 

sampler. PDB samplers utilizing an outer protective 

mesh can be attached to a weighted line by using the 

following procedure:

(a) Insert cable ties through the attachment 

points in the weighted line.

(b) At each end of the PDB sampler, weave 

the ends of the cable ties or clamp through the 

LPDE mesh surrounding the sampler and tighten 

the cable ties. Thus, each end of the PDB 

sampler will be attached to a knot/loop in the 

weighted line by means of a cable tie or clamp. 

The cable ties or clamps should be positioned 

through the polyethylene mesh in a way that 

prevents the PDB sampler from sliding out of the 

mesh.

(c) Trim the excess from the cable tie before 

placing the sampler down the well. Caution 

should be exercised to prevent sharp edges on 

the trimmed cable ties that may puncture the 

LDPE.

Figure 2. Example of multiple PDB 

samplers prepared for deployment.
Passive Diffusion Bag Sampler and Sample Recovery 9

5. When using PDB samplers without the protective outer mesh, the holes punched at the ends of the 

bag, outside the sealed portion, can be used to attach 

the samplers to the weighted line. Stainless-steel spring 

clips have been found to be more reliable than cable 

ties in this instance, but cable ties also work well.

6. Lower the weight and weighted line down the 

well until the weight rests on the bottom of the well 

and the line above the weight is taut. The PDB 

samplers should now be positioned at the expected 

depth. A check on the depth can be done by placing a 

knot or mark on the line at the correct distance from the 

top knot/loop of the PDB sampler to the top of the well 

casing and checking to make sure that the mark aligns 

with the lip of the casing after deployment.

7. Secure the assembly in this position. A suggested method is to attach the weighted line to a hook 

on the inside of the well cap. Reattach the well cap. 

The well should be sealed in such a way as to prevent 

surface-water invasion. This is particularly important 

in flush-mounted well vaults that are prone to flooding. 

8. Allow the system to remain undisturbed as the 

PDB samplers equilibrate. 

PASSIVE DIFFUSION BAG SAMPLER AND 

SAMPLE RECOVERY

The amount of time that the samplers should be 

left 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 environmental 

disturbances caused by sampler deployment to return to 

ambient conditions. The rate that the water within the 

PDB sampler equilibrates with ambient water depends 

on multiple factors, including the type of compound 

being sampled and the water temperature. The concentrations of benzene, cis-1,2-dichloroethene (cDCE), 

tetrachlorethene (PCE), trichloroethene (TCE), toluene, naphthalene, 1,2-dibromoethane (EDB), and total 

xylenes within the PDB samplers equilibrated with the 

concentrations in an aqueous mixture of those 

compounds surrounding the samplers under laboratory 

conditions within approximately 48 hours at 21 °C 

(Vroblesky and Campbell, 2001). A subsequent laboratory study of mixed VOCs at 10 °C showed that PCE 

and TCE were equilibrated by about 52 hours, but other 

compounds required longer equilibration times (T.M. 

Sivavec and S.S. Baghel, General Electric Company, 

written commun., 2000). Chloroethane, cDCE, trans1,2-dichloroethene, and 1,1-dichloroethene were not 

equilibrated at 52 hours, but appeared to be equilibrated 

by the next sampling point at 93 hours. Vinyl chloride, 

1,1,1-trichloroethane, 1,2-dichloroethane, and 1,1-

dichloroethane were not equilibrated at 93 hours, but 

were equilibrated by the next sampling point at 166 

hours. Different equilibration times may exist for other 

compounds. Differences in equilibration times, if any, 

between single-solute or mixed-VOC solutions have 

not yet been thoroughly examined. 

Under field conditions, the samplers should be 

left in place long enough for the well water, contaminant distribution, and flow dynamics to restabilize following sampler deployment. The results of borehole 

dilution studies show that wells can recover to 90 percent of the predisturbance conditions within minutes to 

several hours for permeable to highly permeable geologic formations, but may require 100 to 1,000 hours 

(4 to 40 days) in muds, very fine-grained loamy sands, 

and fractured rock, and may take even longer in fractured shales, recent loams, clays, and slightly fractured 

solid igneous rocks (Halevy and others, 1967). 

In general, where the rate of ground-water 

movement past a diffusion sampler is high, equilibration times through various membranes commonly 

range from a few hours to a few days (Mayer, 1976; 

Harrington and others, 2000). One field investigation 

showed adequate equilibration of PDB samplers to 

aquifer trichloroethene (TCE) and carbon tetrachloride 

(CT) concentrations within 2 days in a highly permeable aquifer (Vroblesky and others, 1999). In other 

investigations, PDB samplers recovered after 14 days 

were found to be adequately equilibrated to chlorinated 

VOCs (Obrien & Gere Engineers, Inc., 1997a, 1997b; 

Hare, 2000); therefore, the equilibration period was 

less than or equal to 14 days for those field conditions. 

Because it appears that 2 weeks of equilibration probably is adequate for many applications, a minimum 

equilibration time of 2 weeks is suggested. When 

applying PDB samplers in waters colder than previously tested (10 °C) or for compounds without sufficient corroborating field data, a side-by-side comparison with conventional sampling methodology is 

advisable to justify the field equilibration time.

In less permeable formations, longer equilibration times may be required. It is probable that water in 

the well bore eventually will equilibrate with the porewater chemistry; however, if the rate of chemical 

change or volatilization loss in the well bore exceeds 

the rate of exchange between the pore water and the 

well-bore water, then the PDB samplers may under-
10 User’s Guide for Polyethylene-Based Passive Diffusion Bag Samplers to Obtain Volatile Organic Compound 

Concentrations in Wells—Part 1: Deployment, Recovery, Data Interpretation, and Quality Control and Assurance

estimate pore-water concentrations. Guidelines for 

equilibration times and applicability of PDB samplers 

in low-permeability formations have not yet been 

established. Therefore, in such situations, a side-byside comparison of PDB samplers and conventional 

sampling methodology is advisable to ensure that the 

PDB samplers do not underestimate concentrations 

obtained by the conventional method. A detailed 

discussion of diffusion rates relevant to diffusion 

sampler equilibrium in slow-moving ground-water 

systems can be found in Harrington and others (2000).

Following the initial equilibration period, the 

samplers maintain equilibrium concentrations with the 

ambient water until recovery. Thus, there is no specified maximum time for sampler recovery. PDB 

samplers have routinely been left in ground waters 

having concentrations of greater than 500 ppm of TCE 

for 3 months at a time with no loss of bag integrity, and 

at one site, the PDB samplers have been left in place in 

VOC-contaminated ground water for 1 year with no 

reported loss of sampler integrity (Paul Hare, General 

Electric Company, oral commun., 2000). The effects of 

long-term (greater than 1 month) PDB-sampler deployment on sampler and sample integrity have not yet 

been thoroughly tested for a broad range of compounds 

and concentrations. Moreover, in some environments, 

development of a biofilm on the polyethylene may be a 

consequence of long-term deployment. Investigations 

of semipermeable membrane devices (SPMDs) have 

shown that the transfer of some compounds may be 

reduced, but not stopped, across a heavily biofouled 

polyethylene membrane (Ellis and others, 1995; 

Huckins and others, 1996; Huckins and others, in 

press). If a heavy organic coating is observed on a 

PDB sampler, it is advisable to determine the integrity 

of the sample by comparing contaminant concentrations from the PDB sampler to concentrations from a 

conventional sampling method before continuing to use 

PDB samplers for long-term deployment in that well. 

Recovery of PDB samplers is accomplished by 

using the following approach:

1. Remove the PDB samplers from the well by 

using the attached line. The PDB samplers should not 

be exposed to heat or agitated.

2. Examine the surface of the PDB sampler for 

evidence of algae, iron or other coatings, and for tears 

in the membrane. Note the observations in a sampling 

field book. If there are tears in the membrane, the 

sample should be rejected. If there is evidence that the 

PDB sampler exhibits a coating, then this should be 

noted the validated concentration data.

3. Detach and remove the PDB sampler from the 

weighted line. Remove the excess liquid from the exterior of the bag to minimize the potential for cross 

contamination. 

4. A variety of approaches may be used to transfer the water from the PDB samplers to 40-mL volatile 

organic analysis (VOA) vials. One type of commercially available PDB sampler provides a discharge 

device that can be inserted into the sampler. If 

discharge devices are used, the diameter of the opening 

should be kept to less than about 0.15 inches to reduce 

volatilization loss. Two options are presently available 

to recover water from the sample using discharge 

devices. One option involves removing the hanger and 

weight assembly from the sampler, inverting the 

sampler so that the fill plug is pointed upward, and 

removing the plug. The water can be recovered by 

directly pouring in a manner that minimizes agitation 

or by pouring through a VOC-discharge accessory 

inserted in place of the plug. The second approach 

involves piercing the sampler near the bottom with a 

small-diameter discharge tube and allowing water to 

flow through the tube into the VOA vials. In each case, 

flow rates can be controlled by tilting or manipulating 

the sampler. Alternatively, the PDB sampler can be cut 

open at one end using scissors or other cutting devices 

which have been decontaminated between use for 

different wells. Water can then be transferred to 40-ml 

VOA vials by gently pouring in a manner that minimizes water agitation. Acceptable duplication has been 

obtained using each method. Preserve the samples 

according to the analytical method. The sampling vials 

should be stored at approximately 4 °C in accordance 

with standard sampling protocol. Laboratory testing 

suggests that there is no substantial change in the VOC 

concentrations in PDB samplers over the first several 

minutes after recovery; however, the water should be 

transferred from the water-filled samplers to the 

sample bottles immediately upon recovery.

5. A cost-effective alternative when using multiple PDB samplers in a single well is to field screen 

water from each sampler using gas chromatography. 

These results can be used to decide which of the multiple PDB samplers should be sent to an EPA-approved 

laboratory for standard analysis. Typically, at least the 

sample containing the highest concentration should be 

analyzed by a laboratory.
Determining Applicability of Passive Diffusion Bag Samplers, and Interpretation of Data 11

6. If a comparison is being made between 

concentrations obtained using PDB samplers and 

concentrations obtained using a conventional sampling 

approach, then the well should be sampled by the 

conventional approach soon after (preferably on the 

same day) recovery of the PDB sampler. The water 

samples obtained using PDB samplers should be sent 

in the same shipment, as the samples collected by the 

conventional approach for the respective wells. Utilizing 

the same laboratory may reduce analytical variability.

7. Any unused water from the PDB sampler and 

water used to decontaminate cutting devices should be 

disposed in accordance with local, state, and Federal 

regulations.

DETERMINING APPLICABILITY OF PASSIVE 

DIFFUSION BAG SAMPLERS AND 

INTERPRETATION OF DATA

When attempting to determine whether the use 

of PDB samplers is appropriate at a particular well, a 

common approach is to do a side-by-side comparison 

with a conventional sampling method during the same 

sampling event. This approach is strongly suggested in 

wells having temporal concentration variability. In a 

well having relatively low temporal concentration variability, comparison of the PDB-sampler results to 

historical concentrations may provide enough information to determine whether the PDB samplers are 

appropriate for the well. In general, if both PDB and 

conventional sampling produce concentrations that 

agree within a range deemed acceptable by local, 

state, and Federal regulatory agencies and meet the 

site-specific data-quality objectives, then a PDB 

sampler may be approved for use in that well to monitor ambient VOC concentrations. If concentrations 

from the PDB sampler are higher than concentrations 

from the conventional method, it is probable that 

concentrations from the PDB sampler adequately 

represent ambient conditions because there usually is a 

greater potential for dilution from mixing during 

sampling using conventional methods than during 

sampling using PDB samplers. 

If, however, the conventional method produces 

concentrations that are significantly higher than those 

obtained using the PDB sampler, then it is uncertain 

whether the PDB-sampler concentrations represent 

local ambient conditions. In this case, further testing 

can be done to determine whether contaminant stratification and/or intra-borehole flow is present. Multiple 

sampling devices can be used to determine the presence of contaminant stratification, and borehole flowmeters can be used to determine whether intraborehole flow is present. When using flowmeters to 

measure vertical flow in screened boreholes, however, 

the data should be considered qualitative because of 

the potential for water movement through the sand 

pack. Borehole dilution tests (Halevy and others, 1967; 

Drost and others, 1968; Grisak and others, 1977; 

Palmer, 1993) can be used to determine whether water 

is freely exchanged between the aquifer and the well 

screen. 

Once the source of the difference between the 

two methods is determined, a decision can be made 

regarding the well-specific utility of the PDB samplers. 

Tests may show that VOC concentrations from the 

PDB samplers adequately represent local ambient 

conditions within the screened interval despite the 

higher VOC concentration obtained from the conventional method. This may be because the pumped 

samples incorporated water containing higher concentrations either from other water-bearing zones induced 

along inadequate well seals or through fractured clay 

(Vroblesky and others, 2000), from other water-bearing zones not directly adjacent to the well screen as a 

result of well purging prior to sampling (Vroblesky and 

Petkewich; 2000), or from mixing of chemically stratified zones in the vicinity of the screened interval 

(Vroblesky and Peters, 2000). 

The mixing of waters from chemically stratified 

zones adjacent to the screened interval during pumping 

probably is one of the more important sources of 

apparent differences between the results obtained from 

PDB sampling and conventional sampling because 

such stratification probably is common. Vertical stratification of VOCs over distances of a few feet has been 

observed in aquifer sediments by using multilevel 

sampling devices (Dean and others, 1999; Pitkin and 

others, 1999), and considerable variation in hydraulic 

conductivity and water chemistry has been observed in 

an aquifer in Cape Cod, Massachusetts, on the scale of 

centimeters (Wolf and others, 1991; Smith and others 

1991; Hess and others, 1992). Multiple PDB samplers 

have been used to show a change in TCE concentration 

of 1,130 (μg/L over a 6-ft vertical screened interval in 

Minnesota (Vroblesky and Petkewich, 2000). Tests 

using PDB samplers in screened intervals containing 

VOC stratification showed that the PDB-sampler data 

appeared to be point-specific, whereas the pumped 

sample integrated water over a larger interval (Vroblesky 

and Peters, 2000). 
12 User’s Guide for Polyethylene-Based Passive Diffusion Bag Samplers to Obtain Volatile Organic Compound 

Concentrations in Wells—Part 1: Deployment, Recovery, Data Interpretation, and Quality Control and Assurance

The decision on whether to use PDB samplers in 

such situations depends on the data-quality objectives 

for the particular site. If the goal is to determine and 

monitor higher concentrations or to examine contaminant stratification within the screened interval, then 

the PDB samplers may meet this objective. If the goal 

is to determine the average concentrations for the 

entire screened interval, then a pumped sample or an 

average from multiple diffusion samplers may be 

appropriate.

As an aid in the decision-making process, the 

following section examines the influences that hydraulic and chemical heterogeneity of an aquifer can have 

on sample quality in long-screened wells. Because 

VOC concentrations from PDB samplers commonly 

are compared to VOC concentrations from other 

sampling methodologies, the second section examines 

the differences in sample quality between these methodologies in situations of hydraulic and chemical 

heterogeneity.

Influences of Hydraulic and Chemical 

Heterogeneity on Sample Quality in 

Long-Screened Wells

Sampling biases and chemical variability in 

long-screened wells, which can be loosely defined as 

wells having significant physical and chemical heterogeneity within the screened interval and in the adjacent aquifer (Reilly and Leblanc, 1998), have been the 

subject of numerous investigations. Sources of chemical variability in such wells include non-uniform flow 

into wells (Robbins and Martin-Hayden, 1991; Reilly 

and Gibs, 1993; Chiang and others, 1995; Church and 

Granato, 1996; Reilly and LeBlanc, 1998), lithologic 

heterogeneity (Reilly and others, 1989; Robbins, 1989; 

Martin-Hayden and others, 1991; Gibs and others, 

1993; Reilly and Gibs, 1993), and in-well mixing. 

In a well open across a chemically or hydraulically 

heterogeneous section of the aquifer, differences in 

the sampling methodology can produce significant 

differences in the sampling results. 

Long-screened wells have the potential to 

redistribute chemical constituents in the aquifer 

where there are vertical hydraulic gradients within the 

screened interval. Water can move into the well from 

one horizon and exit the well at a different horizon 

(Church and Granato, 1996; Reilly and LeBlanc 1998). 

If there is vertical flow in the screened or open interval, and the zone of low hydraulic head (outflow from 

the well) is within the contaminated horizon, then the 

PDB samplers (or any standard sampling methodology) can underestimate or not detect the contamination. The reason is that, in this case, the contaminated 

horizon does not contribute water to the well under 

static conditions. Instead, water from other horizons 

with higher hydraulic head will invade the contaminated horizon by way of the well screen. Under 

pumped conditions, the majority of the extracted water 

will be from the most permeable interval, which may 

not be the contaminated zone. Even when pumping 

induces inflow from the contaminated interval, much 

of that inflow will be a reflection of the residual 

invaded water from other horizons. In this situation, 

a substantial amount of purging would be required 

before water representative of the aquifer could be 

obtained (Jones and Lerner, 1995). Such sampling is 

not likely to reflect a significant contribution from the 

contaminated zone, and concentrations in the contaminated zone probably will be underestimated. 

Similarly, if VOC-contaminated water is flowing into the well and is exiting the well at a different 

horizon, then VOCs will be present along the screened 

interval between the two horizons. In this case, VOC 

concentrations in the screened interval may be representative of aquifer concentrations at the inflow 

horizon, but may not be representative of aquifer 

concentrations near the outflow horizon. 

In areas where vertical stratification of VOC 

concentrations are anticipated, using multiple PDB 

samplers may more fully characterize the contaminated horizon than using a single PDB sampler. This 

is particularly true in wells having screens 10 ft or 

longer; however, significant VOC stratification has 

been observed over intervals of less than 5 ft (Vroblesky 

and Peters, 2000). Because of the increased probability 

of vertical concentration or hydraulic gradients within 

the open interval of long-screened (greater than 10 ft) 

wells, it is advisable to determine the zones of inflow 

and outflow within the screened or open interval of 

these wells using borehole flowmeter analysis (Hess, 

1982; 1984; 1986; 1990; Young and others, 1998).

Comparison of Passive Diffusion Bag 

Sampling Methodology to Conventional 

Methodologies

Traditional sampling methodologies, such as the 

purge-and-sample (or conventional purging method), 

low-flow or low-volume sampling, and using straddle 

packers and multilevel samplers, produce VOC 
Determining Applicability of Passive Diffusion Bag Samplers, and Interpretation of Data 13

concentrations that may differ from VOC concentrations obtained from PDB samplers because the methodologies sometimes are influenced in different ways 

by aquifer hydraulic and chemical heterogeneity. This 

section examines potential sources of concentration 

differences between traditional methodologies and the 

PDB methodology.

The purge-and-sample approach to ground-water 

monitoring differs from the diffusion-sampler 

approach primarily because the area of the screened or 

open interval that contributes water to the purged 

sample typically is greater than for the PDB sampler 

and the potential for mixing of stratified layers is 

higher. When pumping three or more casing volumes 

of water prior to collecting a sample, chemical concentrations in the discharging water typically change as the 

well is pumped (Keely and Boateng, 1987; Cohen and 

Rabold, 1988; Martin-Hayden and others, 1991; 

Robbins and Martin-Hayden, 1991; Reilly and Gibs, 

1993; Barcelona and others, 1994; Martin-Hayden, 

2000), due to mixing during pumping and other factors, 

such as the removal of stagnant water in the casing and 

changing patterns of inflow and outflow under ambient 

and pumping conditions (Church and Granato, 1996). 

The induction of lateral chemical heterogeneity during 

pumping also may produce variations in the sampled 

concentrations. The amount of mixing during purging 

can be highly variable (Barber and Davis, 1987; 

Church and Granato, 1996; Reilly and LeBlanc, 1998; 

Martin-Hayden, 2000), and may result in concentrations that are not locally representative (Reilly and 

Gibs, 1993). Substantial vertical hydraulic gradients, 

even in shallow homogeneous aquifers, have been 

observed to bias sampling using conventional purging 

because the majority of the pumped water may come 

from a particular horizon not related to the contaminated zone and because the intra-well flow that 

intruded the aquifer may not be adequately removed 

during purging (Hutchins and Acree, 2000). Thus, 

differences may be observed between concentrations 

obtained from a pumped sample and from a PDB 

sample in a chemically stratified interval if the pumped 

sample represents an integration of water collected 

from multiple horizons and the PDB sampler represents water collected from a single horizon. 

Low-flow purging and sampling (Barcelona and 

others, 1994; Shanklin and others, 1995) disturbs the 

local ground water less than conventional purge-andsample methods. Thus, samples obtained by PDB 

samplers are likely to be more similar to samples 

obtained by using low-flow purging than to those 

obtained by using conventional purge-and-sample 

methods. Even under low-flow conditions, however, 

purging still can integrate water within the radius of 

pumping influence, potentially resulting in a deviation 

from VOC concentrations obtained by PDB sampling. 

One investigation found that in low hydraulic conductivity formations, low-flow sampling methodology 

caused excessive drawdown, which dewatered the 

screened interval, increased local ground-water velocities, and caused unwanted colloid and soil transport 

into the ground-water samples (Sevee and others, 

2000). The authors suggest that in such cases, a more 

appropriate sampling methodology may be to collect a 

slug or passive sample from the well screen under the 

assumption that the water in the well screen is in 

equilibrium with the surrounding aquifer.

Isolating a particular contributing fracture zone 

with straddle packers in an uncased borehole allows 

depth-discrete samples to be collected from the target 

horizon (Hsieh and others, 1993; Kaminsky and Wylie, 

1995). Strategically placed straddle packers often can 

minimize or eliminate the impact of vertical gradients 

in the sampled interval. However, even within a 

packed interval isolating inflowing fracture zones, 

deviations between VOC concentrations in water from 

PDB samplers and water sampled by conventional 

methods still may occur if the conventional method 

mixes chemically stratified water outside the borehole 

or if the packed interval straddles chemically heterogeneous zones. 

The use of multilevel PDB samplers and other 

types of multilevel samplers (Ronen and others, 1987; 

Kaplan and others, 1991; Schirmer and others, 1995; 

Gefell and others, 1999; Jones and others, 1999) potentially can delineate some of the chemical stratification. 

Diffusion sampling and other sampling methodologies, 

however, can be influenced by vertical hydraulic gradients within the well screen or the sand pack. When 

vertical hydraulic gradients are present within the well, 

water contacting the PDB sampler may not be from a 

horizon adjacent to the PDB sampler. Rather, the water 

may represent a mixing of water from other contributing intervals within the borehole. In a screened well, 

even multilevel samplers with baffles to limit vertical 

flow in the well cannot prevent influences from 
14 User’s Guide for Polyethylene-Based Passive Diffusion Bag Samplers to Obtain Volatile Organic Compound 

Concentrations in Wells—Part 1: Deployment, Recovery, Data Interpretation, and Quality Control and Assurance

vertical flow in the gravel pack outside the well 

screen. Such vertical flow can result from small 

vertical differences in head with depth. A field test 

conducted by Church and Granato (1996) found that 

vertical head differences ranging from undetectable 

to 0.49 ft were sufficient to cause substantial flows 

(as much as 0.5 liters/minute) in the well bore. 

QUALITY CONTROL AND ASSURANCE

The sources of variability and bias introduced 

during sample collection can affect the interpretation 

of the results. To reduce data variability caused during 

sampling, a series of quality-control samples should 

be utilized. 

Replicate samples are important for the quality 

control of diffusion-sampler data. Sample replicates 

provide information needed to estimate the precision 

of concentration values determined from the combined 

sample-processing and analytical method and to 

evaluate the consistency of quantifying target VOCs. 

A replicate sample for water-filled diffusion samplers 

consists of two separate sets of VOC vials filled from 

the same diffusion sampler. Each set of VOC vials 

should be analyzed for comparison. Approximately 

10 percent of the samplers should be replicated.

The length of the PDB sampler can be adjusted 

to accommodate the data-quality objectives for the 

sampling event. The length can be increased if additional volume is required for collection of replicate 

and matrix spike/matrix spike duplicate samples.

Trip blanks are used to determine whether external VOCs are contaminating the sample due to bottle 

handling and/or analytical processes not associated 

with field processing. Trip blanks are water-filled 

VOA vials prepared offsite, stored and transported 

with the other bottles used for collecting the environmental sample, and then submitted for analysis with 

the environmental sample. Consideration also should 

be given to the collection of a predeployment PDB trip 

blank to determine if the PDB samplers are exposed to 

extraneous VOCs prior to deployment. The predeployment trip blank should be a PDB sampler that is stored 

and transported with the field PDB samplers from the 

time of sampler construction to the time of deployment in the wells. An aliquot of the predeployment 

blank water should be collected from the PDB sampler 

in a VOA vial and submitted for analysis at the time of 

sampler deployment. 

Water used to construct the diffusion samplers 

should be analyzed to determine the presence of background VOCs. Although many VOCs accidentally 

introduced into the diffusion-sampler water probably 

will reequilibrate with surrounding water once the 

diffusion samplers are deployed, some VOCs may 

become trapped within the diffusion-sampler water. 

For example, acetone, which is a common laboratory 

contaminant, does not easily move through the polyethylene diffusion samplers (Paul Hare, General Electric Company, oral commun., 1999). Thus, acetone 

inadvertently introduced into the diffusion-sample 

water during sampler construction may persist in the 

samplers, resulting in a false positive for acetone after 

sampler recovery and analysis. 

Consideration should be given to the collection 

of a PDB trip blank to determine if the PDB samplers 

are exposed to extraneous VOCs prior to deployment. 

A trip blank is collected from a PDB sampler that is 

stored and transported with the field PDB samplers 

between the time of sampler construction and deployment in the well. The water for the trip blank is 

collected from the PDB sampler in VOA vials at the 

time of sampler deployment. 

SUMMARY

Water-filled passive diffusion bag (PDB) samplers described in this report are suitable for obtaining 

a variety of VOCs in ground water at monitoring wells. 

The suggested application for PDB samplers is for 

long-term monitoring of VOCs in ground-water wells 

at well-characterized sites. Where the screened interval 

is greater than 10 ft, the potential for contaminant 

stratification and/or intra-borehole flow within the 

screened interval is greater than in screened intervals 

shorter than 10 ft. It is suggested 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 intraborehole flow be determined in wells having screens 

longer than 10 ft. A typical PDB sampler consists of a 

1- to 2-ft-long low-density polyethylene lay-flat tube 

closed at both ends and containing deionized water. 

The sampler is positioned at the target horizon by 

attachment to a weighted line or fixed pipe. 
Summary 15

The amount of time that the samplers should be 

left 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 environmental 

disturbances caused by sampler deployment to return 

to ambient conditions. The rate that water within the 

PDB sampler equilibrates with ambient water depends 

on multiple factors, including the type of compound 

being sampled and the water temperature. Concentrations of benzene, cis-1,2-dichloroethene, tetrachlorethene, trichloroethene, toluene, naphthalene, 1,2-

dibromoethane, and total xylenes within the PDB 

samplers equilibrated with the concentrations in an 

aqueous mixture of those compounds surrounding the 

samplers under laboratory conditions within approximately 48 hours at 21 °C. A subsequent laboratory 

study of mixed VOCs at 10 °C showed that tetrachloroethene and trichloroethene were equilibrated by about 

52 hours, but other compounds required longer equilabration times. Chloroethane, cis-1,2-dichloroethene, 

trans-1,2-dichloroethene, and 1,1-dichloroethene were 

not equilibrated at 52 hours, but appeared to be equilibrated by the next sampling point at 93 hours. Vinyl 

chloride, 1,1,1-trichloroethane, 1,2-dichloroethane, and 

1,1-dichloroethane were not equilibrated at 93 hours 

but were equilibrated by the next sampling point at 166 

hours. Different equilibration times may exist for other 

compounds. Differences in equilibration times, if any, 

between single-solute or mixed-VOC solutions have 

not yet been thoroughly examined. 

The samplers should be left in place long enough 

for the well water, contaminant distribution, and flow 

dynamics to restabilize following sampler deployment. 

Laboratory and field data suggest that 2 weeks of 

equilibration probably is adequate for many applications. Therefore, a minimum equilibration time of 

2 weeks is suggested. In less permeable formations, 

longer equilibration times may be required. When 

deploying PDB samplers in waters colder than 

previously tested (10 °C) or for compounds without 

sufficient corroborating data, a side-by-side comparison with conventional methodology is advisable to 

justify the field equilibration time. 

Following the initial equilibration period, the 

samplers maintain equilibrium concentrations with the 

ambient water until recovery. Thus, there is no specified maximum time for sampler recovery after initial 

equilibration. PDB samplers have routinely been left in 

ground waters having concentrations of greater than 

500 ppm of TCE for 3 months at a time with no loss of 

bag integrity, and at one site, the PDB samplers were 

left in place in VOC-contaminated ground water for 

1 year with no reported loss of sampler integrity. 

The effects of long-term (greater than 1 month) PDBsampler deployment on sampler and sample integrity 

have not yet been thoroughly tested for a broad range 

of compounds and concentrations. In some environments, development of a biofilm on the polyethylene 

may be a consequence of long-term deployment. 

Investigations of semipermeable membrane devices 

(SPMDs) have shown that the transfer of some 

compounds across a heavily biofouled polyethylene 

membrane may be reduced, but not stopped. If a heavy 

organic coating is observed on a PDB sampler, it is 

advisable to determine the integrity of the sample by 

comparing sampler results to a conventional sampling 

method concentrations before continuing to use PDB 

samplers for long-term deployment in that well. 

PDB methodology is suitable for a broad variety 

of VOCs, including chlorinated aliphatic compounds 

and petroleum hydrocarbons. The samplers, however, 

are not suitable for inorganic ions and have a limited 

applicability for non-VOCs and for some VOCs. For 

example, although methyl-tert-butyl ether and acetone 

and most semivolatile compounds are transmitted 

through the polyethylene bag, laboratory tests have 

shown that the resulting concentrations were lower 

than in ambient water. The samplers should not be used 

to sample for phthalates because of the potential for the 

LDPE to contribute phthalates to the water sample.

When attempting to determine whether the use 

of PDB samplers is appropriate at a particular well, a 

common approach is to do a side-by-side comparison 

with a conventional sampling method. This approach is 

strongly suggested in wells having temporal concentration variability. In a well having relatively low temporal concentration variability, comparison of the PDBsampler results to historical concentrations may provide enough information to determine whether the 

PDB samplers are appropriate for the well. In general, 

if the two approaches produce concentrations that 

agree within a range deemed acceptable by the local, 

state, and Federal regulatory agencies, then use of a 

PDB sampler in that well will provide VOC concentrations consistent with the historical record. If concentrations from the PDB sampler are higher than concentrations from the conventional method, then it is probable 

that the concentrations from the PDB sampler are an 

adequate representation of ambient conditions. If, however, the conventional method produces concentrations 
16 User’s Guide for Polyethylene-Based Passive Diffusion Bag Samplers to Obtain Volatile Organic Compound 

Concentrations in Wells—Part 1: Deployment, Recovery, Data Interpretation, and Quality Control and Assurance

that are substantially higher than the concentrations 

found by using the PDB sampler, then the PDB sampler may or may not adequately represent local ambient conditions. In this case, the difference may be due 

to a variety of factors, including mixing or translocation due to hydraulic and chemical heterogeneity of 

the aquifer within the screened or open interval of the 

well and the relative permeability of the well screen. 

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Concentrations in Wells—Part 1: Deployment, Recovery, Data Interpretation, and Quality Control and Assurance

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USER’S GUIDE FOR POLYETHYLENE-BASED PASSIVE DIFFUSION BAG SAMPLERS TO OBTAIN VOLATILE ORGANIC

AND QUALITY CONTROL AND ASSURANCEPART 1: DEPLOYMENT, RECOVERY, DATA INTERPETATION,COMPOUND CONCENTRATIONS IN WELLS

USGS WRIR 01-4060

VROBLESKY