Investigation of Polyethylene Passive Diffusion Samplers for Sampling 

Volatile Organic Compounds in Ground Water at Davis Global 

Communication, Sacramento, California, 

August 1998 to February 1999

USGS Open-File Report 00-307

By Don A. Vroblesky, James W. Borchers, Ted R. Campbell, and Willey Kinsey
Abstract C-1

Investigation of Polyethylene Passive Diffusion 

Samplers for Sampling Volatile Organic Compounds 

in Ground Water at Davis Global Communications, 

Sacramento, California, August 1998 to February 1999

By Don A. Vroblesky1, James W. Borchers2, Ted R. Campbell1, and Willey Kinsey2

ABSTRACT

Fourteen wells were instrumented with 

diffusion samplers as a test to determine whether 

the samplers could be used to obtain representative 

volatile organic compound concentrations at a 

study site in Sacramento, California. Single 

diffusion samplers were placed in 10-foot-long 

well screens, and multiple diffusion samplers were 

positioned in 20-foot-long well screens. Borehole 

geophysical logs and electromagnetic flowmeter 

tests were run in selected wells with 20-foot-long 

well screens prior to deploying the samplers. The 

diffusion samplers were recovered after 25 to 

30 days, and the wells were then sampled by using 

the purge-and-sample method. In most wells, the 

concentrations obtained by using the downhole 

diffusion samplers closely matched those obtained 

by using the purge-and-sample method. In seven 

wells, the concentrations differed between the two 

methods by only 2 micrograms per liter (μg/L) or 

less. In three wells, volatile organic compounds 

were not detected in water obtained by using either 

method. In the four remaining wells, differences 

between the methods were less than 2 μg/L in the 

0.2- to 8.5-μg/L concentration range and from 

1.2 to 8.7 μg/L in the 10- to 26-μg/L concentration range. Greater differences (23 percent or 

14.5 μg/L, 31 percent or 66 μg/L, and 46 percent 

or 30 μg/L) between the two methods were 

observed for tetrachloroethene concentrations, 

which ranged between 30 and 211 μg/L in three 

wells. The most probable explanation for the 

differences is that in some wells, the purging 

induced drawdowns and introduced water that 

differed in volatile organic compound concentrations from the in situ water in contact with the 

screened interval of the well. Alternate explanations include the possibility of unrecorded changes 

in nearby contaminant-extraction-well operation 

during the equilibration period. The data suggest 

that the combined use of borehole flowmeter tests 

and diffusion samplers may be useful in optimizing the radius of capture of contaminated 

ground water by the contaminant-removal wells.

Overall, the data suggest that the use of diffusion 

samplers provided an alternative sampling method 

to the purge-and-sample approach.

1

U.S. Geological Survey, Stephenson Center, Suite 129, 720 Gracern Road, Columbia, South Carolina 29210-7651.

2

U.S. Geological Survey, 6000 J Street, Sacramento, California 95819-6129.
C-2 Investigation of Polyethylene Passive Diffusion Samplers in Ground Water at Davis Global Communications, Sacramento, Calif.

INTRODUCTION

The U.S. Geological Survey (USGS), in 

cooperation with the U.S. Air Force Center for 

Environmental Excellence (AFCEE), began an 

initiative in August 1998 to investigate the suitability of 

using polyethylene water-filled passive diffusion 

samplers to collect volatile organic compound (VOC) 

samples from observation wells at Davis Global 

Communications in Sacramento, California. Passive 

diffusion samplers have been successfully used to 

obtain representative water samples for VOC 

concentrations without the need to purge at a different 

site (Vroblesky and Hyde, 1997). Thus, the diffusion 

samplers offer a potential savings in sampling time and 

expense relative to the purge-and-sample approach. 

The purpose of this report is to present results 

comparing VOC concentrations in water obtained from 

diffusion samplers to concentrations in water obtained 

by using the purge-and-sample approach. 

Site Description

Davis Global Communications (fig. 1) is an 

annex of McClellan Air Force Base (AFB) in 

Sacramento, Calif., approximately 4 miles south of the 

city of Davis. The site, which has been in operation 

since the 1950’s, is used for military communications. 

In 1985, three underground storage tanks were found to 

be leaking diesel fuel. During the course of the field 

investigation for hydrocarbon contamination, the 

presence of chlorinated solvents also was detected in 

the ground water. The source of the chlorinated 

solvents is unknown. 

The geology of the site consists of fine-grained 

flood plain or overbank deposits mixed with lesser 

amounts of sandy stream deposits containing 

discontinuous gravels and sands. Driller’s logs of the 

wells indicate that some of the silty and silty clay layers 

are fractured (CH2M HILL, 1994), possibly providing 

conduits for the vertical movement of ground water. 

Hydraulic testing to determine aquifer properties has 

not been done; however, most of the wells sampled for 

this investigation yielded little water and recovered 

slowly, strongly suggesting that the sampled horizons 

have a relatively low hydraulic conductivity.

Ground-water levels and flow directions at the 

site vary seasonally because of the influence of nearby 

agricultural wells, which typically are from 200- to 

500-feet (ft) deep. Ground-water levels are 

approximately 40 ft below mean sea level (msl) during 

the growing season when the agricultural wells are 

actively pumped; water levels rise about 40 ft during 

the winter when the wells are not used (CH2M HILL, 

1994). In addition, onsite contaminant-removal wells 

are in operation most of the time, resulting in localized 

flow toward these pumped wells. 

Methods

Fourteen wells at the site were instrumented with 

diffusion samplers during December 15–18, 1998. Of 

these wells, eight were equipped with screen lengths of 

20 ft and six had screen lengths of 10 ft (table 1). In the 

wells having 10-ft-long screens, a single diffusion 

sampler was centered vertically in the screened 

interval. In wells having 20-ft-long screens, 9 or 10 

diffusion samplers were placed end-to-end along a 

vertical profile within the screened interval. 

Prior to installing the diffusion samplers, six of 

the 20-ft-long screened intervals were investigated by 

using borehole geophysical and flowmeter logging 

techniques. Logging was performed during the same 

week that the samplers were deployed (December 15–

18, 1998). The depths of the wells and water levels 

were measured prior to installing the logging 

equipment. The wells then were logged using an 

electromagnetic induction and gamma tool and a fluid 

resistivity and temperature tool. An electromagnetic 

flowmeter was used under static and pumped 

conditions to measure vertical flow rates at discrete 

locations within the screened interval. At each tested 

well, a submersible pump was placed directly above the 

screened interval; fluid-resistivity and temperature logs 

were run, and vertical-flow measurements were made 

while the well was pumped. 

The diffusion samplers were allowed to 

equilibrate within the screened interval for 

approximately 25 to 30 days. The samplers were 

recovered by removing them from the well, cutting 

open the polyethylene, and gently pouring the contents 

into 40-milliliter (mL) glass sampling vials with Teflon 

caps. 

Immediately following diffusion-sampler 

recovery, the wells were purged and sampled in 

accordance with the ongoing ground-water protocol at 

the site. Well purging consisted of removing three 

casing volumes of water using a Grundfos submersible 

positive-displacement pump. Water samples then were 

collected from the well by using a bailer.
Introduction C-3

Figure 1. Locations of wells at Davis Global Communications, Sacramento, Calif., January 1999 (modified 

from Radian International, 1999).
C-4 Investigation of Polyethylene Passive Diffusion Samplers in Ground Water at Davis Global Communications, Sacramento, Calif.

All samples were stored on ice and shipped to the 

same laboratory for analysis by using the U.S. 

Environmental Protection Agency (USEPA) Method 

8260b. For 20-ft-long screened wells containing 

multiple diffusion samplers, only one of the diffusion 

samplers was sent to a USEPA-certified laboratory for 

analysis by USEPA Method 8260b. To select the 

representative sample for each 20-ft-long screened 

well, the sampling vials for each diffusion sampler 

were stored on ice and sent by overnight mail to the 

USGS in South Carolina. 

Upon arrival at the USGS, an extra vial from 

each diffusion sampler was analyzed by head-space gas 

chromatography using a Photovac 10S Plus gas 

chromatograph. The remaining vials for the diffusion 

sampler containing the highest VOC concentrations at 

each well were stored on ice and sent by overnight mail 

to the same USEPA-certified laboratory that analyzed 

the water collected by using the purge-and-sample 

method. The samples were analyzed by USEPA 

Method 8260b. The laboratory analytical results for the 

diffusion samplers were used as a standard for the 

concentrations obtained using head-space analysis in 

the respective well. Thus, in the graphs showing 

vertical concentration differences, the concentrations at 

one depth (designated in table 2) at each well 

represents the USEPA Method 8260b analysis of 

diffusion-sampler water. The remaining samples at that 

well represent USGS head-space gas chromatography 

analyses adjusted relative to the head-space analysis 

of the sampler analyzed by USEPA Method 8260b. 

Only the samples analyzed by USEPA Method 8260b 

were used directly to compare the two sampling 

methods in this investigation. 

Table 1. Construction data and number of installed diffusion samplers for tested wells at Davis Global 

Communications, Sacramento, Calif.

[ft, feet; msl, mean sea level; bls, below land surface; in., inches]

Well

Top of casing 

elevation

(ft msl)

Groundsurface 

elevation

(ft msl)

Screened 

interval

(ft bls)

Casing 

diameter 

(in.)

Screen 

length

(ft)

Total well 

depth

(ft)

Number of 

diffusion 

samplers 

recovered

DMW-2 26.88 28.1 61–81 4 20 84 9

DMW-3 28.82 29.86 61–81 4 20 83.5 9

DMW-5 26.47 26.88 59–79 4 20 84 9

DMW-6 25.94 25.26 59–79 4 20 80.5 10

DMW-7 27.02 27.5 61–81 4 20 84 10

DMW-8 26.88 26.5 60–80 4 20 84 10

DMWD-3 28.68 27.06 155–175 4 20 250 10

DMWD-14 28.57 26.33 149–169 5 20 178 10

DMWC-3 29.16 26.94 93–103 4 10 108 1

DMWC-4 27.57 24.64 95–105 4 10 106 1

DMWD-10 29.22 27.02 162–172 5 10 173 1

DMWD-1 31.9 30.2 152–162 4 10 240 1

DMWD-11 29.29 27.42 171–181 5 10 181.5 1

DPC-22 28.11 Not measured 91–101 4 10 104 1
Results and Discussion C-5

Table 2. Analytical results from ground-water samples obtained by using diffusion samplers and 

using the purge-and-sample method, Davis Global Communications, Calif., January 1999

[ft, feet; TCE, trichloroethene; PCE, tetrachloroethene; cDCE, cis-1,2-dichloroethene; 1,1-DCE, 1,1-dichloroethene; 

μg/L, micrograms per liter; Dif, water-filled diffusion sampler; P&S, purge-and-sample; J, analyte concentration considered an estimated value because one or more quality control specifications were not met; < less than]

Location Sample 

type

Date 

sampled

Sample depth

(ft)

TCE

(μg/L)

PCE

(μg/L)

cDCE

(μg/L)

1,1-DCE

(μg/L)

20-foot-long well screens

DMW-2 Dif 1/13/99 67–68 17.3 1.7 4.5 0.8 J

P&S 1/14/99 61–81 26.0 J 3.5 5.4 .7 J

DMW-3 DIF 1/13/99 71–72 10.7 47.2 .3 J 3.6

P&S 1/14/99 61–81 10.7 61.7 .3 J 2.5

DMW-5 DIF 1/13/99 69–70 19.8 145.0 .3 J 10.5

P&S 1/14/99 59–79 23.8 211.0 .2 J 8.8

DMW-6 DIF 1/14/99 61–62 19.7 35.3 1.7 2.9

P&S 1/14/99 59–79 25.1 65.4 1.9 2.6

DMW-7 DIF 1/13/99 69–70 30.9 2.4 8.5 1.3

P&S 1/13/99 61–81 31.6 2.3 8.5 1.1 J

DMW-8 DIF 1/13/99 68–69 3.1 <.6 1.0 J .3 J

P&S 1/13/99 60–80 2.8 1.0 .7 J .1 J

DMWD-3 DIF 1/13/99 161–162 3.2 4.0 <.1 .6 J

P&S 1/14/99 155–175 3.3 4.0 <.1 .4 J

DMWD-14 DIF 1/14/99 153–154 2.5 1.6 .1 J .4 J

P&S 1/15/99 149–169 2.3 1.5 .0 .2 J

10-foot-long well screens

DMWC-3 DIF 1/12/99 97–98 3.3 3.8 0.1 J 0.2 J

P&S 1/12/99 93–103 4.6 5.8 .2 J .3 J

DMWC-4 DIF 1/11/99 99–100 <.1 <.1 <.1 <.1

P&S 1/11/99 95–105 .2 J <.1 <.1 <.1

DMWD-10 DIF 1/12/99 166–167 5.0 2.7 <.1 .7 J

P&S 1/12/99 162–172 4.6 2.8 <.1 .4 J

DMWD-1 DIF 1/11/99 156–157 <.1 <.1 <.1 <.1

P&S 1/11/99 152–162 <.1 <.1 <.1 <.1

DMWD-11 DIF 1/11/99 175–176 <.1 <.1 <.1 <.1

P&S 1/11/99 171–181 <.1 <.1 <.1 <.1

DPC-22 DIF 1/12/99 95–96 <.1 <.1 <.1 <.1

P&S 1/13/99 91–101 <.1 <.1 <.1 <.1

RESULTS AND DISCUSSION

In most wells, the concentrations obtained by 

using downhole diffusion samplers closely matched 

those obtained using the purge-and-sample method 

(fig. 2). In 7 of the 14 wells (DMW-7, DMW-8, 

DMWC-3, DMWC-4, DMWD-10, DMWD-14, and 

DMWD-3), where VOC concentrations ranged from 

near the detection limit (0.1 μg/L) to 31.6 μg/L, the 

concentrations differed by only 2 μg/L or less between 

the two methods (table 2). For most regulatory 

purposes, this difference is negligible. Of these wells, 

four had 20-ft-long well screens (wells DMW-7, 

DMW-8, DMWD-3, and DMWD-14), and the 

remaining had 10-ft-long well screens. No VOCs 

were detected in ground water collected from wells 

DMWD-1, DMWD-11, and DPC-22. The lack of VOC 

detection by either method in these wells indicates that 

the materials used in constructing the diffusion 

samplers did not contribute VOCs to the analytical 

results.
C-6 Investigation of Polyethylene Passive Diffusion Samplers in Ground Water at Davis Global Communications, Sacramento, Calif.

Figure 2. Comparison of volatile organic compound concentrations in ground water obtained from 

diffusion samplers and from the purge-and-sample method at Davis Global Communications, Sacramento, 

Calif., January 1999.
Results and Discussion C-7

Analyses of ground water from the remaining 

four wells equipped with 20-ft-long screens (DMW-5, 

DMW-2, DMW-3, and DMW-6) indicated various 

degrees of comparability between the two sampling 

methods. In all four of these wells, where 

concentrations of individual VOCs were approximately 

10 μg/L or less, comparisons between the two sampling 

methods showed differences of less than 2 μg/L. 

Although some of the comparisons in this low range of 

concentration values constitute a high percent 

difference, the actual difference in micrograms per liter 

is negligible for most regulatory purposes. For 

concentrations of trichloroethene (TCE) ranging from 

about 10 to 26 μg/L, the concentration differences also 

were small (ranging from about 4 to 6 μg/L in three of 

the wells and 8.7 μg/L in the fourth well, DMW-2). 

Greater differences were observed between the two 

methods for tetrachloroethene (PCE) concentrations 

above 30 μg/L. In wells DMW-3, DMW-5, and 

DMW-6, the differences were 23 percent (14.5 μg/L), 

31 percent (66 μg/L), and 46 percent (30 μg/L), 

respectively (table 2). 

Potential explanations for the differences 

between concentrations include insufficient wellequilibration time, water-level variations due to the 

intermittent pumping of onsite contaminant-removal 

wells, the possibility that the two methods sampled 

different water, and experimental errors inherent to 

each method. The hypothesis that insufficient wellequilibration time had elapsed following well testing 

and sampler installation seems unlikely because two of 

the three wells where the poorest matches were 

observed had not been subjected to borehole logging or 

pumping for electromagnetic flowmeter testing. It is 

possible, however, that intermittent pumping at nearby 

contaminant-removal wells during the equilibration 

period produced changes in hydraulic conditions at the 

screened intervals. Although contaminant-recovery 

wells typically operate continuously at the site, none 

were in operation on the day that geophysical logging 

and flowmeter testing were performed in well DMW-5 

(December 15, 1998); some removal wells were in 

operation during part of the next day when well 

DMW-7 was tested; no removal wells were operating 

on the day that wells DMW-2 and DMW-8 were tested 

(December 17, 1998); and four removal wells were 

operating on the day that wells DMWD-3 and 

DMWD-14 were tested (December 18, 1998). Groundwater flow directions may vary substantially depending 

on when the contaminant-removal wells are in 

operation. Because records of the times that the wells 

are on and off typically are not kept, it is possible that 

changes in the operation of contaminant-removal wells 

during the diffusion-sampler equilibration period 

resulted in hydraulic conditions that differed from the 

conditions at the time of sampling. This potentially 

could result in a discrepancy between the two sampling 

methods. Alternate explanations include the possibility 

of unrecorded changes in nearby contaminantextraction well operation during the equilibration 

period.

Although insufficient equilibration time and 

changes in the operation of contaminant-removal wells 

potentially explain the discrepancies observed between 

sampling methods at some wells, water chemistry, 

geophysical logs, borehole flowmeter tests, and 

historical soil-gas data suggest a more probable 

scenario. Water chemistry from the diffusion samplers 

represents water derived from the screened interval, 

whereas the purge-and-sample method may have 

induced the infiltration of water from shallower zones 

above the well screen, thus resulting in the collection of 

mixed waters. A case in point is well DMW-5, where 

concentrations of TCE and PCE were higher in samples 

collected by purging the well than in samples collected 

by the diffusion samplers.

Analysis of water samples from the nine 

diffusion samplers positioned in the screened interval 

of well DMW-5 indicated that under static conditions 

of equilibration, the highest concentrations of TCE 

(19.8 μg/L) and PCE (145.0 μg/L) (table 2) in the 

diffusion samplers were from the center of the screened 

interval, which was the approximate center of an 

adjacent sand layer (fig. 3A, B, C). The data suggested 

that this sand layer was the predominant source of TCE 

and PCE to the well. However, TCE and PCE 

concentrations differed between the two sampling 

methods. TCE and PCE concentrations in water 

obtained by using the purge-and-sample method were 

23.8 μg/L and 211.0 μg/L, respectively (table 2).

A probable explanation for the difference can be 

postulated by examining supportive data. Analysis of 

drilling logs and natural gamma logs indicated that the 

lower 13 ft of the screened interval of well DMW-5 was 

open to a sand layer extending from a depth of 67 to 

80 ft below land surface (fig. 3C, D). Overlying the 

sand was a fractured silty clay with slickensided 

surfaces that extended from a depth of 38.5 to 61.5 ft 

below land surface; sand and gravel composed the 

remainder of the shallow subsurface. Flowmeter tests 
C-8 Investigation of Polyethylene Passive Diffusion Samplers in Ground Water at Davis Global Communications, Sacramento, Calif. Figure 3. (A) Tetrachloroethene and trichloroethene concentrations in diffusion samplers, (B) screened interval, (C) lithology, (D) natural gamma, (E) borehole flowmeter, and (F) fluid-resistivity data at well DMW-5, Davis Global Communications, Sacramento, Calif., January 1999.
Results and Discussion C-9

and geophysical logs run in the well indicated that 

when well DMW-5 was pumped, most of the water 

entered the well bore near the top of the screened 

interval at a depth of about 58 to 62 ft, with a smaller 

volume entering from the adjacent sand layer (fig. 3E). 

Under static conditions, however, the sand layer 

contributed the largest percentage of water moving into 

the screened interval (fig. 3E). Fluid resistivity logs 

provided further evidence that most water entered near 

the top of the well screen; during pumping, fluid 

resistivity increased sharply at the top of the screened 

interval (fig. 3F). The relatively high percentage of 

flow entering the well at the top of the screened interval 

during pumping suggests that purging the well may 

have induced the downward movement of water along 

the annular space of the well bore or along fractures 

within the silty clay material overlying the screened 

interval.

Results of a soil-gas survey conducted in 1994 

showed that substantial amounts of TCE and PCE were 

present at depths of 40 to 60 ft in the subsurface 

materials adjacent to well DMW-5 (CH2M HILL, 

1995, site SGB-4). The presence of TCE and PCE in 

these shallow subsurface materials combined with the 

fact that pumping well DMW-5 created substantial 

drawdown, strongly suggests that pumping well 

DMW-5 resulted in the vertical downward movement 

of water into the screened interval and, ultimately, the 

mixing of water from shallower zones with water from 

the screened interval. Thus, the diffusion samplers 

probably provided a more representative sample of 

water from the screened interval. 

Of the four wells showing VOC concentration 

differences that were greater than 2 μg/L between the 

two sampling methodologies, borehole flowmeter data 

were only available for wells DMW-5 and DMW-2. For 

well DMW-2, there were no substantial vertical 

variations in TCE concentrations within the well screen 

(fig. 4A, B). The borehole flowmeter data showed that 

when the well was pumped, approximately the same 

Figure 4. (A) Trichloroethene concentrations in diffusion samplers, (B) screened interval, (C) lithology, 

and (D) borehole flowmeter data at well DMW-2, Davis Global Communications, Sacramento, Calif., 

January 1999.
C-10 Investigation of Polyethylene Passive Diffusion Samplers in Ground Water at Davis Global Communications, Sacramento, Calif.

amount of water was obtained from the sand near the 

bottom of the screened interval as was obtained from 

the silty clay at the top of the screened interval (fig. 4C, 

D). As with well DMW-5, the apparent inflow of water 

from silty clay at the top of the well screen suggests that 

water may have moved downward from shallower 

zones to the screened interval along the annular space 

of the well bore or through fractures in the overlying 

material. Although no soil-gas profiles were collected 

at well DMW-2, soil-gas data from approximately 100 

ft away showed the presence of TCE and PCE at depths 

of 40 and 60 ft (CH2M HILL, 1995, site SGB-8). Thus, 

as in well DMW-5, it is possible that the two methods 

sampled water from different sources at well DMW-2.

Wells DMW-3 and DMW-6 also showed lower 

PCE concentrations in the diffusion samplers than in 

water obtained by the purge-and-sample method 

(table 2). No borehole flowmeter data were available 

for these wells, but the screened intervals for these 

wells were below fractured clay. The lithologic 

similarity between the sediment overlying the well 

screens in these wells with the sediment overlying the 

screens in wells DMW-5 and DMW-2 again implies the 

possibility that flow through fractures or the downward 

leakage of water from shallower zones during pumping 

influenced the water-quality samples from wells 

DMW-3 and DMW-6 (figs. 5, 6). At well DMW-6, 

when the well was not being pumped, the diffusionsampler data imply that there was a concentration 

gradient in the screened interval with the highest 

concentrations occurring in a sand and fractured silt 

layer near the top of the screened interval. 

The combined approach of using diffusion 

samplers and a borehole flowmeter also provided 

Figure 5. (A) Tetrachloroethene and trichloroethene concentrations in diffusion samplers, (B) screened 

interval, and (C) lithology at well DMW-3, Davis Global Communications, Sacramento, Calif., January 

1999.
Results and Discussion C-11

information on the source of water being removed from 

the aquifer by contaminant-removal wells in operation 

at the site. Water from well DMWD-3 contained low 

concentrations (less than 5 μg/L) of PCE and TCE in 

the screened interval, which was installed in a zone of 

sand and gravel (fig. 7A, B, C). Flowmeter tests 

conducted within the well, however, showed that water 

flowed into the well near the bottom of the screened 

interval and exited the well through the upper half of 

the screen, even when the well was not being pumped 

(fig. 7D). When a pump was placed in the well and 

water was pumped out at 0.96 gallon per minute, water 

still exited the well through the upper part of the 

screened interval. A probable explanation is that 

contaminant-removal well DEWC-3, which was 32.4 ft 

south of well DMWD-3, pumped water from a depth of 

93–108 ft below land surface and may have caused the 

flowthrough by capturing water from the horizon 

screened by well DMWD-3. Because the water in well 

DMWD-3 contained less than 5 μg/L of the target 

compounds, the data suggest that some of the water 

captured by contaminant-removal well DEWC-3 was 

relatively uncontaminated. Thus, a combination of 

diffusion samplers and borehole flowmeter tests may 

be useful in optimizing the contaminant-capture radius 

of contaminant-removal wells. Overall, the data 

suggest that the use of diffusion samplers provides an 

alternative sampling method to the purge-and-sample 

approach used for ground-water investigations.

Figure 6. (A) Tetrachloroethene and trichloroethene concentrations in diffusion samplers, 

(B) screened interval, and (C) lithology at well DMW-6, Davis Global Communications, 

Sacramento, Calif., January 1999.
C-12 Investigation of Polyethylene Passive Diffusion Samplers in Ground Water at Davis Global Communications, Sacramento, Calif.

SUMMARY

Fourteen wells were instrumented with diffusion 

samplers at Davis Global Communications, 

Sacramento, Calif., as a test to determine whether the 

samplers could be used to obtain representative volatile 

organic compound (VOC) concentrations at the site. Of 

these wells, eight had screen lengths of 20 ft and two 

had screen lengths of 10 ft. Single diffusion samplers 

were placed in the 10-ft-long well screens, and multiple 

diffusion samplers were placed in the longer screens. 

The samplers were recovered after a minimum of 

14 days, and the wells were then sampled by using the 

purge-and-sample method. 

In most wells, the concentrations obtained by 

using downhole diffusion samplers closely matched 

those obtained by using the purge-and-sample method. 

In seven wells, the concentrations differed between the 

two methods by only 2 μg/L or less. For most 

regulatory purposes, this difference is negligible. In 

three of the remaining wells, VOCs were not detected 

in water obtained by using either method. 

In the remaining four wells, the degree of 

comparability between the two sampling methods 

varied. In these wells, differences between the methods 

were less than 2 μg/L in the 0.2- to 8.5-μg/L 

concentration range and from 1.2 to 8.7 μg/L in the 10- 

to 26-μg/L concentration range. In wells DMW-3, 

DMW-5, and DMW-6, greater differences (23 percent 

or 14.5 μg/L, 31 percent or 66 μg/L, and 46 percent or 

30 μg/L, respectively) between the two methods were 

observed for PCE concentrations which ranged 

between 30 and 211 μg/L.

Figure 7. (A) Tetrachloroethene and trichloroethene concentrations in diffusion samplers, (B) screened interval, 

(C) lithology, and (D) borehole flowmeter data at well DMWD-3, Davis Global Communications, Sacramento, Calif., 

January 1999.
References C-13

 Potential explanations for the differences 

include insufficient equilibration time for the diffusion 

samplers, hydraulic changes during the equilibration 

period due to possible unrecorded changes in the 

pumping of onsite contaminant-removal wells, and the 

possibility that the two methods sampled water from 

differing sources at some wells. Data collected during 

this investigation, combined with soil-gas data 

collected during a previous investigation, implied that 

at some wells, the two methods sampled water from 

differing horizons.

At wells DMW-2 and DMW-5, the diffusion 

samplers seemed to be sampling water representative 

of the horizon adjacent to the screened interval. 

Lithologic data, borehole fluid resistivity and natural 

gamma logs, borehole flowmeter results, water-level 

measurements, and historical soil-gas data suggest, 

however, that water obtained while using the purgeand-sampling method at wells DMW-2 and DMW-5 

was derived partly from the downward movement of 

water along the annular space of the well bore or 

through fractures in the silty clay. Although borehole 

flowmeter data were not available for the remaining 

two wells (DMW-3 and DMW-6), the lithologic 

similarity between the sediments at these wells and the 

sediments at wells DMW-2 and DMW-5 implied that 

water could move downward from shallower zones into 

the well screen while using the purge-and-sample 

method. Thus, as in this case, the purge-and-sample 

approach may have overestimated concentrations in the 

screened interval. Overall, the data suggest that the use 

of diffusion samplers provided an alternative sampling 

method to the purge-and-sample approach used for 

ground-water investigations.

The data also showed that vertical variations in 

VOC concentrations can exist within the screened 

intervals. In addition, the combined use of borehole 

flowmeter tests and diffusion samplers showed that 

contaminant-removal well DEWC-3 seemed to be 

capturing water from the horizon screened by well 

DMWD-3, indicating that at least some of the water 

captured by contaminant-removal well DMWC-3 

contains VOC concentrations less than 5 μg/L. These 

data may be useful in optimizing the radius of capture 

of contaminated ground water by the contaminantremoval wells.

REFERENCES

CH2M HILL, 1994, Davis Global Communications Site, 

Remedial Investigation/Feasibility Report, 

Appendix S—Well construction data: Consultant’s 

report prepared for McClellan Air Force Base, Calif., 

v. 3 of 3, February.

———1995, Fieldwork summary final, Davis Global Communications Site, Delivery order 7013: Consultant’s 

report prepared for McClellan Air Force Base, Sacramento, Calif., March, 17 p.

Radian International, 1999, Final data summary, Davis 

Global Communications Site, fourth quarter 1998: 

Consultant’s report prepared for McClellan Air Force 

Base, Sacramento, Calif., February 1999, 26 p. 

Vroblesky, D.A., and Hyde, W.T., 1997, Diffusion samplers 

as an inexpensive approach to monitoring VOCs in 

ground water: Ground Water Monitoring and 

Remediation, v. 17, no. 3, p. 177–184.
Field Testing of Passive Diffusion Bag Samplers for Collection of 

Ground-Water Volatile Organic Compound Concentrations at 

Naval Industrial Reserve Ordnance Plant, Fridley, Minnesota, 

November 1999 to May 2000

USGS Water-Resources Investigations Report 00-4246

By Don A. Vroblesky and Matthew D. Petkewich
Introduction D-1

Field Testing of Passive Diffusion Bag Samplers for 

Volatile Organic Compound Concentrations in 

Ground Water, Naval Industrial Reserve 

Ordnance Plant, Fridley, Minnesota, 

November 1999 and May 2000

By Don A. Vroblesky and Matthew D. Petkewich

ABSTRACT

Volatile organic compound concentrations 

from passive diffusion bag samplers were 

compared with concentrations from conventional 

purge (three or more casing volumes) sampling 

and low-flow purge sampling in side-by-side 

tests in 17 wells at the Naval Industrial Reserve 

Ordnance Plant, in Fridley, Minnesota. An initial 

comparison of 1,2-dichloroethene and trichloroethene concentrations obtained by using passive 

diffusion bag samplers and the conventional 

purge method in wells where one passive diffusion bag sampler was deployed showed good 

agreement at several wells but poor agreement at 

others. Collection of data from multiple diffusion 

samplers during the conventional purge 

sampling and during the low-flow sampling, 

however, suggests that the volatile organic 

compound concentrations from the passive diffusion bag samplers accurately reflect the volatile 

organic compound distribution in the screened 

interval, whereas the conventional purge and 

low-flow purge samples reflect mixing during 

pumping. The data also show that contaminant 

stratification was present in some wells. In one 

well, trichloroethene concentrations ranged from 

470 to 1,600 micrograms per liter over a vertical 

distance of approximately 6 feet. 

INTRODUCTION

Low-density polyethylene passive diffusion 

bag (PDB) samplers, filled with deionized water or 

air, can be an inexpensive alternative sampling 

method for volatile organic compounds (VOCs) in 

contaminated wells (Vroblesky and Hyde, 1997; 

Gefell and others, 1999). The use of PDB samplers 

in wells has generated interest because they can be 

used to sample ground water without the need for 

prior well purging. Investigations have shown that 

PDB sampling methods can result in substantial cost 

savings over traditional sampling methods (Parsons 

Engineering Science Inc., 1999; McClellan AFB 

Environmental Directorate, 2000).

The Naval Industrial Reserve Ordnance Plant 

(NIROP), in Fridley, Minnesota, has been in operation since 1940. Activities at the plant resulted in 

ground-water contamination by chlorinated aliphatic 

hydrocarbons, primarily trichloroethene (TCE). The 

TCE contamination has migrated from the NIROP 

property and probably extends to the Mississippi 

River (fig. 1) (Tetra Tech NUS, Inc., 2000). As part 

of an effort to reduce long-term monitoring costs 

associated with well sampling, Southern Division 

Naval Facilities Engineering Command requested 

the U.S. Geological Survey to examine the potential 

for using PDB samplers as a low-cost alternative to 

the standard sampling approaches used at the site. 

The uppermost aquifer system at NIROP 

consists of heterogeneous interbedded medium- to 

coarse-grained sands with layers of fine-grained sand 
D-2 Field Testing of Passive Diffusion Bag Samplers for Volatile Organic Compound Concentrations in Ground Water,

Naval Industrial Reserve Ordnance Plant, Fridley, Minnesota, November 1999 and May 2000

and gravel and low-permeability 

sediment layers ranging from silt to 

clay. The uppermost aquifer is 

referred to as an aquifer system 

because it can be roughly divided into 

two water-bearing zones varying 

substantially in thickness and hydraulic connection across the site. The 

shallowest zone ranges in thickness 

from 41 to 93 feet (ft), and the deeper 

zone ranges in thickness from 19 to 

55 ft (Tetra Tech NUS, Inc., 2000). 

Most of the wells tested during this 

investigation are screened in this 

aquifer system. Beneath the uppermost aquifer system, and separated 

from it by a leaky confining unit, is a 

bedrock dolomitic limestone that 

functions as an aquifer; well 3-PC 

was open to this aquifer (fig. 1).

Purpose and Scope

The purpose of this report is to 

present the findings of an investigation to determine whether the use of 

PDB samplers is a viable VOC 

sampling method for observation 

wells at the site. The investigation 

involved comparing VOC concentrations in water obtained by using PDB 

samplers with VOC concentrations in 

water obtained by using the conventional purge-and-sample method 

routinely used at the site. In addition, 

VOC concentrations in water from 

PDB samples from selected wells 

collected with PDB samplers were 

compared with VOC concentrations 

in water obtained by low-flow 

sampling. PDB samplers were placed 

in 19 observation wells. Multiple PDB 

samplers were placed in 8 of the wells. 

Figure 1. Locations of sampling sites at the 

Naval Industrial Reserve Ordnance Plant 

(NIROP), Fridley, Minnesota, November 1999 

to May 2000.
Methods D-3

Acknowledgments

This investigation was funded by the 

U.S. Navy, Southern Division, Naval Facilities 

Engineering Command. 

METHODS

Each PDB sampler consisted of a 2-inch diameter low-density polyethylene (LDPE) tube containing deionized water and heat-sealed at both ends. 

On the outside of each sampler, LDPE mesh 

provided abrasion protection. This sampling method 

is patented (patent number 5,804,743) and is available for nonexclusive licensing from the U.S. 

Geological Survey Technology Enterprise Office, 

MS-211, National Center, 12201 Sunrise Valley 

Drive, Reston, Virginia (telephone 703-648-4450; 

fax 703-648-4408).

PDB samplers were attached to weighted lines 

by plastic cable ties. In most wells, single PDB 

samplers were deployed at the approximate vertical 

centers of the saturated screened intervals.

PDB samplers were tested in 19 wells at 

NIROP (table 1; fig. 1). During the initial test, the 

samplers were deployed in October 1999, allowed to 

equilibrate approximately 30 days, and recovered in 

November 1999. Seventeen of the wells were instrumented with single PDB samplers, and two wells 

were instrumented with multiple PDB samplers. In a 

second test, multiple PDB samplers were deployed 

in seven wells in April 2000, allowed to equilibrate 

approximately 35 days, and recovered in May 2000. 

Recovery of the PDB samplers consisted of 

removing them from the wells, cutting them open, and 

decanting the water into 40-milliliter (mL) volatile 

organic analysis (VOA) vials. The samples were 

preserved with hydrochloric acid, stored at approximately 4 degrees Celsius (°C), and transferred to a 

commercial laboratory for analysis using the U.S. 

Environmental Protection Agency (USEPA) method 

8260b (U.S. Environmental Protection Agency, 1999). 

During the November 1999 test, the wells 

were purged and sampled by the site contractor using 

the method typical for the site. This method 

consisted of first purging each well by removing at 

least three casing volumes of water and monitoring 

until the pH, specific conductance, and temperature 

stabilized. In well 12-D (fig. 1; table 1), this involved 

removing 300 gallons of water (four casing 

volumes). Once the well was purged, water samples 

for VOC analysis were collected by using the pump 

and then sent for analysis using USEPA method 

8260b (U.S. Environmental Protection Agency, 

1999) to the same laboratory that analyzed the PDB 

samplers. 

A second test (April to May 2000) was done to 

provide further information in wells showing poor 

agreement. During the second test, VOC concentrations 

in multiple PDB samplers were compared with VOC 

concentrations collected by using low-flow methods. 

The low-flow sampling approach (Barcelona 

and others, 1994; Shanklin and others, 1995) was 

used to reduce mixing due to the removal of large 

quantities of water during the purging process. Lowflow sampling for the second test consisted of pumping the wells at a rate of approximately 100 to 300 mL 

per minute until the temperature, pH, and specific 

conductance stabilized and no additional water-level 

drawdowns were observed. Typically, this required 

purging less than a gallon of water over a time period 

of approximately 15 minutes.

Table 1. Well-construction details and number of passive 

diffusion bag samplers deployed, Naval Industrial Reserve 

Ordnance Plant, Fridley, Minnesota

[PDB, passive diffusion bag; DU, deep unconsolidated sediments; 

MU, middle unconsolidated sediments; DLS; deep limestone; SU, 

shallow unconsolidated sediments; NA, data not available]

Well

number

(fig. 1)

Screen 

length 

(feet)

Screened depth 

below top of 

casing (feet) Zone

Number of 

PDB samplers 

deployed

Top Bottom Test 1

(1999)

Test 2

(2000)

2-D 10 102.3 112.5 DU 1 0

3-IS 10 67.4 77.4 MU 1 0

3-PC 27 132.9 159.6 DLS 6 6

3-S 15 19.7 34.8 SU 1 9

4-IS 10 66.9 76.9 MU 1 0

7-D 10 108 118 DU 1 0

8-D 10 118 128 DU 1 6

8-S 10 19.8 29.7 SU 1 4

9-D 10 114.3 124.3 DU 1 0

9-S 10 19.3 29.3 SU 1 0

12-D 10 122.9 132.9 DU 1 0

13-S 10 23.9 33.9 SU 1 0

14-D 10 82.6 92.6 DU 1 0

17-S 10 29 39 SU 1 0

18-S 10 30.8 40.8 SU 1 6

19-S 10 35 45 SU 1 5

24-S 15 21.7 36.7 SU 3 0

25-S 10 NA NA SU 1 0

26-S NA NA NA SU 1 5
D-4 Field Testing of Passive Diffusion Bag Samplers for Volatile Organic Compound Concentrations in Ground Water,

Naval Industrial Reserve Ordnance Plant, Fridley, Minnesota, November 1999 and May 2000

Four to nine PDB samplers were deployed in 

each of the seven wells during the second test. At all 

of the tested wells except well 3-S, a submersible 

positive-displacement pump was deployed at the 

same time as the PDB samplers. In well 3-S, an 

obstruction in the well prevented installation of the 

pump; therefore, a Tygon tube was attached to the 

PDB-sampler line at the time of sampler deployment. One end of the tube was open at the depth of 

the PDB sampler, and the other end extended to the 

surface for attachment to a peristaltic pump.

After field-parameter stabilization, water 

samples were collected for VOC analysis. The 

submersible pumps then were removed from the 

wells with the attached PDB samplers, and the 

water recovered in the PDB samplers was transferred to VOA vials. Both sets of samples were 

sent to a commercial laboratory for analysis using 

USEPA method 8260b (U.S. Environmental 

Protection Agency, 1999). The same general 

approach was used to sample well 3-S; however, 

well 3-S was sampled by low-flow methodology 

using a peristaltic pump.

FIELD TEST RESULTS AND 

CONCLUSIONS

A comparison of total 1,2-dichloroethene 

(1,2-DCE) and TCE concentrations obtained by 

using PDB samplers to concentrations obtained by 

using the conventional purge method in wells where 

one PDB sampler was deployed showed good agreement at several wells and poor agreement in others 

(table 2). For this investigation, good agreement was 

considered to be a concentration difference of less 

than 5 micrograms per liter (μg/L) for 1,2-DCE and 

to be a concentration difference of either less than 

10 μg/L or less than 10 percent for TCE. Of the sites 

Table 2. Comparison of total 1,2 dichloroethene and trichloroethene concentrations obtained by purgeand-sample method to concentrations obtained by using passive diffusion bag samplers in wells where a 

single passive diffusion sampler was deployed at the Naval Industrial Reserve Ordnance Plant, Fridley, 

Minnesota, November 1999

[1,2-DCE, total 1,2-dichloroethene; TCE, trichloroethene; PDB, passive diffusion bag; <, less than; >, greater than. 

Samples obtained by the purge-and-sample method were collected by a private consultant. Concentrations are in 

micrograms per liter (μg/L)]

Well 

number

(fig. 1)

Sample 

date

1,2-DCE concentration TCE concentration 

Well 

volumes 

purged

Gallons of 

water 

purged

PDBsampling 

method

Purge-andsample 

method

PDBsampling 

method

Purge-andsample 

method

2.D 11/3/99 2.8 3.1 2.5 4.4 3 54

3-IS 11/3/99 6.4 4.2 69 42# 4 40

3-S 11/3/99 3.7 260# 38 730# 4 6

4-IS 11/4/99 53 57 860 910 3 27

7-D 11/3/99 <1 4.4 2.2 17# 3 180

8-D 11/4/99 52 12# 23 70# 4 260

8-S 11/3/99 440 620# 240 340# 4 8

9-D 11/3/99 10 9.1 68 62 4 240

9-S 11/3/99 22 19 180 160# 5 10

12-D 11/3/99 22 2# 14 24 4 300

13-S 11/2/99 <2 <2 <1 <1 3 9

14-D 11/2/99 <2 <2 <1 <1 3 156

17-S 11/3/99 71 68 40 42 3 9

18-S 11/4/99 130 650# 570 2,300# 4 12

19-S 11/3/99 15 19 410 610# 3 6

25-S 11/2/99 <2 <2 <1 <1 3 9

26-S 11/4/99 52 38# 2,900 3,800# 3 6

#Sites that showed poor comparison between methods (>5 μg/L difference between methods for total 1,2-DCE, >10 μg/L or 

>10 percent difference between methods for TCE).
Field Test Results and Conclusion D-5

that did not meet these criteria, samples from well 

3-IS had higher 1,2-DCE and TCE concentrations in 

water from the PDB sampler than from the pumped 

sample, implying that higher concentrations were 

present in the well than were indicated by the pumped 

sample, and the PDB sampler better represented the 

higher concentrations. Samples from well 9-S had 

TCE concentrations that differed by 11 percent, but 

the concentration was higher in water from the PDB 

sampler than from the pumped sample, again implying that the PDB sampler better represented the 

higher concentrations. Samples from well 7-D had 

higher concentrations of TCE in water from the 

purged sample than in water from the PDB sample 

(table 2); however, some differences probably are to 

be expected after purging 180 gallons of water from 

the well.

Other wells showed substantially poorer agreement in VOC concentrations between the PDB 

samples and the conventional purge samples. In 

particular, wells 3-S, 8-S, 18-S, 19-S, and 26-S 

showed substantially higher VOC concentrations in 

water from the conventional purge sampling than in 

water from the PDB samplers (table 2). Examination 

of the data from two wells where multiple diffusion 

samplers were deployed during the first test (table 3) 

provides some clues as to a possible source of the 

poor agreement in some of the wells. At well 24-S, 

the analytical data show no significant concentration 

difference between sampling methods for 1,1-dichloroethane (1,1-DCA), 1,1-dichloroethene (1,1-DCE), 

1,2-DCE, and TCE. Data from the PDB sampler, 

however, indicates the presence of relatively high 

1,2-DCE concentrations and relatively low TCE 

concentrations near the top of the screen and the 

opposite near the base of the screen (fig. 2A). This 

may mean that the dechlorination potential is higher 

in sediment near the base of the screen than near the 

top of the screen or it may be the product of differential transport. In any case, it is apparent that the 

1,2-DCE and TCE concentrations change over the 

screened interval. Thus, pumping the well would 

lead to mixing of these differing concentrations. 

Although the 1,2-DCE and TCE concentrations 

obtained by the conventional purging approach 

differs from the concentrations found in the PDB 

sampler from the same depth, it appears that the 

pumped sample represents an approximate average 

concentration across the screened interval 

(fig. 2A, table 3). The data suggest that 

some of the differences in results between 

the two methods may be from mixing of 

stratified contaminant concentrations by 

the purging of three or more casing 

volumes.

At well 3-PC, no contaminant 

stratification was apparent from the PDBsampler data (fig. 2B); however, the tetrachloroethene (PCE) concentrations from 

the PDB sampler were approximately 

15 μg/L lower than in water from the 

pumped sample. After purging 285 

gallons of water, the contributing areas 

and sources of water sampled are probably much different between the conventional purged sample and waters in 

contact with the PDB sampler, thus leading to differences in concentrations of 

PCE. TCE and 1,2-DCE were not significant components in the contamination at 

well 3-PC.

Table 3. Concentrations of selected volatile organic compounds in water 

from multiple passive diffusion bag samplers and conventional purge 

sampling, Naval Industrial Reserve Ordnance Plant, Fridley, Minnesota, 

November 1999

[1,1-DCA, 1,1-dichloroethane; 1,1-DCE, 1,1-dichloroethene; 1,2-DCE, total 

1,2-dichloroethene; PCE, tetrachloroethene; TCE, trichloroethene; PDB, passive 

diffusion bag sampler; P&S, conventional purge-and-sample method (samples 

were collected by private consultant); <, less than. Concentrations are in 

micrograms per liter] 

Well

number

(fig. 1)

Sampling 

method

Depth 

below 

top of 

casing 

(feet)

1,1-DCA 1,1-DCE 1,2-DCE PCE TCE

3-PC PDB 134.8 <1 <1 <2 22 1.7

3-PC PDB 138.8 <1 <1 <2 23 1.7

3-PC PDB 142.8 <1 <1 <2 25 1.7

3-PC PDB 146.8 <1 <1 <2 24 1.7

3-PC PDB 150.8 <1 <1 <2 24 1.6

3-PC PDB 154.8 <1 <1 <2 25 1

3-PC P&S 144.7 <1 <1 <2 40 2.7

24-S PDB 24.3 3.5 1.9 780 <1 200

24-S PDB 28.3 3.6 2.1 600 <1 350

24-S PDB 32.3 2.4 1.4 380 <1 500

24-S P&S 28.3 3.4 1.6 520 <1 330
D-6 Field Testing of Passive Diffusion Bag Samplers for Volatile Organic Compound Concentrations in Ground Water,

Naval Industrial Reserve Ordnance Plant, Fridley, Minnesota, November 1999 and May 2000

To determine whether the differences between 

the methods could be attributed to mixing as a result 

of the conventional purging approach, seven of the 

wells where poor agreement was observed were 

resampled by using a combination of multiple PDB 

samplers and a low-flow purging method (table 4). 

At well 3-PC, where PCE concentrations differed by 

15 μg/L between the conventional purge and the 

PDB samplers, resampling showed that the PCE 

concentrations in water from the PDB samplers and 

the low-flow sampling differed by only about 2 μg/L. 

These data suggest that purging 285 gallons during 

the first sampling adversely affected the results. 

Well 18-S showed substantial differences in 

concentration between methods during the first 

sampling (table 2). Concentrations of 1,2-DCE and 

TCE were 650 and 2,300 μg/L, respectively, in 

water from the conventional purge method but only 

130 and 570 μg/L, respectively, in water from the 

PDB sampler (table 2). During the second sampling, 

the PDB samplers showed that substantial stratification of VOCs is present over a vertical interval of 

approximately 6 ft (fig. 3A). TCE concentrations 

ranged from 470 μg/L at a depth of 31.48 ft to 

1,600 μg/L at a depth of 37.61 ft (table 4). The 

1,2-DCE concentration changed from 240 to 480 μg/L 

over the same interval. Although the TCE concentration from the low-flow sampling (1,000 μg/L) 

differs from the closest PDB samplers (1,300 and 

1,600 μg/L), the low-flow-sampling concentrations 

are consistent with what would be expected from 

mixing the concentrations over the screened interval 

during pumping (fig. 3A). During the first round of 

sampling, the PDB sampler was positioned in the 

center of the well screen in an area where concentrations were substantially lower than near the base 

of the screened interval. Therefore, mixing of water 

across the screened interval during pumping could 

produce pumped concentrations exceeding those in 

the single PDB sampler, as shown in table 2. The 

data suggest that the VOC concentrations from the 

PDB samplers accurately reflect the VOC distribution in the screened interval. The data also suggest 

that the discrepancy between PDB, conventional 

Figure 2. Comparison between passive diffusion bag (PDB) sampling method and 

conventional purge sampling method, Naval Industrial Reserve Ordnance Plant, Fridley, 

Minnesota, November 1999.
Field Test Results and Conclusion D-7

Table 4. Concentrations of selected volatile organic compounds in water from diffusion and low-flow 

sampling, Naval Industrial Ordnance Reserve Plant, Fridley, Minnesota, May 2000

[1,1,1-TCA, 1,1,1-trichloroethane; 1,1-DCA, 1,1-dichloroethane; 1,2-DCE, 1,2-total dichloroethene; PCE, tetrachloroethene; TCE, trichloroethene; PDB, passive diffusion bag; LF, low-flow; <, less than. Concentrations are in micrograms 

per liter]

Well Sampling 

method

Depth below 

top of casing 

(feet)

1,1,1-TCA 1,1-DCA 1,2-DCE Benzene PCE TCE

3-PC PDB 135.6 <1.0 <1.0 <2.0 <1.0 8.3 1.6

3-PC PDB 140.2 <1.0 <1.0 <2.0 <1.0 8.5 1.5

3-PC PDB 145.3 <1.0 <1.0 <2.0 <1.0 8.8 1.7

3-PC PDB 150.2 <1.0 <1.0 <2.0 <1.0 8.9 1.6

3-PC PDB 157.1 <1.0 <1.0 <2.0 <1.0 9.5 1.7

3-PC LF+ 157.6 <1.0 <1.0 <2.0 <1.0 11 1.3

3-S PDB 24.11 22 4.2 87 <2.0 <2.0 360

3-S PDB 25.36 21 4.5 85 <2.0 <2.0 360

3-S PDB 26.51 21 4.6 90 <2.0 <2.0 360

3-S PDB 28.01 17 5 84 <2.0 <2.0 330

3-S PDB 29.41 5.3 10 120 <2.0 <2.0 220

3-S PDB 30.26 <2.0 16 160 <2.0 <2.0 280

3-S PDB 31.41 <2.0 15 190 <2.0 2.5 390

3-SR PDB 31.41 <2.0 15 190 <2.0 2.8 410

3-S PDB 32.61 <5.0 <5.0 180 <5.0 <5.0 380

3-S LF# 30.11 15 8.3 130 <2.0 <2.0 360

8-D PDB 119.1 <1.0 1 40 5 <1.0 16

8-D PDB 121.2 <1.0 1.1 41 5 <1.0 17

8-D PDB 122.9 <1.0 1 40 4.8 <1.0 17

8-D PDB 124.9 <1.0 1 40 4.9 <1.0 17

8-D PDB 126.5 <1.0 1.1 41 4.8 <1.0 17

8-D LF+ 124.9 <1.0 1.1 38 3.1 1.1 26

8-S PDB 22.45 <2.0 <2.0 110 <2.0 3.5 230

8-S PDB 25.2 <2.0 <2.0 110 <2.0 3.9 260

8-S PDB 30.1 <5.0 <5.0 640 <5.0 <5.0 170

8-S LF+ 29.45 <10 <10 740 <10 <10 230

18-S PDB 31.48 <5.0 <5.0 240 <5.0 <5.0 470

18-S PDB 32.66 <5.0 <5.0 260 <5.0 <5.0 500

18-S PDB 33.71 <10 <10 220 <10 <10 940

18-S PDB 35.71 <10 <10 450 <10 <10 1,300

18-S PDB 37.61 <10 <10 480 <10 <10 1,600

18-S LF+ 36.86 <10 <10 380 <10 <10 1,000

19-S PDB 35.86 <1.0 3.1 8.5 <1.0 3.1 130

19-S PDB 38.11 <2.0 3.4 15 <2.0 <2.0 310

19-S PDB 39.86 <1.0 2.9 8.7 <1.0 2.8 140

19-S PDB 43.06 <1.0 3.5 9.1 <1.0 3.1 120

19-S LF+ 42.26 <1.0 2.6 11 <1.0 2.4 160

26-S PDB 32.76 <20 <20 86 <20 <20 2,700

26-S PDB 34.71 <20 <20 53 <20 <20 3,400

26-S PDB 39.91 <20 <20 56 <20 <20 2,900

26-S LF+ 39.16 <20 <20 <40 <20 <20 2,000

26-SR LF+ 39.16 <1.0 <1.0 37 <1.0 <1.0 2,000

RReplicate sample.

+Sample collected using submersible positive-displacement pump.

#Sample collected using peristaltic pump. 
D-8 Field Testing of Passive Diffusion Bag Samplers for Volatile Organic Compound Concentrations in Ground Water,

Naval Industrial Reserve Ordnance Plant, Fridley, Minnesota, November 1999 and May 2000

Figure 3. Comparison between passive diffusion bag (PDB) samples and low-flow samples in resampled wells, Naval 

Industrial Reserve Ordnance Plant, Fridley, Minnesota, May 2000.
Field Test Results and Conclusion D-9

purge, and low-flow purge sample concentrations is 

caused by mixing of the stratified contamination or 

of areally heterogeneous concentrations during 

pumping.

A similar argument can be postulated to 

explain the substantial differences in concentration 

between the two methods at well 8-S during the first 

round of sampling (table 2). The resampling with 

multiple PDB samplers shows that concentrations 

shallower than 26 ft deep differ from those at 30 ft 

deep (fig. 3B). The TCE concentration in the lowflow sample is an approximate average of the 

concentrations measured across the length of the 

well screen with the PDB samplers. The 1,2-DCE 

concentration is higher in water from the low-flow 

sample than from the PDB sampler; however, this 

may be expected in an area where the 1,2-DCE 

concentration increased with depth from 110 to 

640 μg/L over an interval of 5 ft (fig. 3B). Because 

the low-flow sample was collected from the base of 

the interval, it is not unreasonable to suppose that 

concentrations continued to increase with depth 

below the sampled interval, and that the low-flow 

sample represents an integration of water that 

included higher concentrations than were evident 

from the PDB samplers. Thus, the data are again 

consistent with the hypothesis that the PDB samplers 

accurately reflected the ambient VOC concentrations.

Well 19-S is another well where discrepancies 

were noted between TCE concentrations from the 

conventional-purge samples and the PDB samples 

(table 2). Resampling the well by using multiple 

PDB samplers and low-flow sampling again showed 

substantial discrepancies in the TCE concentrations; 

however, examination of the vertical TCE-concentration distribution strongly suggests that the lowflow sample represents a mixing of stratified TCE 

layers in the screened interval (fig. 3C). The data 

again suggest that the PDB samplers provided an 

accurate representation of ambient concentrations. 

The difference in TCE concentrations measured 

between the two sampling events (410 to 610 μg/L 

in November 1999 and 130 to 310 μg/L in May 

2000) is consistent with observation that TCE 

concentrations have shown substantial temporal 

variability in previous years (Keith Henn, Tetra Tech 

NUS, written commun., 2000). 

Substantial discrepancies in the 1,2-DCE and 

TCE concentrations between the PDB sampling and 

the conventional sampling also were observed in 

water from well 3-S during the first test (table 2). 

A possible explanation for the poor agreement is that 

the diffusion sampler was not properly positioned. 

When the sampler was recovered, at least 5 ft of line 

that was intended to be below the water table was 

dry, implying that the diffusion sampler was inadvertently placed too shallow and possibly even 

partially above the water table. Thus, a comparison 

of the two approaches during the first test may be 

inappropriate. During the second test at the well, 

however, data from the PDB samplers showed that 

concentrations of 1,1,1-trichloroethane (1,1,1-TCA), 

1,1-dichloroethane (1,1-DCA), 1,2-DCE, and TCE 

were stratified along the length of the well screen 

(fig. 3D, table 4). The data suggest that the low-flow 

sampling represents an averaging of concentrations, 

whereas the PDB sampler represents concentrations 

at points (fig. 3D).

In wells 8-D and 26-S, no substantial vertical 

concentration stratification was observed (figs. 3E, 

3F), despite the substantial difference in concentrations obtained by the PDB samplers and the conventional purge sampling (table 2). When well 8-D was 

sampled by using low-flow purging, the concentrations from the pumped sample were similar to those 

in the PDB samplers (0.1-μg/L difference for 

1,1-DCA, 3-μg/L difference for 1,2-DCE, 1.8-μg/L 

difference for benzene, and 9-μg/L difference for 

TCE) (table 4). The concentration discrepancy 

obtained while using the conventional purge approach 

probably was a reflection of pumping 260 gallons of 

water and may represent lateral mixing of chemically heterogeneous water. At well 26-S, TCE 

concentrations obtained by using the conventional 

purge approach were substantially higher than in 

water from the PDB sampler (table 2), and concentrations obtained by using low-flow sampling were 

substantially lower than those in water from the PDB 

sampler (fig. 3F). These data and the previous 

discussion suggest that the pumped sample integrated water with different concentrations than those 

that were present in the screened interval of the well.
D-10 Field Testing of Passive Diffusion Bag Samplers for Volatile Organic Compound Concentrations in Ground Water,

Naval Industrial Reserve Ordnance Plant, Fridley, Minnesota, November 1999 and May 2000

SUMMARY

VOC concentrations from PDB samplers were 

compared to VOC concentrations from conventional purge sampling and low-flow purge sampling 

in side-by-side tests at NIROP, in Fridley, Minnesota. PDB samplers were tested in 19 wells at 

NIROP. The samplers were deployed in October 

1999, allowed to equilibrate approximately 30 days, 

and recovered in November 1999. In a second test, 

PDB samplers were deployed in 7 wells in April 

2000, allowed to equilibrate approximately 35 days, 

and recovered in May 2000. 

A comparison of 1,2-DCE and TCE concentrations obtained by using PDB samplers and the conventional purge method in wells where one PDB sampler 

was deployed showed good agreement at several wells 

but poor agreement in others. For this investigation, 

good agreement was considered to be less than 5 μg/L 

difference for 1,2-DCE and less than 10 μg/L or less 

than 10 percent difference for TCE. Of the sites that 

did not meet these criteria, some sites (wells 3-IS and 

9-S) had higher concentrations in water from the PDB 

sampler than in water from the conventional purge 

sample, implying that the PDB sampler more accurately reflected the local concentrations. 

Concentration data from multiple diffusion 

samplers during the conventional-purge sampling and 

during the low-flow sampling indicates that the VOC 

concentrations in many of the wells are stratified 

within the screened interval. At well 18-S, TCE 

concentrations ranged from 470 to 1,600 μg/L over a 

vertical distance of approximately 6 ft. In this and 

other wells where stratification was observed, the 

concentration in the pumped sample appears to represent a mixing of waters having differing concentrations, whereas the PDB samplers represent localized 

concentrations. Other discrepancies between VOC 

concentrations from the conventional purge method 

and the PDB samplers appear to be related to the 

removal of multiple casing volumes of water (in some 

cases more than 100 gallons) prior to sampling. The 

data suggest that the VOC concentrations from the 

PDB samplers accurately reflect the VOC distribution 

in the screened interval of the tested wells. The data 

further suggest that PDB samplers are a viable method 

for sampling ground-water VOCs at NIROP. Multiple 

diffusion samplers may be required at some sites 

where contaminant stratification is present in the 

screened or open interval. 

REFERENCES

Barcelona, M., Wehrmann, H.A., and Varljen, M.D., 

1994, Reproducible well-purging procedures and 

VOC stabilization criteria for ground-water sampling: Ground Water, v. 32, no. 1, p. 12-22.

Gefell, M.J., Hamilton, L.A., and Stout, D.J., 1999, 

A comparison between low-flow and passivediffusion bag sampling results for dissolved volatile 

organics in fractured sedimentary bedrock, in Proceedings of the Petroleum and Organic Chemicals in 

Ground Water—Prevention, Detection, and Remediation Conference, November 17-19, 1999, Houston, 

Texas: National Ground Water Association and the 

American Petroleum Institute, p. 304-315. 

McClellan AFB Environmental Directorate, 2000, Passive 

diffusion membrane samplers technology application 

analysis report: Sacramento, Calif., McClellan AFB, 

78 p. 

Parsons, Engineering Science, Inc., 1999, Final technical 

report for the evaluation of groundwater diffusion 

samplers: Consultant’s report to the Air Force Center 

for Environmental Excellence Technology Transfer 

Division, December 1999, 51 p.

Shanklin, D.E., Sidle, W.C., and Ferguson, M.E., 1995, 

Micro-purge low-flow sampling of uranium-contaminated ground water at the Fernald Environmental 

Management Project: Ground Water Monitoring and 

Remediation, v. 15, no. 3, p. 168-176.

Tetra Tech NUS, Inc., 2000, 1999 annual monitoring 

report, Naval Industrial Reserve Ordnance Plant 

(NIROP), Fridley, Minnesota: Consultant’s report 

to Southern Division Naval Facilities Engineering 

Command, North Charleston, South Carolina, March 

2000, 56 p.

Vroblesky, D.A., and Hyde, W.T., 1997, Diffusion 

samplers as an inexpensive approach to monitoring 

VOCs in ground water: Ground Water Monitoring 

and Remediation, v. 17, no. 3, p. 177-184. 

U.S. Environmental Protection Agency, 1999, On-line SW846 methods: accessed December 21, 1999, at URL 

http: //search.epa.gov/epaoswer/hazwaste/test/

tx8xxx.htm#8XXX.
Evaluation of a Diffusion Sampling Method for Determining 

Concentrations of Volatile Organic Compounds in 

Ground Water, Hanscom Air Force Base, 

Bedford, Massachusetts

USGS Water-Resources Investigations Report 00-4242

Peter E. Church
Introduction E-1

Evaluation of a Diffusion Sampling 

Method for Determining Concentrations of 

Volatile Organic Compounds in Ground 

Water, Hanscom Air Force Base, 

Bedford, Massachusetts

By Peter E. Church

Abstract

Ground-water samples were collected 

in May 1999 at the Hanscom Air Force Base, 

Bedford, Massachusetts, with a method involving 

water diffusion samplers and a conventional 

low-flow sampling method to evaluate the use 

of diffusion samplers as an inexpensive and reliable 

alternative method for monitoring volatile organic 

compounds at the base. The principal compounds 

detected by both sampling methods were 1,2-

dichloroethylene isomers, which ranged in 

concentration from not detected to nearly 

7,000 micrograms per liter, and trichloroethylene, 

which ranged in concentration from not detected to 

nearly 5,000 micrograms per liter. A Sign test, 

applicable to these highly skewed concentrations, 

indicates that with a probablity of 95 percent, it is 

equally likely to have diffusion sample concentrations of 1,2-dichloroethylene isomers and 

tricholorethylene greater than low-flow sample 

concentrations as it is to have diffusion sample concentrations of these compounds less than low-flow 

sample concentrations.

Analysis of the distribution of 1,2-dichloroethylene isomers and trichloroethylene concentrations in samples from long-screen wells (screen 

length 10 feet or greater) with multiple-diffusion 

samplers indicates that vertical concentration 

variations within well screens differ substantially 

from sampled wells at the base. These concentration variations can be attributed to concentration 

stratification in the aquifer adjacent to the well 

screen; however, data from borehole-flowmeter 

logs from selected long-screen wells suggest that 

wellbore flow also may be a factor affecting concentration variations. Where water quality varies 

vertically along a well screen, water sampled with 

multiple diffusion samplers may better characterize 

water quality in the well than low-flow samples.

INTRODUCTION

Volatile organic compounds (VOCs) are 

present in ground water at Hanscom Air Force Base 

(AFB), Massachusetts. These VOCs include chlorinated solvents, petroleum hydrocarbons, and their biodegradation products. Remediation efforts to remove 

VOCs from ground water have been ongoing at the 

base since 1991. Considerable amounts of time and 

money are spent each year to collect water samples 

from monitoring wells using conventional low-flow 

techniques as part of a remediation-monitoring 

programs at sites such as this. An alternative, lowercost sampling method that will save time and yield reliable results would be advantageous to all services and 

agencies involved in such activities.
E-2 Evaluation of a Diffusion Sampling Method for Determining Concentrations of VOCs in Ground Water, Hanscom AFB, Mass.

Vroblesky and Hyde (1997) describe an inexpensive and effective sampling method that uses water-towater polyethylene-membrane diffusion samplers 

(referred to as diffusion samplers in this report) placed 

in wells. Although this passive method has yielded 

promising results in some settings (Vroblesky and 

Hyde, 1997), additional testing is needed to evaluate its 

suitability as a long-term monitoring tool at Hanscom 

AFB. Also of concern at the base are the vertical distribution of VOCs in long-screen wells (screen length 

10 ft or greater) and possible redistribution of VOCs 

caused by vertical flow in these wells.

The U.S. Geological Survey (USGS), in cooperation with the Air Force Center for Environmental 

Excellence (AFCEE), Brooks Air Force Base, San 

Antonio, Texas, and in consultation with the Restoration Program Manager at Hanscom AFB, designed a 

ground-water-sampling and borehole-logging program 

to compare VOC concentrations in water samples collected with the diffusion sampling method and a lowflow sampling method. To support interpretation of the 

water-quality data, multiple diffusion samplers were 

placed in long-screen wells and an open borehole in 

bedrock to examine the vertical distribution of VOC 

concentrations and to evaluate possible effects of flow 

in well screens on the vertical distribution of VOCs in 

selected wells.

The USGS installed diffusion samplers in wells 

during April 1999; samplers were retrieved in May 

1999. The IT Corporation, Hopkinton, Mass., under 

contract to the Hanscom AFB, collected ground-water 

samples with the low-flow sampling method in May 

1999, soon after retrieval of diffusion samplers. Vertical flow was measured by the USGS in four longscreen wells with a borehole flowmeter in June 1999.

The diffusion sampling method was chosen for 

testing as a possible alternative method over the current 

low-flow method used at the Hanscom AFB because 

diffusion samplers were expected to require less overall 

time for sampling, and lower costs for equipment and 

labor. Low-flow sampling methods, designed for collection of ground-water samples adjacent to well 

screens, while minimizing disturbance to the aquifer 

and drawdowns in the well casings (Puls and Barcelona, 1995), require purging the well-screen water until 

various water-quality parameters stabilize, collection 

and disposal of the purged water, and decontamination 

of the downhole sampling equipment before collection 

of a sample from another well. The diffusion sampling 

method eliminates the monitoring of water-quality 

parameters and generates little to no waste water for 

disposal. Therefore, assuming that the quality of water 

in the well screen is representative of the water quality 

in the adjacent aquifer, the diffusion sampling method 

may prove to be a reliable alternative to the low-flow 

method.

This report compares a diffusion sampling 

method to a low-flow sampling method for monitoring 

of VOCs in ground water at the Hanscom AFB. The 

report also describes the possible effects of vertical 

variations of VOCs and borehole flow in long-screen 

wells on sampling with diffusion and low-flow 

methods.

The author thanks personnel of the Air Force 

Center for Environmental Excellence, Brooks 

AFB, San Antonio, Texas and Tom Best, Restoration 

Program Manager, Hanscom AFB, for their cooperation in developing the study program. Tom 

Best provided pertinent site information and assistance 

in the field, and personnel of IT Corporation, 

Hopkinton, Mass., collected the low-flow samples. 

The helpful comments throughout this study from 

Richard Willey, Office of Site Remediation and 

Restoration, U.S. Environmental Protection Agency, 

Region I, Boston, Mass., and the reviews of the report 

by Richard Willey and Javier Santillan, Air Force 

Center for Environmental Excellence, are greatly 

appreciated. William J. Andrade, Analytical Specialist 

and Joe Montanaro, Analyist, U.S. Environmental 

Protection Agency, Region I, Lexington, Mass., also 

are acknowledged for analyzing both the diffusion 

and low-flow samples and for providing guidance on 

quality-assurance procedures during the collection 

of water samples.

DESCRIPTION OF STUDY AREA

The study area is in the northeastern part of the 

Hanscom AFB in Bedford, Mass. (fig. 1). Physical, 

hydrogeological, and hydraulic characteristics of this 

area have been described by Haley & Aldrich, Inc. 

(1996, 1998). The land surface ranges in altitude from 

about 110 to 125 ft in most of the study area. In the 

west-central part of the study area, near well A-3, landsurface altitude increases to greater than 145 ft. 

Swamps occupy the north-central and eastern part of 

the study area. Surface drainage at the Hanscom AFB 

is controlled by storm culverts and swales that drain to 

the northwest, northeast, and east.
Introduction E-3

Base from the U.S. Geological Survey 7.5 x15 minute quadrangle topographic map

Maynard, Massachusetts base map, 1:25,000, 1987

Based on Massachusetts coordinate system

42o28'45"

71o17'15" 71o17' 71o

16'45"

42o28'30"

Treatment

 Facility

Campground

Aircraft 

landing

strip

0

0 100 METERS

500 FEET

121

120

123

125

119

119

125

120

130

140

118

118

117

116

117

119

EXPLANATION

117

B245

B249

2

SWAMP RAP1-6T

WATER-TABLE CONTOURS—Shows 

altitude of water table in May 1998 (Haley 

and Aldrich, Inc., 1998). 

Contour interval is variable. Datum is 

sea level.

WELLS SAMPLED BY WATER 

DIFFUSION METHOD AND 

IDENTIFIER

WELLS SAMPLED BY WATER

DIFFUSION AND LOW-FLOW 

METHOD AND IDENTIFIER

WELLS SAMPLED BY WATER 

DIFFUSION AND LOW-FLOW METHOD 

AND IDENTIFIER; VERTICAL FLOW 

MEASURED 

RESTORATION WELLS AND 

IDENTIFIER

DIRECTION OF FLOW

0

0 50

50

MILES

 KILOMETERS

MASSACHUSETTS

CAPE COD

42o30'

42o

73o 71o

B240

RAPI-7

B254

B245

B244A

B255

B126

B248

B249

B240

A-3

B113

RAP2-2R

3

6

4

5

2

1

B111

RAP2-1T

RAP2-1R

RAP2-3T

B251

B107

B108

PO2-2R

RAP1-6T

RAP1-6R

RAP1-6S

Figure 1. Location of study area, altitude of water table in May 1998, location of wells sampled with 

diffusion and low-flow sampling methods, wells logged with a borehole flowmeter, and restoration wells, 

Hanscom Air Force Base, Bedford, Massachusetts.
E-4 Evaluation of a Diffusion Sampling Method for Determining Concentrations of VOCs in Ground Water, Hanscom AFB, Mass.

VOCs are present in surficial aquifers, a shallow 

aquifer that is unconfined, a deep confined aquifer, and 

the underlying fractured bedrock aquifer that also is 

confined. The shallow aquifer consists of fine sand and 

silt of glacial outwash deposits. The deep confined 

aquifer consists of a wide range of particle sizes from 

silt to boulders [previously described as glacial till by 

Haley & Aldrich, Inc. (1996), and hereafter referred to 

as till] below a confining layer of lacustrine silt. The 

bedrock is composed primarily of granitic gneiss and 

schists. The bedrock surface slopes from a depth of 

about 20 to 30 ft below land surface in the northern part 

of the study area to a depth of about 100 to 120 ft 

below land surface in the southern part of the study 

area. The outwash deposits at the surface range in 

thickness from about 8 to 28 ft and grade downward 

from silty, fine to medium sand to silty, fine to coarse 

sand. The lacustrine deposit ranges in thickness from 

less that 1 ft to about 48 ft and grades downward from 

fine sand and silt to clayey silt. Lacustrine sediments 

are not present in the west-central part of the study near 

well A-3 where the outwash is directly underlain by the 

sandy and gravelly till, which ranges in thickness from 

about 8 to 60 ft. In this report, the outwash deposits are 

referred to as the surficial aquifer, the confined till 

deposit as the till aquifer, and the fractured bedrock as 

the bedrock aquifer.

Water-level measurements in May 1998 (Haley 

& Aldrich, Inc., 1998) indicate that the water table 

within the study area ranged in altitude from about 116 

to 140 ft (fig. 1). The water table is primarily in the 

surficial aquifer at depths of 0 to about 12 ft below land 

surface. Ground-water flow in the surficial aquifer is 

generally from the southwest to the northeast. In the 

west-central part of the site, where the lacustrine 

deposit is not present and the till is directly overlain 

by the outwash deposits, a cone of depression in the 

water table is formed by the continuous pumping of 

the bedrock aquifer by Restoration Well No. 6 (fig. 1). 

The water table in the southeastern part of the site 

appears to be affected by continuous pumping from 

Restoration Well No. 5 in the till aquifer. Pumping 

from Restoration Wells Nos.1, 2, 3, and 4 (fig. 1) 

have formed a depression in potentiometric surfaces 

in the till and bedrock aquifers from the southeastern 

to the northwestern parts of the site (potentiometric 

surfaces in the till and bedrock aquifer are not shown 

on fig. 1). Aquifer-test data from selected wells indicate 

horizontal hydraulic conductivities range from about 

5 to 65 ft/d in the till aquifer and from about 0.1 to 

0.6 ft/d in the bedrock aquifer (Haley & Aldrich, Inc., 

1996).

SAMPLING METHODS

Water-quality samples were collected with the 

diffusion sampling method and a low-flow sampling 

method. Duplicate samples, and equipment and trip 

blank samples, were collected for each sampling 

method to assess the quality of the data collected. Vertical flow was measured in selected long-screen wells 

with a borehole flowmeter.

Diffusion Samplers

Diffusion samplers were constructed based on 

the method described by Vroblesky and Hyde (1997). 

Polyethylene sleeves, 2-inch wide by 18-inch long, and 

4 mil thick, were heat sealed at one end, filled with 

about 300 mL of deionized water, and then closed by 

heat sealing the other end after the elimination of any 

air space. The water-filled polyethylene tubes were slid 

into 24-inch long, 1.5-inch diameter polyethylenemesh tubing and secured to plastic-covered cords at 

both ends with plastic cable ties. The diffusion samplers then were lowered into wells with weights 

attached to the cords, either to depths within well 

screens or to an open borehole in bedrock. The depths 

were measured from the midpoint of the samplers to 

the top of the well casing. The samplers remained 

in the wells for about 3 weeks before recovery to 

allow time for VOCs diffusing into the samplers to 

equilibrate with VOCs in the aquifer.

Upon retrieval, the polyethylene mesh was partially cut open, a small slit was made at the top of a 

sampler, and the water samples were decanted into 

40-milliliter glass vials. Hydrochloric acid (about 

0.1 mL) was added to the vials to preserve the sample. 

Once capped, the vials were packed in ice. Samples 

were hand delivered to the nearby U.S. Environmental 

Protection Agency (USEPA) laboratory in Lexington, 
Sampling Methods E-5

Mass., at the end of each day for analysis of VOCs by 

USEPA method 8260 (U.S. Environmental Protection 

Agency, 1996).

Seventy diffusion samplers were placed in 23 

wells on April 21 and 22, 1999. Fourteen of these wells 

had screens that were at least 10 ft long or longer and 

multiple diffusion samplers were placed in these wells. 

Five wells with 10-foot screens each contained three 

diffusion samplers; in each well, one sampler was 

placed about 1 ft above the bottom of the screen, one 

at the middle of the screen, and one about 1 ft below 

the top of the screen. Eight wells with screens longer 

than 10 ft each contained five diffusion samplers 

that were equally spaced from about 1 ft above the 

bottom of the screen to about 1 ft below the top of the 

screen. Five samplers also were placed in the open 

bedrock well; these were equally spaced as in the 

long-screen wells.

Each of the eight remaining wells, which had 

screens 10 ft long or shorter, contained a single diffusion sampler placed at the midpoint of the screen. In 

the case where the water level was below the top of the 

screen, the diffusion sampler was placed at the midpoint between the water level and the bottom of the 

screen. At well RAP1-6S, a long-screen well open to 

the water table, only two diffusion samplers were 

installed in the 6 ft of water within the 14.5-foot long 

screen.

Diffusion samplers were retrieved during 

May 10–13, 1999, generally in order of increasing 

VOC concentration as determined from results of previous sampling (Haley & Aldrich, Inc., 1998). The 

comparison between the diffusion and the low-flow 

sampling methods was made using the midpoint diffusion sampler in wells where multiple samplers were 

installed. Because a diffusion sampler was not placed 

at the midpoint between the water level and the bottom 

of the screen in well RAP1-6S, the depth at which the 

low-flow sample was obtained, concentrations from 

this well were not used in the comparison of diffusion 

and low-flow sampling method. Relevant diffusion 

sampling information are summarized in Church and 

Lyford (2000).

Low-Flow Sampling

A bladder pump was used by IT Corporation, 

Hopkinton, Mass., to collect water samples with the 

low-flow sampling method. The pump intake was 

placed at the midpoint of each well screen. In the case 

where the water level was below the top of the screen, 

the pump intake was placed at the midpoint between 

the water level and the bottom of the screen. Purge 

rates were adjusted from about 0.1 to 1.0 L/min 

(0.26 gal/min) according to the rate of inflow to 

each well to minimize drawdown. Drawdowns measured during sampling ranged from negligible to 1.42 

ft, however, drawdowns in 86 percent of the wells sampled were less than 0.5 ft. Water-quality field parameters, water temperature, specific conductance, pH, and 

turbidity were monitored at 5-minute intervals, and a 

sample was collected after these field parameters stabilized. The stabilization criteria for these field parameters are: water temperature, ±1 degree Celsius; specific 

conductance, ±5 percent microsiemens per centimeter; 

pH, ±0.1 pH unit; turbidity, ±10 nephelometric units. 

Samples were processed and analyzed using the same 

procedures that were used with the diffusion samples.

Samples were collected from 21 wells with the 

low-flow sampling method May 10–14, 1999, after 

the diffusion samplers were retrieved. The wells 

B244A and B245, from which diffusion samples 

were obtained, were not sampled by the low-flow 

method because of the difficulty in transporting sampling equipment to these wetland locations. Low-flow 

water samples generally were collected within one day 

after the diffusion samples were collected and in the 

same order that the samplers were retrieved from the 

wells. Relevant low-flow sampling information 

are summarized in Church and Lyford (2000).

Borehole Flowmeter

Vertical flow in wells was measured using a 

borehole flowmeter, which consists of a downhole 

probe with heat sensors located equidistant above and 

below a heat source at the bottom of the probe (Keyes, 

1990). The heat source is a thin metal mesh through 

which water flows. A pulse of electricity causes this 

mesh to increase in temperature, thereby increasing the 

temperature of a small parcel of water. Travel time of 

the heated water is measured as it passes either of the 
E-6 Evaluation of a Diffusion Sampling Method for Determining Concentrations of VOCs in Ground Water, Hanscom AFB, Mass.

heat sensors, and vertical direction is determined by 

the sensor that detected the heated water. The annular 

space between the probe’s heat source and the well 

screen or casing must be sealed to direct vertical 

flow, if any, through the metal mesh. Travel times 

are calibrated to well diameter, and flow rates are 

expressed in gallons per minute. If the annular space 

between the heat source and the well screen has been 

properly sealed and the water-level changes caused 

by introducing the probe have stabilized, accuracies 

of ±5 percent can be obtained for vertical-flow 

measurements under static conditions. The minimum 

flow rate that can be detected by the borehole flowmeter used at this site is reported as 0.03 gal/min by 

the manufacturer (Mount Sopris Instruments, Golden, 

Colo.). Field experience with this flowmeter indicates 

that flow rates as low as 0.01 gal/min can be detected 

before the measurement is affected by thermal 

convection (B.P. Hansen, U.S. Geolgical Survey, oral 

commun., 1999)

Borehole-flowmeter logging was conducted 

under ambient (unstressed) and pumping (stressed) 

conditions in five wells at the base; two screened in the 

till aquifer, two screened in bedrock aquifer, and one in 

the open borehole in bedrock aquifer. Reliable flowmeter measurements under unstressed conditions were not 

obtained from the two wells screened in bedrock 

because the water levels in the well casing had not stabilized 2 hours after water was displaced by lowering 

the logging probe. In the open borehole in bedrock, the 

annular space between the probe and the bedrock wall 

could not be sealed, and reliable data under unstressed 

and stressed conditions could not be obtained.

EVALUATION OF SAMPLING 

METHODS

The principal VOCs detected with both sampling 

methods were 1,2-dichloroethylene isomers (1,2-DCE) 

and trichloroethylene (TCE). Concentrations of 1,2-

DCE in diffusion samples ranged from below the minimum reporting limit of 5 micrograms per liter (μg/L) to 

6,800 μg/L for 1,2-DCE and to 4,900 μg/L for TCE. 

Concentrations in water samples collected with the 

low-flow method ranged from below the minimum 

reporting limit of 5 μg/L to 6,400 μg/L for 1,2-DCE 

and 4,900 μg/L for TCE (table 1). Other VOCs 

detected, but generally at lower concentrations, include 

acetone, vinyl chloride, 1,1-dichloroethylene (1,1-

DCE), and 1,1-dichloroethane (1,1-DCA) (Church and 

Lyford, 2000).

Samples were collected at the midpoint of well 

screens (and at the midpoint of the open hole in bedrock) in 20 wells with both methods. Because VOCs 

were not detected in all wells, and many of the VOCs 

detected had concentrations outside of the calibration 

ranges of analytical instruments, the number of wells 

with paired samples for comparison of sampling methods was reduced to10 for concentration of 1,2-DCE 

and 16 for concentration of TCE (table 2, figs. 2 and 3). 

Concentrations of vinyl chloride were detected within 

analytical instrument calibration ranges in samples at 

the midpoint of well screens with both methods from 

only two wells, and 1,1-DCA from only one well. Acetone, commonly detected in laboratory blank samples 

(Church and Lyford, 2000), and 1,1-DCE were not 

detected in any samples from the midpoint of well 

screens with either sampling method. Therefore, only 

1,2-DCE and TCE concentrations are used to evaluate 

the diffusion sampling method as an alternative to the 

low-flow sampling method. Concentrations of 1,2-DCE 

used in this comparison of methods ranged from 8.2 to 

2,500 μg/L in diffusion samples and 5.9 to 2,600 μg/L 

in low-flow samples. Concentrations of TCE ranged 

from 12 to 4,900 μg/L in diffusion samples and 11 to 

4,900 μg/L in low-flow samples (table 2).

Quality Assurance for 

Sampling Methods

Quality assurance for water samples collected 

with diffusion samplers included an equipment blank, 

daily trip blanks, and duplicate samples for about 7 

percent of the samples collected. The equipment blank 

was the deionized water contained in a diffusion sampler exposed to air for about one week. Quality assurance for water samples collected with low-flow method 

included daily equipment blanks, a trip blank, and 

duplicate samples for about 14 percent of the samples. 

The USEPA Laboratory quality-assurance procedures 

included matrix spike samples made from selected diffusion samples and low-flow samples, and lab blanks 

(Church and Lyford, 2000).
Evaluation of Sampling Methods E-7

Table 1. Concentrations of volatile organic compounds in ground-water samples collected with diffusion and low-flow sampling methods from wells at Hanscom Air Force Base, Bedford, Massachusetts, May 10–14, 1999 —Continued

Well

name

Water diffusion 

sampler name

Diffusion sampler 

depth, in feet below 

land surface

Low-flow sample 

depth, in feet below 

land surface

1,2-Dichloroethylene isomers

(μg/L)

Trichloroethylene

(μg/L)

Diffusion sample Low-flow sample Diffusion sample Low-flow sample

A-3 A-3 48.0 48.0 --(5) --(5) --(5) --(5)RAP1-6S RAP1-6S-A 8.1 11.0 37 12 9.2 2.8(L)RAP1-6S RAP1-6S-B 12.1 59 16B107 B107 13.3 13.5 --(5) --(5) --(5) --(5)B255 B255 99.5 99.5 --(5) --(5) --(5) --(5)B254 B254 64.5 64.5 1.4(L) --(5) 7.6 2.6(L)RAP1-7 RAP1-7-A 39.0 5.1 56RAP1-7 RAP1-7-B 44.75 --(100) 280RAP1-7 RAP1-7-C 50.5 50.5 --(50) 8.2(L) 180 180RAP1-7 RAP1-7-D 56.25 --(50) 220RAP1-7 RAP1-7-E 62.0 9.7(L) 190B126 B126-A 52.7 8.4 15B126 B126-B 56.7 56.5 14 11 22 19B126 B126-C 60.7 11 20B111 B111-A 58.0 7.8 65B111 B111-B 61.8 62.0 8.2 5.9 85 47B111 B111-C 65.6 9.1 77B244A B244A-A 42.0 63 8.1B244A B244A-B 46.5 65 5B244A B244A-C 51.0 55 47B244A B244A-D 55.5 61 46B244A B244A-E 60.0 82 19

Table 1. Concentrations of volatile organic compounds in ground-water samples collected with diffusion and low-flow sampling methods from wells at Hanscom Air Force Base, Bedford, Massachusetts, May 10–14, 1999 [Blank spaces indicate that only one low-flow sample was collected per well; samples were not collected from wells B244A and B245 with the low-flow sampling method. B, analyte found in lab blank; E, estimated value exceeds calibration range; L, estimated value is below calibration range; μg/L, micrograms per liter; --(5), not detected at reporting limit of 5 μg/L]
E-8 Evaluation of a Diffusion Sampling Method for Determining Concentrations of VOCs in Ground Water, Hanscom AFB, Mass. B245 B245 17.5 15 7.4 B251 B251 72.5 72.5 1(L) 4.3(L) 18 22 B249 B249 95.0 95.0 2.9(L) --(5) 35 18 B248 B248 59.5 59.5 170 130 470 260 B113 B113-A 54.7 98 32 B113 B113-B 58.7 58.5 100 51 30 11 B113 B113-C 62.7 99 34 PO2-2R PO2-2R-A 103.5 10 43 PO2-2R PO2-2R-B 110.5 12 48 PO2-2R PO2-2R-C 117.5 117.5 16 25 56 68 PO2-2R PO2-2R-D 124.5 140 350 PO2-2R PO2-2R-E 131.0 140 320 RAP2-3T RAP2-3T-A 67.6 170 160 RAP2-3T RAP2-3T-B 70.8 14(L) 86 RAP2-3T RAP2-3T-C 74.0 74.0 35 77 200 170 RAP2-3T RAP2-3T-D 77.2 28 160 RAP2-3T RAP2-3T-E 80.4 13(L) 71 B108 B108-A 69.0 22 21 B108 B108-B 73.0 73.0 25 7.4 12 16 B108 B108-C 77.0 26 14 RAP2-1R RAP2-1R-A 107.0 340 840 RAP2-1R RAP2-1R-B 110.5 300 890 RAP2-1R RAP2-1R-C 114.1 114.1 270 470 780 750 RAP2-1R RAP2-1R-D 117.7 280 540 RAP2-1R RAP2-1R-E 121.2 260 490 RAP2-1T RAP2-1T-A 59.3 --(25) 55 RAP2-1T RAP2-1T-B 63.5 15(L) 230 RAP2-1T RAP2-1T-C 67.7 67.5 95 --(250) 900 880 RAP2-1T RAP2-1T-D 71.8 82 1300 RAP2-1T RAP2-1T-E 76.0 97 990

Table 1. Concentrations of volatile organic compounds in ground-water samples collected with diffusion and low-flow sampling methods from wells at Hanscom 

Air Force Base, Bedford, Massachusetts, May 10–14, 1999 —Continued

Well

name

Water diffusion 

sampler name

Diffusion sampler 

depth, in feet below 

land surface

Low-flow sample 

depth, in feet below 

land surface

1,2-Dichloroethylene isomers

(μg/L)

Trichloroethylene

(μg/L)

Diffusion sample Low-flow sample Diffusion sample Low-flow sample
Evaluation of Sampling Methods E-9

RAP2-2R RAP2-2R-A 82.9 1400 320RAP2-2R RAP2-2R-B 87.5 1800 300RAP2-2R RAP2-2R-C 92.0 92.0 1800 2,200 280 190RAP2-2R RAP2-2R-D 96.2 1800 270RAP2-2R RAP2-2R-E 101.2 1900 350RAP1-6T RAP1-6T-A 30.6 2400 410RAP1-6T RAP1-6T-B 33.9 6800 1600RAP1-6T RAP1-6T-C 37.0 37.0 6800(E) 6,200(E) 1600 1,500RAP1-6T RAP1-6T-D 40.4 6600(E) 1800RAP1-6T RAP1-6T-E 43.7 6200(E) 1600RAP1-6R RAP1-6R-A 52.5 5100(E) 1000RAP1-6R RAP1-6R-B 57.0 5400(E) 1100RAP1-6R RAP1-6R-C 61.6 61.5 6400(E) 6,400(E) 1400 1,200RAP1-6R RAP1-6R-D 66.2 6300(E) 1300RAP1-6R RAP1-6R-E 70.7 5400 1100B240 B240-A 57.0 2200(B) 4400B240 B240-B 61.0 61.0 2500 2,600 4900 4,900B240 B240-C 65.0 2500 4600

Table 1. Concentrations of volatile organic compounds in ground-water samples collected with diffusion and low-flow sampling methods from wells at Hanscom 

Air Force Base, Bedford, Massachusetts, May 10–14, 1999 —Continued

Well

name

Water diffusion 

sampler name

Diffusion sampler 

depth, in feet below 

land surface

Low-flow sample 

depth, in feet below 

land surface

1,2-Dichloroethylene isomers

(μg/L)

Trichloroethylene

(μg/L)

Diffusion sample Low-flow sample Diffusion sample Low-flow sample
E-10 Evaluation of a Diffusion Sampling Method for Determining Concentrations of VOCs in Ground Water, Hanscom AFB, Mass.

Table 2. Concentrations, differences in concentrations, and estimated error in concentrations due to sampling and analytical 

processes of volatile organic compounds in ground-water samples collected with diffusion and low-flow sampling methods at 

Hanscom Air Force Base, Bedford, Massachusetts, May 10–14, 1999

[Comparison of range or error between sampling methods: >, range of error in diffusion sample is greater than the range of error in the low-flow sample; 

=, range of error in diffusion sample overlaps range of error in low-flow samples; <, range of error in diffusion sample is less than range of error in low-flow 

sample. Wells are listed in order of increasing low-flow sample concentration. μg/L, micrograms per liter]

Well

Concentration

Relative 

percent

difference 

(RPD)

Concentration Comparison 

of range

of error 

between 

sampling 

methods

Diffusion

sample

Low-flow 

sample

Diffusion 

sample minus 

low-flow sample

Range of error due to sampling and analytical 

processes (±10 percent)

Diffusion sample Low-flow sample

Low High Low High

 1,2-dichloroethylene isomers (μg/L)

B111 8.2 5.9 2.3 33 7.38 9.02 5.31 6.49 >

B108 25 7.4 17.6 109 22.5 27.5 6.66 8.14 >

B126 14 11 3 24 12.6 15.4 9.9 12.1 >

PO2-2R 16 25 -9 44 14.4 17.6 22.5 27.5 <

B113 100 51 49 65 90 110 45.9 56.1 >

RAP2-3T 35 77 -42 75 31.5 38.5 69.3 84.7 <

B248 170 130 40 27 153 187 117 143 >

RAP2-1R 270 470 -200 54 243 297 423 517 <

RAP2-2R 1,800 2,200 -400 20 1,620 1,980 1,980 2,420 <

B240 2,500 2,600 -100 4 2,250 2,750 2,340 2,860 =

Average........................................................................ 45

Trichloroethylene (μg/L)

B113 30 11 19 93 27 33 9.9 12.1 >

B108 12 16 -4 29 10.8 13.2 14.4 17.6 <

B249 35 18 17 64 31.5 38.5 16.2 19.8 >

B126 22 19 3 15 19.8 24.2 17.1 20.9 =

B251 18 22 -4 20 16.2 19.8 19.8 24.2 <

B111 85 47 38 58 76.5 93.5 42.3 51.7 >

PO2-2R 56 68 -12 19 50.4 61.6 61.2 74.8 =

RAP2-3T 200 170 30 16 180 220 153 187 =

RAP1-7 180 180 0 0 162 198 162 198 =

RAP2-2R 280 190 90 38 252 308 171 209 >

B248 470 260 210 58 423 517 234 286 >

RAP2-1R 780 750 30 4 702 858 675 825 =

RAP2-1T 900 880 20 2 810 990 792 968 =

RAP1-6R 1,400 1,200 200 15 1,260 1,540 1,080 1,320 =

RAP1-6T 1,600 1,500 100 6 1,440 1,760 1,350 1,650 =

B240 4,900 4,900 0 0 4,410 5,390 4,410 5,390 =

Average........................................................................ 27
Evaluation of Sampling Methods E-11

5 10 100 1,000 10,000

CONCENTRATION OF LOW-FLOW SAMPLE,

IN MICROGRAMS PER LITER

 

5

10,000

7

10

20

30

40

50

70

100

200

300

400

500

700

1,000

2,000

3,000

4,000

5,000

7,000

CONCENTRATION OF DIFFUSION SAMPLE,

IN MICROGRAMS PER LITER

DIFFUSION AND

LOW-FLOW SAMPLES

± 10 PERCENT OF LINE

OF PERFECT AGREEMENT

Line of perfect agreement

5 10 100 1,000 10,000 5

10,000

7

10

20

30

40

50

70

100

200

300

400

500

700

1,000

2,000

3,000

4,000

5,000

7,000

CONCENTRATION OF LOW-FLOW SAMPLE,

IN MICROGRAMS PER LITER

 

CONCENTRATION OF DIFFUSION SAMPLE,

IN MICROGRAMS PER LITER

DIFFUSION AND

LOW-FLOW SAMPLES

± 10 PERCENT OF LINE

OF PERFECT AGREEMENT

Line of perfect agreement

The VOCs 1,2-DCE and TCE were not detected 

above reporting limits in the diffusion and low-flow 

trip blank samples. They also were not detected above 

reporting limits in the diffusion sampling equipment 

blank sample and in most of the low-flow sampling 

equipment blank samples. Concentrations of 1,2-DCE 

(12 μg/L) and TCE (56 μg/L) were detected in the lowflow equipment blank sample on the last day of sampling when wells with the highest 1,2-DCE and TCE 

concentrations were sampled. Assuming the equipment blank concentrations were derived from the first 

well sampled on this day (RAP2-2R), concentrations 

of the second sample collected, and perhaps the two 

additional samples collected on this day, may be 

affected by contamination of the low-flow sampling 

equipment. In this case, the 1,2-DCE concentration of 

the equipment blank would be about 0.2 percent of the 

concentrations in the low-flow samples from wells 

RAP1-6T and RAP1-6R (6,200 μg/L and 6,400 μg/L), 

and about 0.5 percent of the concentration in the 

sample from well B240 (2,600 μg/L). Although the 

1,2-DCE concentrations in samples from wells RAP1-

6T and RAP1-6R exceed the calibration range of the 

analytical instrument, and as such are qualitative estimates, they demonstrate, as does the concentration in 

the sample from well B240, that the contaminated 

equipment blank has minimal effect on the 1,2-DCE 

concentrations in samples from these wells. The TCE 

concentration of the equipment blank, however, may 

account for about 4 percent of the TCE concentration 

in the sample from well RAP1-6T (1,500 μg/L), 5 percent of the sample from well RAP1-6R (1,200 μg/L), 

and 1 percent of the sample well B240 (4,900 μg/L).

Thirteen laboratory blank samples were analyzed during the period that diffusion and low-flow 

samples were analyzed. The VOCs 1,2-DCE and TCE, 

as well as vinyl chloride, 1,1-DCA, and 1,1-DCE, 

were not detected in any of the laboratory blank 

samples.

Duplicate samples for 1,2-DCE and TCE concentrations were obtained with the diffusion sampling 

method in four wells (B111, B113, RAP2-1R, and 

RAP1-6R) that also were sampled with the low-flow 

method. Concentrations of 1,2-DCE in the original and 

duplicate samples in well RAP1-6R were reported as 

estimated values because the concentrations exceeded 

Figure 2. Comparison of 1,2-dichloroethylene isomers in 

ground water collected with diffusion and low-flow sampling 

methods, Hanscom Air Force Base, Bedford, Massachusetts, 

May 1999.

Figure 3. Comparison of trichloroethylene in ground water 

collected with diffusion and low-flow sampling methods, 

Hanscom Air Force Base, Bedford, Massachusetts, May 1999.
E-12 Evaluation of a Diffusion Sampling Method for Determining Concentrations of VOCs in Ground Water, Hanscom AFB, Mass.

the calibration range of analytical instrument. Concentrations 1,2-DCE from this well, therefore, are not 

incuded in the duplicate sample analysis for 1,2-DCE, 

nor in any other quantitative analyses. Relative percent 

differences (RPDs) in 1,2-DCE concentrations between 

the original and duplicate samples from the three 

remaining wells ranged from 7.7 to 8.3 percent, with 

an average of 8.1 percent. Concentration of 1,2-DCE 

in two of these duplicate samples are less than the concentrations in the original samples, and one is greater. 

RPDs for the original and duplicate sample concentrations of TCE from the four wells ranged from 1.2 to 

15.4 percent, with an average of 6.1 percent. The RPD 

of 15.4 percent (from well RAP1-6R) appears anomalous compared to the other three RPDs, which ranged 

from 1.2 to 6.5 percent with an average of 3 percent. 

Concentration of TCE in two of these duplicate samples are less than those in the original samples, and two 

are greater. There appears to be no positive or negative 

bias in duplicate sample concentrations sampled with 

the diffusion method.

Duplicate samples were obtained with the lowflow sampling method in three wells (B254, B240, and 

RAP1-6R). Concentrations of 1,2-DCE in the original 

and duplicate samples from well RAP1-6R exceeded 

the calibration range, concentrations of 1,2-DCE in 

well B254 were not detected in original and duplicate 

samples, and concentrations of TCE in well B254 were 

estimated below the calibration range. As a result, lowflow duplicate sample analysis of 1,2-DCE concentrations is represented by samples from one well (B240), 

and in this case, the original and duplicate sample concentrations are the same (2,600 μg/L). TCE duplicate 

analysis is represented by concentrations from two 

wells (B240 and RAP1-6R); RPDs are 2.1 and 8.0 percent, with an average of about 5 percent. Concentration 

of TCE in one duplicate sample is less than that in the 

original sample, and TCE concentrations in the other 

duplicate sample is greater than that in the original 

sample.

The error in sample concentrations attributable 

to sampling methods and analytical processes is 

estimated as within ±10 percent for both sampling 

methods, based on analyses of trip, equipment, and laboratory blank samples and duplicate samples. Concentrations of TCE in low-flow samples from wells RAP1-

6T, RAP1-6R, and B240, however, contain additional 

error, as much as 5 percent, due to the TCE detected in 

the equipment blank sample that represents the day that 

samples were collected from these wells.

Comparison of Concentrations of

1,2-DCE and TCE in Diffusion and 

Low-flow Samples

Concentrations of 1,2-DCE and TCE in samples 

collected with diffusion and low-flow methods, differences in concentrations, relative percent differences in 

concentrations, and ranges of error due to sampling and 

analytical processes are provided in table 2. These data 

show a wide range of concentrations, and a wide range 

of differences in concentrations of 1,2-DCE and TCE 

sampled with the diffusion and low-flow methods. 

Average RPD for 1,2-DCE concentrations from 

samples collected with both methods is about 45 percent, whereas the average RPD for TCE concentrations 

is about 27 percent (table 2), indicating substantially 

smaller differences between TCE concentration from 

diffusion and low-flow samples than differences 

between 1,2-DCE concentrations from both methods. 

With the estimated error attributable to sampling and 

analytical processes of ±10 percent applied to each 

sample, and sample concentrations from both methods 

are considered to be the same if their ranges of error 

overlap, concentrations of 1,2-DCE in diffusion 

samples are greater than those in low-flow samples in 

5 wells, are the same in one well, and are less in 4 

wells. TCE concentrations in diffusion samples are 

greater than those low-flow samples in 5 wells, are the 

same in 9 wells, and are less in 2 wells (table 2).

Because the 1,2-DCE and TCE concentrations 

determined from both methods are highly skewed, 

even with a log10 transformation, a Sign test, a nonparametric statistical test that can be applied to paired, 

non-normally distributed data sets with non-normally 

distributed differences (Helsel and Hirsch, 1992), was 

used to compare the concentrations from each method. 

The concentration data applied to this test include the 

estimated error of ±10 percent for each diffusion and 

low-flow sample. Results of these statistical tests indicate, at a probability of 95 percent, that it is equally 

likely to have diffusion sample concentrations of 

1,2-DCE and TCE greater than low-flow sample 
Distribution of Volatile Organic Compounds in Long-Screen Wells E-13

concentrations as it is to have diffusion sample concentrations of 1,2-DCE and TCE less than low-flow 

sample concentrations. Therefore, results from evaluation of the diffusion sampling method indicate that use 

of diffusion samplers for collection of VOCs containing 1,2-DCE and TCE, and thus other VOCs, may be a 

viable alternative to the low-flow sampling method currently being used at this base.

It is useful to note, however, that the highest concentrations of 1,2-DCE and TCE in long-screen wells, 

which were determined from samples collected with 

multiple diffusion samplers placed in these wells, are 

not necessarily at the midpoint of well screens where 

low-flow samples were obtained (table 1). A nonmidpoint sample concentration is considered to 

be higher than the midpoint sample concentration if 

the ranges of uncertainty (±10 percent) in concentrations for each sample do not overlap. The highest concentrations of 1,2-DCE were detected in diffusion 

samples either above or below the midpoint sample in 

36 percent of the long-screen wells. The highest concentrations of TCE were detected in samples either 

above or below the midpoint sample in 43 percent of 

the long-screen wells. This result demonstrates that, if 

the goal is to determine the highest concentrations of 

VOCs in a long-screen well, even if only to select 

where along the well screen a sample should be collected with another sampling method, use of diffusion 

samplers can be very effective in monitoring, or 

assisting in monitoring of VOCs in ground water.

DISTRIBUTION OF VOLATILE 

ORGANIC COMPOUNDS IN 

LONG-SCREEN WELLS

Analysis of concentrations of water samples 

obtained with multiple diffusion samplers in longscreen wells (screen length 10 ft or greater) indicate 

that vertical variations of concentrations of 1,2-DCE 

and TCE within well screens differ considerably in 

samples from well to well at this site. Because concentrations of 1,2-DCE and TCE also range widely at this 

site, standard deviations of 1,2-DCE and TCE concentrations within each well, normalized by their respective average concentrations, were calculated as 

indicators of the relative variations of concentrations 

among these wells. These normalized standard deviations (NSDs) are shown in figure 4, ordered first by 

aquifer, second by increasing screen length, and third 

by increasing NSD for TCE concentrations. A low 

NSD indicates a small variation in concentrations in a 

well screen. An NSD was not calculated for 1,2-DCE 

concentrations in wells RAP2-3T, RAP2-1T, and 

RAP1-7 because 1,2-DCE was not detected above 

reporting limits in some of the diffusion samples in 

these wells.

Comparison between NSDs in these wells suggests increasing variations in concentrations of 

1,2-DCE and TCE with increasing screen length 

(fig. 4). Vertical variations in concentrations among 

well screens of similar screen length and similar variations in concentrations within well screens of different 

length, however, also are apparent. The variations of 

1,2-DCE and TCE concentrations in these wells may 

reflect the distribution of these concentrations in the 

aquifer adjacent to the wells. Wellbore flow also may 

have an appreciable effect on the distribution of contaminant concentrations in the long-screen wells at this 

site. Explanations of these variations in 1,2-DCE and 

TCE concentrations are discussed below for wells from 

which borehole-flowmeter data were obtained.

Till Aquifer

At well RAP1-6T (15.1 ft screen), 1,2-DCE and 

TCE concentrations in the upper diffusion sample are 

appreciably less than those in lower four samples 

(1,2-DCE concentrations in the bottom three samples 

are estimated above calibration range) (fig. 5). This difference in concentrations is likely due to the upper part 

of the screens placed in the fine-grained lacustrine 

deposit. Borehole-flowmeter data under unstressed 

conditions indicate a uniform upward flow of about 

0.025 gal/min within the till, and then decreases to less 

than 0.01 gal/min in the overlying lacustrine deposit 

(fig. 5). Under pumping conditions, borehole flowmeter 

data indicate that most of the water pumped to the surface is from the lower third of the well screen (near the 

middle of the till deposit) and that little, if any, flow is 

contributed from the lacustrine deposit. The flowmeter 

data are consistent with the lithologic data in indicating 

that the till is more hydraulically conductive than the 

overlying lacustrine deposit.
E-14 Evaluation of a Diffusion Sampling Method for Determining Concentrations of VOCs in Ground Water, Hanscom AFB, Mass.

(20.7)

1.2

1

0.8

0.6

0.4

0.2

0

WELL NAME

(10)

(10)

(10)

(10)

(15.1) (15.2)

(20.7)

(10)

(16.5)

(20)

TILL AQUIFER BEDROCK AQUIFER

RAP1-7

PO2-2R

RAP1-6RRAP2-2R

B244A

RAP2-1R

B240

RAP1-6TRAP2-3TRAP2-1T

B113B111B126B108

STANDARD DEVIATION / MEAN CONCENTRATION

(20.3)

(25)

(30)

(20.2)

E

E

E

EXPLANATION

(10)

1,2-DCE

TCE

SCREEN LENGTH

(in feet)

CONCENTRATIONS THAT EXCEED CALIBRATION

RANGE OF ANALYTICAL INSTRUMENTS INCLUDED

IN NORMALIZED STANDARD DEVIATION

Although the apparent uniform distribution of 

VOC concentrations with depth in the till also may be 

similar in the aquifer, it also is likely due to the upward 

unstressed flow in the well homogenizing the concentration in the part of the well screened in the till (fig. 5). 

The lower concentrations observed in the well screen 

opposite the lacustrine deposit probably indicates that 

most of the upward moving waters containing VOCs 

exit the well screen below the lacustrine deposit. 

Concentrations of TCE in the midpoint diffusion 

sample and the low-flow samples are similar (fig. 5). 

The relative percent differences of these concentrations 

resulting from application of both methods are about 6 

percent. These similar concentrations suggest that the 

same waters are sampled with both methods, but the 

source of water, whether from the aquifer adjacent to 

the sampling devices in the screen or from lower in the 

aquifer because of wellbore flow, is uncertain.

The vertical distribution of TCE concentrations 

at well RAP2-1T (fig. 6), where the upper 4 ft of the 

20.7-foot well screen is in the lacustrine deposit, are 

similar to those observed at well RAP1-6T. Concentrations of TCE in the bottom four diffusion samples, 

where the well screen is in the till, are substantially 

higher than in the upper sample where the well is 

screened in the lacustrine deposit. Borehole flowmeter 

data indicate downward flow in the till under nonpumping (unstressed) conditions with a maximum flow 

of about 0.06 gal/min near the middle of the screen 

(fig. 6), indicating that the horizontal hydraulic conductivity of the till may be higher in this zone than in the 

overlying and underlying till. Although measurements 

of flow under unstressed conditions were not made in 

the upper part of the screen in the lacustrine deposit, 

the first measurement in the till, near the contact with 

the lacustrine deposit, was about 0.01 gal/min, indicating that little to no flow occurred in the lacustrine 

Figure 4. Normalized standard deviations of variations of 1,2-dichloroethylene isomers (1,2-DCE) and 

trichloroethylene (TCE) concentrations from multiple diffusion samples in long-screen wells, Hanscom Air 

Force Base, Bedford, Massachusetts.
Distribution of Volatile Organic Compounds in Long-Screen Wells E-15

20

50

25

30

35

40

45

-0.2 0 0.2 0.4 0.6 100 1,000 10,000

TRICHLOROETHYLENE

1,2-DICHLOROETHYLENE

ISOMERS

VERTICAL FLOW UNDER

NON-PUMPING CONDITIONS

VERTICAL FLOW UNDER

PUMPING CONDITIONS,

ABOUT 0.6 GALLONS PER

MINUTE 

Well

casing

Well

screen

VERTICAL FLOW IN WELL, IN GALLONS PER

MINUTE (+, UPWARD FLOW; -, DOWNWARD FLOW)

CONCENTRATION, IN MICROGRAMS

PER LITER 

DEPTH BELOW LAND SURFACE, IN FEET

SILT

fine to

coarse sand

(lacustrine)

SAND

fine to coarse,

silt, gravel,

cobbles, and

boulders

(till)

BEDROCK

DIFFUSION

SAMPLES

LOW-FLOW

SAMPLE

E

E

E

E

E

CONCENTRATIONS THAT

EXCEED CALIBRATION

RANGE OF ANALYTICAL

INSTRUMENTS

deposit. Flow under pumping (stressed) conditions 

exhibits a nearly uniform increase in volume of water 

contributed to the well with decreasing depth in the till. 

The flow rate measured at the top of the till approximates the rate at which water was being pumped from 

the well, indicating that very little water, if any, was 

contributed from the lacustrine deposit.

The flowmeter data from this well are consistent 

with the lithologic data that indicate the point of contact between the lacustrine and till deposits. The 

unstressed flow data suggest that contaminants would 

flow preferentially within a zone near the middle of the 

part of the well screened in the till. Therefore, the relatively uniform TCE concentrations observed below this 

zone probably reflect the downward flow in the well 

screen. As in well RAP1-6T, the relatively lower concentration of TCE measured in the lacustrine deposit 

could be the result of lower concentrations in this unit 

or insufficient time for the well water to equilibrate 

with the aquifer water after installing the diffusion 

samplers in the slower moving water of the lacustrine 

deposit.

The concentrations of TCE from the midpoint 

diffusion sample and the low-flow sample show little 

variation (fig. 6). The relative difference is about 2 percent. Because different dilution factors were used in 

these analyses, concentrations of 1,2-DCE were 

detected above the reporting limit of 25 μg/L in the diffusion sample and was not detected above the reporting 

limit of 250 μg/L in the low-flow sample (fig. 6), and, 

therefore, cannot be compared directly. The close correlation between TCE concentrations from both methods suggests that waters from the same source are 

being sampled, and flowmeter data suggest that much 

Figure 5. Lithology, well casing and screen, borehole-flowmeter data, and concentrations of 1,2-dichloroethylene 

isomers and trichloroethylene in multiple diffusion samples and in the low-flow sample at well RAP1-6T in the till aquifer 

at Hanscom Air Force Base, Bedford, Massachusetts.
E-16 Evaluation of a Diffusion Sampling Method for Determining Concentrations of VOCs in Ground Water, Hanscom AFB, Mass.

50

80

55

60

65

70

75

-0.2 0 0.2 0.4 0.6 1 5,000 10 100 1,000

casing

Well

Well

screen

VERTICAL FLOW IN WELL, IN GALLONS PER

MINUTE (+, UPWARD FLOW; -, DOWNWARD FLOW)

CONCENTRATION, IN MICROGRAMS

PER LITER 

DEPTH BELOW LAND SURFACE, IN FEET

1,2-Dichloroethylene isomers not detected above reporting

limit in two diffusion samples and in the low-flow sample

SILT

some clay

and

fine sand

(lacustrine)

SAND

medium, trace

fine and

coarse sand,

trace silt

and gravel;

cobbles and

boulders at

depth

(till)

VERTICAL FLOW UNDER

NON PUMPING

CONDITIONS

VERTICAL FLOW UNDER

PUMPING CONDITIONS,

ABOUT 0.6 GALLONS

PER MINUTE 

TRICHLOROETHYLENE

1,2-DICHLOROETHYLENE

ISOMERS

DIFFUSION

SAMPLES

LOW-FLOW

SAMPLE

1,2-DICHLOROETHYLENE

ISOMER MINIMUM

REPORTING LIMIT

of this water is from the aquifer adjacent to a midpoint 

zone in the screen. Downward flow in the screen of 

lower concentration water from the upper part of the 

till aquifer, and possibly from the lacustrine deposit, 

however, could dilute concentrations in water in the 

mid-section of the screen, and result in lower concentrations than in the adjacent aquifer.

Bedrock Aquifer

Variations in concentrations of 1,2-DCE and 

TCE at well RAP1-6R (20.2-ft well screen) are relatively small (fig. 7) (1,2-DCE concentrations in the 

upper four samples are estimated above calibration 

range) and are comparable to those in 10-foot screens 

in the till. Unstressed flow was not measured in this 

well because the water level in the well casing had 

not stabilized after 2 hours since placing the flowmeter 

probe in the well. Vertical flow in the well screen, 

however, is suggested as a possible cause for the 

nearly uniform distribution of 1,2-DCE and TCE concentrations. Borehole flowmeter data under pumping 

conditions indicate that most of the water pumped was 

contributed from the bottom 6 ft of the screen; especially from a thin zone about 4 to 6 ft from the bottom 

(fig. 7). This contribution would be consistent with a 

fracture, or fracture zone in the bedrock in this vicinity. 

Although the driller’s log describes the bedrock as 

highly fractured, the flowmeter data indicate that the 

only substantial water-bearing fractures are near the 

bottom of the well screen. The nearly uniform distribution of concentrations with depth in the well screen 

could be the result of downward flow of contaminants 

Figure 6. Lithology, well casing and screen, borehole-flowmeter data, and concentrations of 1,2-dichloroethylene 

isomers and trichloroethylene in multiple diffusion samples and in the low-flow sample at well RAP2-1T in the till aquifer 

at Hanscom Air Force Base, Bedford, Massachusetts.
Distribution of Volatile Organic Compounds in Long-Screen Wells E-17

VERTICAL FLOW UNDER

PUMPING CONDITIONS,

ABOUT 0.25 GALLONS PER

MINUTE 

TRICHLOROETHYLENE

1,2-DICHLOROETHYLENE

ISOMERS

DIFFUSION

SAMPLES

LOW-FLOW

SAMPLE

VERTICAL FLOW IN WELL, IN GALLONS PER

MINUTE (+, UPWARD FLOW; -, DOWNWARD FLOW)

CONCENTRATION, IN MICROGRAMS

PER LITER 

E

E

E

E E

E

CONCENTRATIONS THAT

EXCEED CALIBRATION

RANGE OF ANALYTICAL

INSTRUMENTS

40

80

45

50

55

60

65

70

75

-0.1 0 0.1 0.2 100 10,000 1,000

Well

casing

Well

screen

DEPTH BELOW LAND SURFACE, IN FEET

SAND

fine to coarse,

silt, gravel,

cobbles, and

boulders

(till)

BEDROCK

granite,

highly

fractured

Bottom of

borehole

possible

fracture zone

entering the upper part of the screen from a source in 

the upper bedrock or lower part of the till, or upward 

flow originating from the fracture identified or from 

fractures at depths below the well screen. It also is possible that this nearly uniform distribution of concentrations with depth in the screen reflects the distribution in 

the formation.

Concentrations of 1,2-DCE in the low-flow 

sample (also estimated above the calibration range) 

and in the midpoint diffusion sample appear to be the 

same, and the TCE concentrations from both methods 

are similar (fig. 7). The respective relative percent difference for concentrations of TCE is 15 precent. 

Although the entry point, or zone, of these waters is 

uncertain, both the diffusion and low-flow samples 

appear to be from the same source.

At well PO2-2R (30-ft well screen) (fig . 8), 

the large relative variations in concentrations of 

1,2-DCE and TCE compared to those in samples from 

the other wells examined at this base (fig. 4) appear 

to result from the well screen intersecting a fracture 

or fracture zone. Although reliable ambient flow data 

were not obtained, borehole flowmeter data under 

pumping conditions indicate that most of the water 

pumped to the surface is contributed from a zone at 

and below a previously defined fracture (Tom Best, 

Restoration Program Manager, Hanscom Air Force 

Base, written commun., 1999). The depth to the top 

of this fracture, or fracture zone, was reported as 

116 ft below land surface, but its downward extent 

was not provided in the drillers log. No flow was measured in the bottom 8 ft of the well screen, and little to 

Figure 7. Lithology, well casing and screen, borehole-flowmeter data, and concentrations of 1,2-dichloroethylene isomers 

and trichloroethylene in multiple diffusion samples and in the low-flow sample at well RAP1-6R in the bedrock aquifer at 

Hanscom Air Force Base, Bedford, Massachusetts.
E-18 Evaluation of a Diffusion Sampling Method for Determining Concentrations of VOCs in Ground Water, Hanscom AFB, Mass.

90

140

95

100

105

110

115

120

125

130

135

DEPTH BELOW LAND SURFACE, IN FEET

-0.2 0 0.2 0.4

VERTICAL FLOW IN WELL, IN GALLONS PER

MINUTE (+, UPWARD FLOW; -, DOWNWARD FLOW)

10 100 1,000 5,000

CONCENTRATION, IN MICROGRAMS

PER LITER

SAND

fine to coarse,

some fine

to medium

gravel

(till)

BEDROCK

granite

Bottom of

borehole

BEDROCK

granite

fracture

Well

casing

Well

screen

1

VERTICAL FLOW UNDER

PUMPING CONDITIONS,

ABOUT 3.5 GALLONS PER

MINUTE 

TRICHLOROETHYLENE

1,2-DICHLOROETHYLENE

ISOMERS

DIFFUSION

SAMPLES

LOW-FLOW

SAMPLE

no flow was measured in the upper 10 ft of the well 

screen. Therefore, this fracture, or fracture zone, may 

extend 5 or 6 ft below its reported upper level and 

account for the differences in concentrations above and 

below this zone. Although concentrations differ, nearly 

uniform distributions of 1,2-DCE and TCE concentrations are present above and below the fracture zone. 

This distribution of concentrations could reflect concentrations in the formation, however, other explanations are possible. Water in the zone below the fracture 

appears to be stagnant because no water was contributed to the flow while the well was pumped. Therefore, 

the lower concentrations above the fracture could represent temporal concentration changes that were not 

propagated into the zone below the fracture. The nearly 

uniform distribution of contaminants in the well screen 

above the fracture could result from downward flow of 

contaminants from above into the fracture or upward 

flow from the fracture.

The relative difference between 1,2-DCE concentrations from the midpoint diffusion sample and the 

low-flow sample at well PO2-2R is 44 percent and for 

TCE concentrations is 19 percent. These differences in 

concentrations between methods, however, are 

much smaller than the differences in diffusion sample 

concentrations above and below the fracture. The relative difference of 1,2-DCE concentrations from above 

and below the fracture is about 170 percent and for 

TCE concentrations is about 150 percent. Similar to the 

other wells examined, the entry point(s), or zone(s), of 

these waters into the well screen is uncertain, however, 

both the diffusion and low-flow method appear to be 

sampling water from the same source.

Figure 8. Lithology, well casing and screen, borehole-flowmeter data, and concentrations of 1,2-dichloroethylene 

isomers and trichloroethylene in multiple diffusion samples and in the low-flow sample at well PO2-2R in the bedrock 

aquifer at Hanscom Air Force Base, Bedford, Massachusetts.
Summary and Conclusions E-19

SUMMARY AND CONCLUSIONS

This study has compared a diffusion sampling 

method to a low-flow sampling method for monitoring 

VOCs in ground water at the Hanscom Air Force Base, 

Bedford, Mass. In addition, the possible effects of vertical variations of VOCs and borehole flow in longscreen wells on sampling with diffusion and low-flow 

methods were examined.

Diffusion samplers have been shown to be a 

viable alternative to the low-flow sampling method currently being used at Hanscom AFB for monitoring 

VOCs in ground water. Concentrations of 1,2-DCE 

and TCE in samples collected at the midpoint of well 

screens with the diffusion sampling method were compared with concentrations of 1,2-DCE and TCE 

in samples collected at the same depths in wells with 

the low-flow sampling method. Concentrations of 

1,2-DCE range from 8.2 to 2,500 μg/L in diffusion 

samples and 5.9 to 2,600 μg/L in low-flow samples. 

Concentrations of TCE range from 12 to 4,900 μg/L in 

diffusion samples and 11 to 4,900 μg/L in low-flow 

samples. A Sign test, applicable to these highly skewed 

concentrations, indicates that with a probablity of 95 

percent, it is equally likely to have diffusion sample 

concentrations of 1,2-DCE and TCE greater than lowflow sample concentrations as it is to have diffusion 

sample concentrations of 1,2-DCE and TCE less than 

low-flow sample concentrations.

Analysis of the distribution of 1,2-DCE and TCE 

concentrations in long-screen wells (screen length 10 ft 

or greater) in a till aquifer composed of a wide range of 

particle sizes from silt to boulders and in a bedrock 

aquifer with multiple diffusion samplers demonstrated 

that variations in concentrations within well screens 

differ significantly from well to well at the base. 

The vertical distribution of these concentrations in 

the long-screen wells may reflect the distribution of 

concentrations in the aquifer adjacent to the well 

screens. Borehole flowmeter data, however, indicate 

that the distribution of concentrations in samples from 

wells with long screens may be substantially affected 

by ambient vertical borehole flow.

In cases where there is either downward or 

upward borehole flow throughout most of the well 

screen, the concentrations of VOCs in the water that 

enter the screen are likely to predominate throughout 

the screen length as water(s) of different concentrations 

from other depths may be prevented from entering 

the screen. Analytical results from a single low-flow 

sample should be similar to the analytical results 

from a single diffusion sample obtained from almost 

anywhere in the screen. Concentrations of VOCs in 

the well may be adequately characterized with 

both methods, however, the results may not be representative of the VOCs in the aquifer adjacent to the 

screen. Where vertical borehole flow is not present, 

VOCs in the well and aquifer adjacent to the screen 

may be adequately characterized with both methods at 

any specific depth. Delineation of the vertical distribution of VOCs with multiple diffusion samples may be 

needed, however, to determine an optimal depth for 

sampling with the low-flow method. If concentrations 

vary substantially with depth, however, an optimal 

depth may not exist and a single low-flow sample from 

any depth within the screen may not adequately characterize the VOCs in the well or the VOCs in the aquifer 

adjacent to the well. In a situation where borehole flow 

varies along the length of the screen, multiple diffusion 

samples may be able to characterize the vertical variations of VOCs in the well, but this depth profile may 

not represent the distribution of VOCs in the aquifer.

These observations reinforce results from previous studies that have demonstrated the difficulty of collecting representative ground-water samples in wells 

completed with long screens (Reilly and others, 1989; 

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

A single sample collected with any method may not be 

representative of the formation water. Use of multiple 

diffusion samplers in conjunction with borehole flowmeter logs in long-screen wells may be useful in defining the vertical distribution of VOCs in the screened 

interval of an aquifer and evaluating if meaningful 

water-quality data can be obtained.

Other advantages of the diffusion sampling 

method over low-flow sampling methods include less 

overall time for collection of samples, no need for 

monitoring stabilization parameters to signal when 

sampling may begin, and minimal waste water. In addition, diffusion samplers have a distinct advantage over 

low-flow methods in evaluating the distribution of 

VOCs in wells because multiple samples can be 

obtained with minimal additional time over the time 

needed for collection of one sample. Multiple diffusion 

sampling in a well also can be effective even if diffusion samples are used only to select a location along 

the well screen where a sample should be taken with 

another method.
E-20 Evaluation of a Diffusion Sampling Method for Determining Concentrations of VOCs in Ground Water, Hanscom AFB, Mass.

REFERENCES

Church, P.E., and Granato, G.E., 1996, Bias in ground-water 

data caused by well-bore flow in long-screen wells: 

Ground Water, v. 34, no. 2, p. 262–273.

Church, P.E., and Lyford, F.P., 2000, Sampling of volatile 

organic compounds in ground water by diffusion 

samplers and a low-flow method, and collection of 

borehole-flowmeter data, Hanscom Air Force Base, 

Massachusetts: U.S. Geological Survey Open-File 

Report 00-207, 18 p.

Haley & Aldrich, Inc., 1996, Architect-engineer field 

investigation report, sampling round no. 9, June–July 

1996, Long-term sampling program, Hanscom Air 

Force Base, Bedford, Massachusetts, v. 1 and 2: 

Cambridge, Mass., Haley & Aldrich, Inc., variously 

paged.

_____1998, Architect-engineer field investigation report, 

sampling round no. 11, May 1998, Long-term sampling 

program, Hanscom Air Force Base, Bedford, 

Massachusetts: Boston, Mass., Haley & Aldrich, Inc., 

variously paged.

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods in 

water resources: New York, Elsevier, 522 p.

Keyes, W.S., 1990, Borehole geophysics applied to groundwater investigations: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 2, 

chap. E2, 150 p.

Puls, R.W., and Barcelona, M.J., 1995, Low-flow (minimal 

drawdown ) ground-water sampling procedures: Ada, 

Okla., U.S. Environmental Protection Agency Ground 

Water Issues Report, EPA/540/S-95/504, 12 p.

Reilly, T.E, Franke, O.L., and Bennett, G.D., 1989, Bias in 

groundwater samples caused by wellbore flow: Journal 

of Hydraulic Engineering, v. 115, no. 2, p. 270–276.

Reilly, T.E., and LeBlanc, D.R., 1998, Experimental 

evaluation of factors affecting temporal variability of 

water samples obtained from long-screen wells: Ground 

Water, v. 36, no. 4, p. 566–576.

U.S. Environmental Protection Agency, 1996, Test method 

for evaluating solid waste, physical/chemical methods 

SW-846 (3d ed.): revision 2, v. IB, chap. 4, section 

4.3.2, final update III, December 1996, p. 1–86.

Vroblesky, D.A. and Hyde, W.T., 1997, Diffusion samplers 

as an inexpensive approach to monitoring VOCs in 

ground water: Ground Water Monitoring and 

Remediation, v. 17, no. 3, p.177–184.
Summarization (by Walter Berger, MitreTech, Inc.) of Passive Diffusion 

Membrane Samplers, Final, August 7, 2000: 

Technology Application Analysis Report: 

McClellan Air Force Base Environmental Management, 

Sacramento, California

By McClellan Air Force Base Environmental Management Directorate
F-1

Summarization (by Walter Berger, MitreTech, Inc.) of 

Passive Diffusion Membrane Samplers, Final, August 7, 

2000: Technology Application Analysis Report

By McClellan Air Force Base Environmental Management Directorate, 2000, 

McClellan Air Force Base Environmental Management, 

Sacramento, California

The Technology Application Analysis Report summarizes the findings of a field demonstration using 

passive diffusion membrane samplers as an innovative approach to monitoring volatile organic compound (VOC) 

contamination in groundwater. The main objective of the demonstration was to compare the cost and performance 

of the diffusion samplers to conventional groundwater purge-and-sample techniques. The demonstration was 

conducted at McClellan AFB from May 12 to June 4, 1999, using 188 diffusion samplers in 30 groundwater 

monitoring wells contaminated with VOCs. The saturated screen length was 8 ft in one well, 10 ft in 17 wells, 

and between 10 and 20 ft in 12 wells. The tested compounds were trichloroethene; tetrachloroethene; 1,1-

dichloroethane; 1,2-dichloroethane; 1,1-dichloroethene; cis-1,2-dichloroethene; carbon tetrachloride; chloroform; and Freon 113. 

Statistical analysis of the demonstration data showed that the diffusion samplers produced sample results 

comparable to the results from conventional purge-and-sample samples. A cost comparison showed that diffusion 

sampling has the potential to dramatically reduce the costs associated with long-term monitoring. 

Multiple diffusion samplers deployed along the length of the well screen showed significant vertical 

concentration variations in nine of the tested wells. The results implied that conventional sampling may not accurately represent the contaminant concentrations at points along the well screen because of mixing and preferential 

flow through coarse formations. The vertical contaminant concentration gradients shown by the diffusion 

samplers implied that the diffusion-sampler results were more representative of formation concentrations adjacent 

to the well screen than were the conventional-sampling results. The report concluded that for wells exhibiting 

vertical concentration gradients, guidance for optimal placement of the diffusion samplers along the length of the 

well screen still needs to be developed because placing a single diffusion sampler without knowledge of contaminant concentration gradients within the well could result in differences between conventional purge-and-sample 

and diffusion-sample results. The report also concluded that these diffusion samplers may only be appropriate for 

monitoring wells with hydraulic gradients sufficient to allow water within the well casing to exchange with 

formation water without purging.

The demonstration was conducted under the guidance of the Strategic Environmental Research and 

Development Program (SERDP) National Environmental Technology Test Site (NETTS) program at 

McClellan AFB.
This document was developed in cooperation with the Interstate Technology and Regulatory

Cooperation (ITRC) Work Group and is printed with permission from the U.S. Geological Survey.

DSP-1