United States Environmental Monitoring

Environmental Protection Systems Laboratory

Agency Las Vegas, NV 89193-3478

E AP Project Summary

Evaluation of Sampling and

Field-Filtration Methods for the

Analysis of Trace Metals in

Ground Water

Research and Development EPA/600/SR-94/119 September 1994

Selected ground water sampling and

field-filtration methods were evaluated

to determine their effects on field parameters and trace metal concentrations in samples collected under several types of field conditions. The study

focused on conditions where traditional

approaches may produce turbid

samples, which often leads to filtration

of suspended particles from the sample

before laboratory chemical analysis.

However, filtration may also remove

colloidal particles that may be important to the transport of hydrophobic

organic contaminants and trace metals. The specific sampling and filtration variables investigated in this study

were (1) filtration with 0.45-μm or 5.0-

μm pore size filters versus no filtration; (2) sampling device, specifically,

bladder pump, submersible-centrifugal

pump, and bailer; and (3) sampling

pump discharge rate during purging

and sample collection using a “low”

rate of 300 mL/min and a “moderate”

rate of 1000 mL/min. Three field sites

were visited: an active municipal solid

waste landfill in Wisconsin, a closed

solid waste landfill in Washington, and

a site contaminated by industrial waste

in Nevada. The evaluation included

three wells each at the Wisconsin and

Washington sites and two wells at the

Nevada site. Filtration with 5.0-μm filters was conducted only at one well at

each site.

The effects of field filtration were

most evident for the bailer, which often

produced trace metal concentrations in

unfiltered samples that were orders-ofmagnitude higher than in 0.45-μm-filtered samples. The largest differences

occurred at the most turbid wells and

in samples containing the highest particle concentrations. Similar effects

were observed in some samples collected by pumps from the most turbid

wells, particularly the low yield well.

For most pump sampling, however, differences in concentrations between

0.45-μm-filtered and unfiltered samples

were not significant and particle concentrations were significantly lower

than those produced by the bailer. Bailers caused more disturbance of the

sampling zone than the three pumping

methods as evidenced by measurements of field parameters and concentrations of particles, major ions, and

trace metals. Little variation was observed in the analytical determinations

between the pumped samples but some

variation existed in the field indicator

parameters—primarily, temperature,

dissolved oxygen, and turbidity. Trace

metal concentrations in 0.45-μm-filtered

samples were generally independent of

sampling method, suggesting that these

constituents were present as dissolved

species and not associated with particles, or associated with particles

smaller than 0.4 μm. At wells where

5.0-μm filtration was conducted, physical and hydrochemical conditions resulted in minimal differences between

trace metal concentrations in the 5.0-

μm-filtered, 0.45-μm-filtered, and unfiltered samples.

This Project Summary was developed

by EPA’s Environmental Monitoring

Systems Laboratory, Las Vegas, NV, to

announce key findings of the research

Karl F. Pohlmann, Gary A. Icopini, Richard D. McArthur, and Charlita G. Rosal
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project that is fully documented in a

separate report of the same title (see

Project Report ordering information at

back).

Introduction

Historically, ground water contaminants

were considered to be partitioned between

two phases, a mobile phase composed of

dissolved (aqueous) solutes in water transported by natural ground water flow and a

normally immobile solid phase composed

of the matrix materials of the water-bearing zone. The action of purging and sampling a monitoring well installed in unconsolidated materials may provide sufficient

energy to suspend matrix materials that

have accumulated in the sampling zone

and well bore and incorporate them in

ground-water samples. Inclusion of metals associated with these normally immobile matrix particles may bias analytical

determinations, leading to elevated and

improbable concentrations of mobile contaminants if suspended particle concentrations are very high. As a result, ground

water samples are commonly filtered in

the field to remove these suspended particles. Filtration has been considered particularly necessary under turbid conditions

where high particle (sediment) loadings

might lead to significant analytical bias

through inclusion of large quantities of

matrix metals in the analysis. Alternatively,

the presence of particles in samples might

also bias analytical determinations through

removal of metal ions from solution during

shipment and storage as a result of interactions with particle surfaces.

Unfortunately, indiscriminant use of field

filtration ignores the presence of particles,

known as colloids, in ground water that

may exist between the extremes of solutes and sediments. Potential association

of metals with colloids has important implications for the practice of field filtration

because the boundary between the particulate and dissolved has been operationally defined at 0.45μm. This boundary

presumes that the component retained on

a 0.45 μm filter represents suspended solids, while the component that passed

through the filter represents dissolved metals.

Collection of ground water samples for

analysis of metals concentrations is required under several U.S. environmental

regulations, including CERCLA (Superfund), RCRA Subtitle C (Hazardous

Waste), and RCRA Subtitle D (Solid

Waste). As a result, the debate regarding

ground water metals samples impacts a

wide range of sampling programs and a

large number of sites, suggesting the need

for further research. This study was undertaken to investigate how concentrations

of trace metals were affected by selected

methods of sample collection and fieldfiltration. The objectives of the study were

to provide a survey of the impacts of the

following aspects of ground-water sampling:

1)Impacts of sample collection method

on determinations of field parameters.

2)Impacts of filtration with 0.45-μm or

5.0 μm pore size filters versus no

filtration on trace metal concentrations.

3)Impacts of sampling device—specifically, bailer, bladder pump, submersible-centrifugal pump (at a “low” discharge rate of 300 mL/min), and submersible-centrifugal pump (at a “moderate” discharge rate of 1000 mL/min),

on trace metal concentrations.

4)Impacts of sampling device on particle size distribution and total concentration.

The study focused on sampling in conventional standpipe monitoring wells under conditions where traditional approaches to sampling may produce turbid

samples.

Procedure

The monitoring wells sampled were constructed of polyvinyl chloride, and were

5.08 cm in diameter, with the exception of

one 10.2 cm diameter well. The top of the

well screens ranged from 2 to 19 m below

ground surface, with well screen lengths

of 0.6 to 6.0 m. The static water level

ranged from 1 to 14 m below ground surface. Volumes of water within the well

screens ranged from 1.2 to 50 L. Although

certainly not representative of geologic and

hydrogeochemical conditions at all solid

waste landfills and hazardous waste sites,

these sites provided typical field conditions where traditional approaches to

ground-water sampling produce turbid

samples.

Four methods of collecting samples from

conventional standpipe monitoring wells

were evaluated using three types of sampling devices and pump discharge rates.

These methods were utilized at eight of

the nine wells. The first method used a

dual-check valve bailer with a volume of

approximately 0.4 L. Samples were transferred from the bailer directly to the sample

bottles for unfiltered samples or to a filtration vessel for filtered samples. Compressed nitrogen gas was used to drive

the samples through either membrane filters or disposable cartridge filters. The

second sampling method was a submersible-centrifugal pump (CP1) operated

at a flow rate of approximately 300 mL/

min. Filtration was conducted in-line with

disposable cartridge filters. The third

method was a bladder pump (BP) operated at a flow rate of approximately 500

mL/min at the Wisconsin site or 1000 mL/

min at the other sites. The fourth method

was a submersible-centrifugal pump (CP2)

operated at a flow rate of approximately

1000 mL/min. Discharge rates were measured at ground surface and were controlled by the pump speed rather than by

flow restrictors or valves. These discharge

rates were used for both purging and sampling. Filtration for methods three and four

was conducted in the same manner as for

method two. The pumps and bailer were

positioned to collect samples from about

0.6 m below the top of the well screen.

Measurements of turbidity, dissolved

oxygen (DO), temperature, electrical conductivity (EC), and pH of the pump discharge were made in-line, while measurements of these parameters for the bailer

discharge were made off-line. Stabilization of these parameters provided an indication of equilibrium between incoming

ground water, the action of the sampler,

and stagnant water in the well; thereby

suggesting that purging was complete. The

relative values of these parameters also

provided a means for comparing the sampling methods with respect to their ability

to minimize disturbance in the sampling

zone. Estimates of particle size distribution were determined gravimetrically by

serial ultrafiltration using microfilters of 5.0

μm, 0.4 μm, 0.1 μm, and 0.03 μm pore

size.

 Results and Discussion

The results of the study demonstrate

three important factors that influence the

accuracy of field parameters measurements during sampling from conventional

standpipe monitoring wells: measurement

techniques, sampling method, and hydraulics of the well. Impacts related to measurement techniques were considered minor because a single individual conducted

all the field measurements and all procedures followed established protocol. In contrast, sampling method and well hydraulics had impacts on values of field parameters in some of the sampling events that

masked all other factors. The relative disturbance in the sampling zone caused by

a sampling method was most evident in

the field measurements of turbidity and

DO, particularly under low-yield conditions.

When the discharge rate exceeded the

well yield, the increasing hydraulic gradient between the formation and the well

mobilized large quantities of particles,
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thereby elevating turbidity values. Continued removal of water from the well dewatered the filter pack, leading to gravity

drainage of pore water and sediments

and continually increasing turbidity values.

Bailer turbidity values were further elevated

by the surging action of the bailer. Elevated DO values of the bailer and BP at

1 L/min in low-yield wells reflect the formation of a large air-water interface which

increased the potential for oxygenation of

incoming ground water as the filter pack

was dewatered. The bailer caused additional aeration of the samples as a result

of the increased exposure to the atmosphere during sample collection and transfer. The lower discharge rate of 0.3 L/

min, which was generally closer to the

well yield, resulted in less variability and

more representative values of turbidity and

DO, as well as lower purged volumes.

Somewhat less variable results were

observed between sampling methods in

wells where purging and sampling rate did

not exceed the well yield. Under these

conditions, hydraulic gradients into the well

were minimal, the filter pack was not dewatered, and turbidity was generally lower.

The two pumping methods produced similar values of most field measurements,

while the surging action of the bailer produced turbidity values that were approximately two orders-of-magnitude higher

than those produced by the pumps. Likewise, DO values in bailed samples were

elevated with respect to the pump values,

an artifact of the bailing process. The

pumps produced equilibrium DO and turbidity conditions with relatively low purged

volumes, while the bailer produced high

values of these parameters and did not

reach equilibrium after greater purged volumes. Results of the study indicate that

DO is sensitive to the purging process

and further suggest that DO may be an

important indicator of the volume required

to remove stagnant water from the sampling system.

As with DO, turbidity exhibited a strong

dependence on sampling method. The

highest turbidity values were obtained with

the bailers, while the lowest turbidities were

obtained with the pumps. Equilibration of

turbidity, like DO and oxidation-reduction

conditions (Eh), is often related to sample

collection method.

Values of pH showed little variation between pump methods with most values

falling within the range of ±0.2 pH units

for a given well. Bailed pH values were

also within this range but were usually

higher than the pumped values, possibly

reflecting degassing of CO2

 from the

samples during collection and pH measurement. In addition, pH reached equilibrium at lower purged volumes than all the

other parameters, independent of sampling method. Although pH is an important

indicator of the speciation of trace metals

in ground water, the relatively uniform values across devices at individual wells do

not alone suggest that similar metals species might be present.

In almost every case, samples collected

by bailer contained higher particle concentrations than those collected by the

pumps, with the greatest differences occurring at the most turbid wells. Furthermore, the size distribution of particles in

most bailed samples was highly skewed

toward larger particles, with over 96%

larger than 0.45 μm, and generally over

93% larger than 5.0 μm. The quantities

and sizes of these particles suggest that

they were not mobile in ground water under natural flow conditions but were primarily the artifacts of well construction,

development, and purging and were mobilized by agitation in the sampling zone

caused by bailing. The particle size distribution in samples pumped from the most

turbid (low-yield) wells were also skewed

toward larger particles, but total particle

concentrations were much lower than in

the bailed samples. In the less turbid (highyield) wells, total particle concentrations in

pumped samples were orders-of-magnitude lower than in bailed samples, reflecting the lower degree of agitation caused

by the pumping methods. Also, particle

sizes in the pumped samples were generally more uniformly distributed; approximately 50% of the particles were larger

than 0.45 μm.

Differences in metal concentrations between filtered and unfiltered samples were

most evident in low-yield and highly-turbid

wells, particularly when the samples were

collected by bailer. In fact, several metals

present in unfiltered bailed samples were

below detection levels in the corresponding filtered samples. The large differences

in concentration between filtered and unfiltered bailed samples reflect the association of metals with the high concentrations

of artifactual particles entrained during bailing. For example, iron in the sampling

zone likely existed as iron hydroxide particles, particles containing elemental iron,

and ferrous iron sorbed to particle surfaces. Removal of the majority of particles

during filtration therefore greatly reduced

iron concentrations in the filtered samples.

Other metals likely existed as aqueous

species sorbed to particle surfaces, or as

elemental components of particles originating as aquifer solids, and their concentrations were similarly reduced by filtration. Additionally, ferrous iron may have

oxidized and precipitated during bailing,

transfer, and filtering of the samples, and

then removed during filtration. Finally, the

formation of a thick filter cake during filtration of bailed samples likely reduced the

effective pore size of the filter membrane,

thereby blocking passage of some particles smaller than 0.45 μm; this would

further reduce the concentrations of associated metals in the sample.

Trace metal concentrations in unfiltered

samples pumped from low-yield and highly

turbid wells were generally lower than in

unfiltered samples bailed from the same

wells. This reflects the lower degree of

agitation associated with pumping and, as

a result, the lower artifactual particle concentrations. Removal of the larger particles in the pumped samples did, however, cause filtered samples to contain

lower metal concentrations than unfiltered

samples, though the differences in concentration were much lower than in bailed

samples. Unfiltered metal concentrations

in samples pumped at 1 L/min were often

slightly higher than in samples pumped at

0.3 L/min, but the concentrations in the

filtered samples from both pumps were

essentially the same. Furthermore, metals

concentrations in filtered pumped samples

did not differ significantly from those in

filtered bailed samples.

In less turbid and high-yield wells, unfiltered bailed samples usually contained

the highest metal concentrations of all

samples, but the differences between

these concentrations and concentrations

in filtered samples were much smaller than

for low-yield and turbid wells. Several metals showed only slight differences between

filtered and unfiltered results in bailed

samples. These results reflect the lower

proportion of artifactual particles removed

during filtration as compared to the lowyield and turbid wells, but also are related

to the metal speciation at each well. Differences between filtered and unfiltered

pumped samples were minimal, and the

concentrations were essentially the same

as those in the filtered bailed samples,

despite the variability in proportion of particles smaller than 0.45 μm. This suggests

that many metals existed primarily as dissolved species and/or were associated with

particles smaller than 0.45 μm in the less

turbid and high-yield wells included in this

study.

Conclusions and

Recommendations

Field determinations of unstable parameters DO and turbidity were the most

sensitive to disturbance of the sampling

zone, with values produced by bailing often orders-of-magnitude higher than those

produced by the pumps. Variations in in-
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dicator parameters EC and pH were insignificant between the four sampling methods, suggesting they were less related to

disturbance of the sampling zone than

DO and turbidity. Temperature showed

little variation between the bladder pump

and bailer but was highly elevated by the

operation of the submersible centrifugal

pump at low discharge rates.

The relationship of turbidity to particle

concentration and its sensitivity to the purging process, relative to other indicator parameters, suggests that turbidity may be a

useful indicator of relative particle concentrations between wells and of stabilization of particle concentrations during

monitoring well purging. If mobile particles

are thought to be important to transport of

contaminants in ground water, use of field

parameters such as pH, temperature, or

EC as criteria for determining adequate

sampling conditions may result in

underpurging.

The effects of field filtration on trace

metal concentrations were most evident

when a bailer was used to sample lowyield and/or turbid wells. Concentrations

in unfiltered bailed samples were up to

several orders-of-magnitude higher than

in filtered bailed, filtered pumped, and unfiltered pumped samples. Elevated metal

concentrations in unfiltered bailed samples

reflected the entrainment of large quantities of normally immobile artifactual particles and their associated matrix metals,

and unknown quantities of contaminant

metals. Pumping at low to moderate rates

in low-yield and/or turbid wells resulted in

less agitation in the sampling zone, lower

particle concentrations, and reduced effects of field filtration on metal concentrations.

The effects of field filtration were the

least evident in high-yield wells and/or lowturbidity wells. Samples bailed from these

wells exhibited much smaller differences

between unfiltered and 0.45-μm-filtered

samples. However, bailing clearly mobilized artifactual particles that caused elevated metal concentrations in most unfiltered bailed samples. Samples collected

by the bailer and immediately filtered exhibited trace metal concentrations that

were roughly equivalent to those produced

by the pumps and in-line filtration.

Samples pumped from these wells exhibited virtually no differences between unfiltered and filtered samples, reflecting the

minimal entrainment of artifactual particles larger than 0.45 μm during sampling

at low to moderate pumping rates. Concentrations in filtered samples bailed from

high-yield wells and/or low-turbidity wells

were generally equivalent to concentrations in pumped samples. This reflects the

removal of larger, normally immobile artifactual particles and associated metals

from the bailed samples.

Although the three sample-collection

methods generally produced similar results when samples from less turbid wells

were filtered, the pumping methods produced the most consistent overall results.

Most metals showed little variation between filtered and unfiltered pumped

samples, reflecting the minimal agitation

in the sampling zone and sample during

purging and sample collection. Use of submersible pumps at low speeds may reduce the uncertainty in results when collecting samples of inorganic ground-water

constituents that have the potential to associate with particles in ground water.

Since this study included only a limited

number of wells at three sites, it does not

represent the wide variety of geologic and

hydrogeochemical conditions likely to be

present at all solid waste or hazardous

waste landfills. As a result, more information is required from a variety of sites

regarding the presence of colloidal particles and the importance of these particles in the transport of trace metals and

other contaminants in ground water. A

better understanding of colloidal transport

processes in ground-water environments

could be gained from research focused

on describing hydrogeochemical conditions

and colloid size distribution, composition,

movement, and association with trace

metals at a variety of solid waste and

hazardous waste sites.

The information in this document has

been funded wholly or in part by the United

States Environmental Protection Agency

under Cooperative Agreement Number

CR815774 to the Water Resources Center of the Desert Research Institute. It has

been subjected to the Agency’s peer and

administrative review, and it has been approved for publication as an EPA document. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
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Charlita G. Rosal (also the EPA Project Officer, see below) is with the Environmental Monitoring Systems Laboratory, Las Vegas, NV 89193-3478, K. F. Pohlman,

G.A. Icopini, and R.D. McArthur are with Desert Research Institute, University of

Nevada at Las Vegas, Las Vegas, NV 89119.

The complete report, entitled "Evaluation of Sampling and Field-Filtration Methods

for the Analysis of Trace Metals in Ground Water," (Order No. PB94-201993/AS;

Cost: $19.50, subject to change) will be available only from:

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

Telephone: 703-487-4650

The EPA Project Officer can be contacted at:

Environmental Monitoring Assessment Laboratory

U.S. Environmental Protection Agency

Las Vegas, NV 89193-3478

 • •

• •

United States

Environmental Protection

Agency

Center for Environmental Research Information

Cincinnati, OH 45268

Official Business

Penalty for Private Use $300

EPA/600/SR-94/119

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