United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas, NV 89193-3478
Research and Development
EPA/600/SR-94/119 September 1994
EPA Project Summary
Evaluation of Sampling and
Field-Filtration Methods for the
Analysis of Trace Metals in
Ground Water
Karl F. Pohlmann, Gary A. Icopini, Richard D. McArthur, and Charlita G. Rosal
Selected ground water sampling and
field-filtration methods were evaluated
to determine their effects on field pa-
rameters and trace metal concentra-
tions in samples collected under sev-
eral 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 impor-
tant to the transport of hydrophobic
organic contaminants and trace met-
als. The specific sampling and filtra-
tion variables investigated in this study
were (1) filtration with 0.45-^m or 5.0-
|am pore size filters versus no filtra-
tion; (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-jjm fil-
ters 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-of-
magnitude higher than in 0.45-jjm-fil-
tered samples. The largest differences
occurred at the most turbid wells and
in samples containing the highest par-
ticle concentrations. Similar effects
were observed in some samples col-
lected by pumps from the most turbid
wells, particularly the low yield well.
For most pump sampling, however, dif-
ferences in concentrations between
0.45-jjm-filtered and unfiltered samples
were not significant and particle con-
centrations were significantly lower
than those produced by the bailer. Bail-
ers caused more disturbance of the
sampling zone than the three pumping
methods as evidenced by measure-
ments of field parameters and concen-
trations of particles, major ions, and
trace metals. Little variation was ob-
served 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-|am-filtered
samples were generally independent of
sampling method, suggesting that these
constituents were present as dissolved
species and not associated with par-
ticles, or associated with particles
smaller than 0.4 |am. At wells where
5.0-jjm filtration was conducted, physi-
cal and hydrochemical conditions re-
sulted in minimal differences between
trace metal concentrations in the 5.0-
(jm-filtered, 0.45-|jin-filtered, and unfil-
tered samples.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory, Las Vegas, NV, to
announce key findings of the research
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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 trans-
ported by natural ground water flow and a
normally immobile solid phase composed
of the matrix materials of the water-bear-
ing zone. The action of purging and sam-
pling a monitoring well installed in uncon-
solidated 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 met-
als associated with these normally immo-
bile matrix particles may bias analytical
determinations, leading to elevated and
improbable concentrations of mobile con-
taminants if suspended particle concen-
trations are very high. As a result, ground
water samples are commonly filtered in
the field to remove these suspended par-
ticles. Filtration has been considered par-
ticularly 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 inter-
actions 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 sol-
utes and sediments. Potential association
of metals with colloids has important im-
plications for the practice of field filtration
because the boundary between the par-
ticulate and dissolved has been opera-
tionally defined at 0.45|om This boundary
presumes that the component retained on
a 0.45 |im filter represents suspended sol-
ids, while the component that passed
through the filter represents dissolved met-
als.
Collection of ground water samples for
analysis of metals concentrations is re-
quired under several U.S. environmental
regulations, including CERCLA (Super-
fund), 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 un-
dertaken to investigate how concentrations
of trace metals were affected by selected
methods of sample collection and field-
filtration. The objectives of the study were
to provide a survey of the impacts of the
following aspects of ground-water sam-
pling:
1) Impacts of sample collection method
on determinations of field parameters.
2) Impacts of filtration with 0.45-|am or
5.0 |im pore size filters versus no
filtration on trace metal concentrations.
3) Impacts of sampling device—specifi-
cally, bailer, bladder pump, submers-
ible-centrifugal pump (at a "low" dis-
charge rate of 300 mL/min), and sub-
mersible-centrifugal pump (at a "mod-
erate" discharge rate of 1000 mL/min),
on trace metal concentrations.
4) Impacts of sampling device on par-
ticle size distribution and total con-
centration.
The study focused on sampling in con-
ventional standpipe monitoring wells un-
der conditions where traditional ap-
proaches to sampling may produce turbid
samples.
Procedure
The monitoring wells sampled were con-
structed 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 sur-
face. 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 condi-
tions 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 sam-
pling 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 trans-
ferred from the bailer directly to the sample
bottles for unfiltered samples or to a filtra-
tion vessel for filtered samples. Com-
pressed nitrogen gas was used to drive
the samples through either membrane fil-
ters or disposable cartridge filters. The
second sampling method was a sub-
mersible-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) oper-
ated 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 mea-
sured at ground surface and were con-
trolled by the pump speed rather than by
flow restrictors or valves. These discharge
rates were used for both purging and sam-
pling. 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 con-
ductivity (EC), and pH of the pump dis-
charge were made in-line, while measure-
ments of these parameters for the bailer
discharge were made off-line. Stabiliza-
tion of these parameters provided an indi-
cation 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 sam-
pling methods with respect to their ability
to minimize disturbance in the sampling
zone. Estimates of particle size distribu-
tion were determined gravimetrically by
serial ultrafiltration using microfilters of 5.0
|j,m, 0.4 |j,m, 0.1 |j,m, and 0.03 |j,m pore
size.
Results and Discussion
The results of the study demonstrate
three important factors that influence the
accuracy of field parameters measure-
ments during sampling from conventional
standpipe monitoring wells: measurement
techniques, sampling method, and hydrau-
lics of the well. Impacts related to mea-
surement techniques were considered mi-
nor because a single individual conducted
all the field measurements and all proce-
dures followed established protocol. In con-
trast, sampling method and well hydrau-
lics had impacts on values of field param-
eters in some of the sampling events that
masked all other factors. The relative dis-
turbance 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 gradi-
ent between the formation and the well
mobilized large quantities of particles,
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thereby elevating turbidity values. Contin-
ued removal of water from the well dewa-
tered 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. El-
evated DO values of the bailer and BP at
1 L/min in low-yield wells reflect the for-
mation 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 addi-
tional aeration of the samples as a result
of the increased exposure to the atmo-
sphere during sample collection and trans-
fer. 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 de-
watered, and turbidity was generally lower.
The two pumping methods produced simi-
lar values of most field measurements,
while the surging action of the bailer pro-
duced turbidity values that were approxi-
mately two orders-of-magnitude higher
than those produced by the pumps. Like-
wise, 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 tur-
bidity conditions with relatively low purged
volumes, while the bailer produced high
values of these parameters and did not
reach equilibrium after greater purged vol-
umes. 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 sam-
pling 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 be-
tween 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 mea-
surement. In addition, pH reached equilib-
rium at lower purged volumes than all the
other parameters, independent of sam-
pling method. Although pH is an important
indicator of the speciation of trace metals
in ground water, the relatively uniform val-
ues across devices at individual wells do
not alone suggest that similar metals spe-
cies might be present.
In almost every case, samples collected
by bailer contained higher particle con-
centrations than those collected by the
pumps, with the greatest differences oc-
curring at the most turbid wells. Further-
more, the size distribution of particles in
most bailed samples was highly skewed
toward larger particles, with over 96%
larger than 0.45 |im, and generally over
93% larger than 5.0 urn. The quantities
and sizes of these particles suggest that
they were not mobile in ground water un-
der natural flow conditions but were pri-
marily the artifacts of well construction,
development, and purging and were mo-
bilized by agitation in the sampling zone
caused by bailing. The particle size distri-
bution 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 (high-
yield) wells, total particle concentrations in
pumped samples were orders-of-magni-
tude lower than in bailed samples, reflect-
ing the lower degree of agitation caused
by the pumping methods. Also, particle
sizes in the pumped samples were gener-
ally more uniformly distributed; approxi-
mately 50% of the particles were larger
than 0.45 |om
Differences in metal concentrations be-
tween 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 correspond-
ing filtered samples. The large differences
in concentration between filtered and un-
filtered bailed samples reflect the associa-
tion of metals with the high concentrations
of artifactual particles entrained during bail-
ing. For example, iron in the sampling
zone likely existed as iron hydroxide par-
ticles, particles containing elemental iron,
and ferrous iron sorbed to particle sur-
faces. 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 origi-
nating as aquifer solids, and their concen-
trations were similarly reduced by filtra-
tion. 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 filtra-
tion of bailed samples likely reduced the
effective pore size of the filter membrane,
thereby blocking passage of some par-
ticles smaller than 0.45 urn; this would
further reduce the concentrations of asso-
ciated 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 con-
centrations. Removal of the larger par-
ticles in the pumped samples did, how-
ever, cause filtered samples to contain
lower metal concentrations than unfiltered
samples, though the differences in con-
centration 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, unfil-
tered 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 met-
als 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 low-
yield and turbid wells, but also are related
to the metal speciation at each well. Dif-
ferences 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 par-
ticles smaller than 0.45 |jm This suggests
that many metals existed primarily as dis-
solved species and/or were associated with
particles smaller than 0.45 \im in the less
turbid and high-yield wells included in this
study.
Conclusions and
Recommendations
Field determinations of unstable param-
eters DO and turbidity were the most
sensitive to disturbance of the sampling
zone, with values produced by bailing of-
ten orders-of-magnitude higher than those
produced by the pumps. Variations in in-
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dicator parameters EC and pH were insig-
nificant between the four sampling meth-
ods, 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 purg-
ing process, relative to other indicator pa-
rameters, suggests that turbidity may be a
useful indicator of relative particle con-
centrations between wells and of stabili-
zation 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 low-
yield and/or turbid wells. Concentrations
in unfiltered bailed samples were up to
several orders-of-magnitude higher than
in filtered bailed, filtered pumped, and un-
filtered pumped samples. Elevated metal
concentrations in unfiltered bailed samples
reflected the entrainment of large quanti-
ties of normally immobile artifactual par-
ticles 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 ef-
fects of field filtration on metal concentra-
tions.
The effects of field filtration were the
least evident in high-yield wells and/or low-
turbidity wells. Samples bailed from these
wells exhibited much smaller differences
between unfiltered and 0.45-|im-filtered
samples. However, bailing clearly mobi-
lized artifactual particles that caused el-
evated metal concentrations in most un-
filtered bailed samples. Samples collected
by the bailer and immediately filtered ex-
hibited trace metal concentrations that
were roughly equivalent to those produced
by the pumps and in-line filtration.
Samples pumped from these wells exhib-
ited virtually no differences between unfil-
tered and filtered samples, reflecting the
minimal entrainment of artifactual par-
ticles larger than 0.45 |im during sampling
at low to moderate pumping rates. Con-
centrations in filtered samples bailed from
high-yield wells and/or low-turbidity wells
were generally equivalent to concentra-
tions in pumped samples. This reflects the
removal of larger, normally immobile arti-
factual particles and associated metals
from the bailed samples.
Although the three sample-collection
methods generally produced similar re-
sults when samples from less turbid wells
were filtered, the pumping methods pro-
duced the most consistent overall results.
Most metals showed little variation be-
tween filtered and unfiltered pumped
samples, reflecting the minimal agitation
in the sampling zone and sample during
purging and sample collection. Use of sub-
mersible pumps at low speeds may re-
duce the uncertainty in results when col-
lecting samples of inorganic ground-water
constituents that have the potential to as-
sociate 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 informa-
tion is required from a variety of sites
regarding the presence of colloidal par-
ticles and the importance of these par-
ticles 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 Cen-
ter of the Desert Research Institute. It has
been subjected to the Agency's peer and
administrative review, and it has been ap-
proved for publication as an EPA docu-
ment. Mention of trade names or commer-
cial products does not constitute endorse-
ment or recommendation for use.
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Charlita G. Rosal (also the EPA Project Officer, see below) is with the Environmen-
tal Monitoring Systems Laboratory, Las Vegas, NV 89193-3478, K. F. Pohlman,
G.A. Icopini, and R.D. McArthurare 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
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