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

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