United States
Environmental Protection
Agency
Office of
Research and
Development
Office of Solid Waste
and Emergency
Response
EPA/540/4-89/001
March 1989
&EPA Superfund
Ground Water Issue
Ground Water Sampling for Metals Analyses
Robert W. Puls and Michael J. Barcelona
The Regional Superfund Ground Water Forum is a group of
ground-water scientists, representing EPA's Regional Superfund
Offices, organized to exchange up-to-date information related
to ground-water remediation at Superfund sites.
Filtration of ground-water samples for metals analysis is an is-
sue identified by the Forum as a concern of Superfund deci-
sion-makers. Inconsistency in EPA Superfund cleanup prac-
tices occurs where one EPA Region implements a remedial
action based on unfiltered ground-water samples, while another
Region may consider a similar site to be clean based on filtered
ground-water samples. RSKERL-Ada and EMSL-Las Vegas
have convened a technical committee of experts in the areas of
ground-water geochemistry, inorganic chemistry, colloidal trans-
port and ground-water sampling technology to examine this is-
sue and provide technical guidance based on current scientific
information.
Members of the committee were Robert W. Puls, Bert E. Bledsoe
and Don A. Clark of RSKERL; Michael J. Barcelona, Illinois State
Water Survey; Phillip M. Gschwend, Massachusetts Institute of
Technology; Terry F Rees, USGS-Denver; John W. Hess, Desert
Research Institute (EMSL-LV); and NicholousT Loux, ERL-Ath-
ens.
This document was written by Robert W. Puls and Michael J.
Barcelona and edited by all members of the committee.
For further information contact Robert Puls, RSKERL-Ada, FTS
743-2262; Bert Bledsoe, RSKERL-Ada, FTS 743-2324; Jane
Denne, EMSL-LV, FTS 545-2655.
The findings and recommendations of the committee were that
use of a 0.45 micron* filter was not useful, appropriate or repro-
ducible in providing information on metals mobility in ground-
water systems, nor was it appropriate for determination of truly
"dissolved" constituents in ground water. A dual sampling ap-
proach was recommended, with collection of both filtered and
unfiltered samples. If the purpose of the sampling is to deter-
mine possible mobile contaminant species, the unfiltered
samples should be given priority. This means that added em-
phasis is placed on appropriate well construction methods,
materials and ground-water sampling procedures. For accu-
rate estimations of truly "dissolved" species concentrations, fil-
tration with a nominal pore size smaller than 0.45 microns was
recommended. It was further concluded that filtration could not
compensate for inadequate construction or sampling proce-
dures.
Background/Support Information
Filtration of ground-water samples for metal analyses will not
provide accurate information concerning the mobility of metal
contaminants. This is because some mobile species are likely
to be removed by filtration before chemical analysis. Metal con-
taminants may move through fractured and porous media not
only as dissolved species, but also as precipitated phases, poly-
meric species or adsorbed to inorganic or organic particles of
colloidal dimensions. Colloids are generally considered as par-
ticles with diameters less than 10 microns (Stumm and Morgan,
1981). Numerous investigators have suggested the facilitated
transport of contaminants in association with mobile colloidal
particles. Kim et al. (1984) suggested that sorption to ground-
water colloidal material caused the mobilization of some radio-
nuclides in Gorleben ground waters. Saltelli et al. (1984) stud-
ied americium percolation in glauconitic sand columns and at-
tributed the unretained fractions to migrating colloidal species.
* Micron = mm = 10'6 meter
Superfund Technology Support Centers for Ground Water
Robert S. Kerr Environmental
Research Laboratory
Ada, OK
Environmental Monitoring
Systems Laboratory
Las Vegas, NV
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These colloids were either homogeneous hydrous precipitates,
or were formed from the adsorption of the radionuclide onto
colloidal size mineral particles. Colloidal particles generated in
batch experiments by Sheppard et al. (1979) were shown to
adsorb significant quantities of radionuclides. Further work by
Sheppard et al. (1980) concluded that the transport of radionu-
clides by colloidal clay particles must be considered in any con-
taminant transport model. Champlin and Eichholz (1968)
showed that the movement of radioactive sodium and ruthe-
nium in sand beds was associated with particulate matter of
micron dimensions. Gschwend and Reynolds (1987) demon-
strated that submicron ferrous phosphate colloids were sus-
pended and presumably mobile in a sand and gravel aquifer.
Studies by Yao et al. (1971) and O'Melia (1980) indicate that
colloidal particles in the range 0.1 to 1.0 micron may be most
mobile in a sandy, porous medium. Kovenya et al. (1972) con-
cluded that particles in the range 0.1 to 0.5 mm were most mo-
bile in soil column studies. As much as 200 ppb copper, lead
and cadmium was found associated with colloidal material in
size range 0.015-0.450 mm by Tillekeratne et al. (1986). Rapid
transport of plutonium (Pu) in core column studies by Champ et
al. (1982) was attributed to colloidal transport, with 48% of the
Pu associated with colloids in the size range 0.003-0.050 mm
and 23% in the range 0.050-0.450 mm. Reynolds (1985) using
carboxylated polystyrene beads ranging from 0.10 to 0.91 mm
in size, recovered 45% of the 0.91 mm size beads, and greater
than 70% of 0.10 and 0.28 mm size beads in laboratory sand
column effluents.
Lake and estuarine studies by Baker et al. (1986) and Means
and Wijayaratne (1982) demonstrated the importance of natu-
ral colloidal material in the transport of hydrophobic contami-
nants. Carter and Suffet (1982) found that a significant fraction
of "dissolved"DDT in surface waters was bound to colloidal hu-
mic material. Takayanagi and Wong (1984) found over 70% of
the total inorganic colloidal particles.
Analytical methods used to determine "dissolved" metal con-
centrations have historically used 0.45 micron filters to sepa-
rate dissolved and particulate phases. If the purpose of such
determinations is an evaluation of "mobile" species in solution,
significant underestimations of mobility may result, due to col-
loidal associations. On the other hand, if the purpose of such
filtration is to determine truly dissolved aqueous species, the
passage of colloidal material less than 0.45 microns in size may
result in the overestimation of dissolved concentrations
(Bergseth, 1983; Kim et al. 1984; Wagemann and Brunskill,
1975). Kennedy et al. (1974) found errors of an order of magni-
tude or more in the determination of dissolved concentrations
of aluminum, iron, manganese and titanium using 0.45 micron
filtration. Sources of error were attributed to filter passage of
fine-grained clay particles. Additionally, filtration of anoxic
ground-water samples is very difficult without iron oxidation and
colloid formation, causing a removal of previously dissolved
species to be filtered. Filter loading and clogging of pores with
fine particles may also occur, reducing the nominal size
(Danielsson, 1981). Filtration should be viewed as only one
approach for determining the "true" solution geochemistry of
ground water, and others should be applied whenever possible.
Purpose of Sampling
It is important to identify the purpose of ground-water sampling
before decisions regarding filtration, centrifugation or other phase
separation techniques are made. Is it to determine the mobility
of contaminants or to determine in situ aqueous geochemistry?
The following definitions are also useful for consideration of this
issue:
(1) Total Contaminant Load Per Unit Volume of
Aquifer = Mobile + Immobile Species.
(2) Mobile Species = Dissolved + Suspended
Species.
(3) Dissolved = Free Ions + Inorganic Complexes
+ Low Molecular Weight Organic Complexes.
(4) Suspended = Adsorbed + Precipitated +
Polymeric + High Molecular Weight Organic
Complexes.
For an assessment of mobility, all mobile species must be con-
sidered, including suspended particles acting as adsorbents for
contaminants. While not all suspended species may necessar-
ily be sufficiently mobile or toxic to pose a health risk, a conser-
vative approach is proposed at this time until more definitive
data are available. Contaminant transport models which ac-
count for an additional aqueous mobile colloidal phase have
been proposed by Avogadro and DeMarsily (1984) and Enfield
and Bengsston (1988).
A principle objective in a sampling effort for testing a geochemi-
cal speciation model is to obtain estimates of the free ion activi-
ties of the major and trace elements of interest. Since there are
relatively few easily performed analytical procedures for mak-
ing these experimental estimates, an alternative procedure is
to test the analytically determined dissolved concentrations with
model predictions including both free and complexed species.
More and more remedial investigations are utilizing such mod-
els to make predictions about contaminant behavior based on
dissolved concentrations. It is not the purpose of this report to
suggest how to perform these analytical determinations, but as
noted above, the use of a 0.45 micron filter as the operational
definition of "dissolved" may be inappropriate. Analytical tech-
niques such as ion selective electrodes, ion exchange and po-
larography may be more accurate. Research utilizing these
and other techniques to correlate "dissolved" with filter size is
recommended.
If one adopts the conservative approach with no filtration for
contaminant mobility estimations, increased importance is placed
on proper well construction, and purging and sampling proce-
dures to eliminate or minimize sources of sampling artifacts.
Sources of Sampling Artifacts vs. "real" Ground-water
Environment
The disturbance of the subsurface environment as a result of
well construction and sampling procedures presents serious
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obstacles to the interpretation of ground-water quality results.
Some degree of disturbance of natural conditions is inevitable.
However, the impact of improper well construction and sam-
pling techniques can permanently bias the usefulness and in-
tegrity of wells as sampling points. Several aspects of well con-
struction and sampling procedures must be carefully consid-
ered to avoid errors associated with the introduction of foreign
particles or the alteration of ambient subsurface conditions which
may affect natural dissolved or suspended materials.
Well Construction
The design, drilling, and construction of monitoring wells have
been identified as particularly important steps in the collection
of representative water chemistry and hydrologic data. Several
references have emphasized the minimization of both the dis-
turbance and the introduction of foreign materials (USEPA,
OSWER-9950.1, 1986; Barcelona, et al., 1983; Barcelona et
al., 1985) because of the potential impact on water chemistry.
The RCRA Technical Enforcement Guidance Document
(USEPA, OSWER-9950.1, 1986) suggests that the well must
allow for sufficient ground-water flow for sampling, minimize
passage of formation materials into the well, and exhibit suffi-
cient structural integrity to prevent collapse of the intake struc-
ture. It should be recognized, however, that the well must first
provide a representative hydraulic connection to the geologic
formation of interest. Without the assurance of this hydraulic
integrity, the water chemistry information cannot be interpreted
in relation to the dynamics of the flow system or the transport of
chemical constituents.
More specific guidance is therefore necessary to maintain or
restore the natural hydraulic conductivity of the formation in the
vicinity of the screened portion of monitoring wells through the
drilling, construction and development procedures. The litera-
ture on water well technology can be most helpful in this regard
since minimal disturbances of the subsurface is a common goal
in maximizing both the yield of water supply wells and the rep-
resentativeness of water samples and hydraulic information from
monitoring wells (Driscoll, 1986).
To insure the long-term integrity of monitoring wells, particularly
with respect to excluding foreign particles and permitting the
passage of mobile (i.e., dissolved and suspended) contaminants,
specific items which should be observed are:
1) If no alternative to the use of drilling muds or fluids exists,
these materials must be removed from the well bore and adja-
cent formations by careful well development (Driscoll, 1986).
This guidance also applies to the removal of the low permeabil-
ity "skin" which is caused by abrasion, oxidation and invasive
muds which may seal the well bore from the screened interval
and bias in situ determininations of hydraulic conductivity (Faust
and Mercer, 1984; Moench and Hsieh, 1985; Faust and Mercer,
1985). Pumping rates during development should be docu-
mented and care should be taken not to exceed these rates
during purging or sampling since further development and well
damage may aggravate suspended particulate and turbidity
problems even in properly designed wells.
2) The emplacement of grouts and seals to isolate the
screened interval must be carefully done. The use of tremie
pipes and frequent checking of the depth of emplacement of
clay or cement grouts during well construction are strongly en-
couraged.
It is also important to take care to follow manufacturer's guide-
lines on the hydration of cement or expanding cement as grouts
or seals. Excess water addition and grading of cement compo-
nents or materials due to free fall through standing water can
permanently damage the well's integrity (Evans and Ellingson,
1988).
3) Casing and screen materials must be selected to retain
their integrity in the subsurface environment (i.e., avoid iron,
steel), minimize bias to water samples and insure that screen
openings are not reduced by the buildup of corrosion products
or by compression (USEPA, OSWER-9950.1, 1986). These
effects can be checked by repeat determinations of in situ hy-
draulic conductivity over the useful life of the well. Redevelop-
ment and replacement of the well should be considered if dete-
rioration or significant changes in hydraulic conductivity are
observed. Erratic water level readings and sudden changes in
turbidity or purging behavior of monitoring wells prior to sam-
pling are warning signs of possible loss of material integrity.
4) Well design fundamentals with regard to the selection of a
filter pack and screen size are among the most important is-
sues in obtaining representative hydraulic and water quality in-
formation. The exclusion of fines, clays, and silts can be
achieved by selecting the grain-size distribution for the filter
pack by multiplying the 50-percent retained size of the finest
formation sample by a factor of two (Driscoll, 1986). The filter
pack material should be cleaned and washed free of fines to
insure that extraneous contaminants or particles are removed.
The well screen slot openings should be chosen to retain 90%
of the filter pack material after development. In natural packed
wells it may be advisable to select a screen slot size which will
retain at least 50% of the finest material in the screened inter-
val. Minimizing slot screen width however, often leads to greater
time and energy spent in well development. The need to docu-
ment well development procedures cannot be overemphasized.
Maintenance of the hydraulic performance of monitoring wells
and the connection of wells to the zones of greatest hydraulic
conductivity, where contaminant transport is most probable,
should take equal importance to the collection of representative
water quality data.
Purging and Sampling
Water that remains in the well casing between sampling peri-
ods is unrepresentative of water in the formation opposite the
screened interval. It must be removed by purging or isolated
from the collected sample by a packer arrangement prior to the
collection of representative water samples. Water level read-
ings must be made carefully to avoid the disturbance of fines or
precipitates which may enter or form in the well due to chemical
reactions or microbial processes and accumulate on the inte-
rior walls of the well casing screen or at the bottom of the well.
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Similarly, it is important to purge the stagnant water at flow rates
below those used in development to avoid further development,
well damage or the disturbance of accumulated corrosion or
reaction products in the well. The use of certain sampling de-
vices, particularly bailers and air-lift arrangements, should be
discouraged in order to avoid the entrainment of suspended
materials which are not representative of mobile chemical con-
stituents in the formation of interest.
A note of caution should be voiced to encourage repetitive sam-
pling of monitoring wells prior to judging the representativeness
of determinations of hydraulic conductivity , water level read-
ings and water quality data. The effects of the inevitable "trauma"
due to drilling, sealing and development of monitoring wells can
bias observations of water chemistry until the subsurface is al-
lowed to equilibrate sufficiently (Walker, 1983). Estimates of
the time to achieve equilibration vary substantially, particularly
when drilling fluids are used in highly permeable formations
(Brobst, 1984; Driscoll, 1986); however, periods of weeks to
several months may be necessary before even major ionic con-
stituents of ground water equilibrate to previous levels
(Barcelona, et al., 1988).
Recommendations for Sampling
In general, the zone of interest must be isolated, the sample
pumped slowly to minimize turbidity and sample collected in
such manner as to eliminate O2 and CO2 exchange with the at-
mosphere. No filtration for mobile metals determination is rec-
ommended. If the unfiltered values exceed maximum contami-
nant level concentrations for ground-water quality, additional
analyses and re-evaluation of sampling artifacts are required.
It should be emphasized that extreme differences between un-
filtered and 0.45 mm filtered samples does not preclude the use
of unfiltered data for risk assessment decisions. Significant
particulate mobility may be occurring at such a site, and addi-
tional analyses with other larger filters (e.g. >0.45 mm) may be
most appropriate given the current size estimates for upper lim-
its for mobile particles.
Isolation of Sampling Zone
Isolation ofthe samplingzone is necessary to minimizethe purge
volume as well as to minimize air contact. This is especially
important since Eh/pH conditions of the formation waters are
notoriously sensitive to dissolved gases content. Inflatable pack-
ers can be used to achieve isolation ofthe sampling zone.
Pumping for Sample Collection
It is recommended that a positive displacement pump can be
used. Othertypes of sample collection (e.g., bailing) may cause
displacement of non-mobile particles or significantly alter ground
water chemistry leading to colloid formation (e.g., vacuum
pumps). Surging must be avoided, and a flow rate as close to
the actual ground-water flow rate should be employed. Acknowl-
edging that this may be impossible or impractical in some in-
stances, a pumping flow rate based on the linear ground-water
flow rate and open screen area is proposed, where
pumping flow rate -linear GW flow rate x 2 x screen ht. x
well radius x 10
While an initial approximation, flow rates around 100 ml/min
have been used to successfully sample ground-waters in a qui-
escent mode.
Additional research is needed in this area, particularly with re-
spect to the appropriateness of this generic equation. An inex-
pensive flow-through type cell set-up utilizing this approach was
described by Garske and Schock (1986).
Assessment of Water Constituents While Sampling
Monitoring of the pumped ground water for dissolved oxygen,
temperature, conductivity and pH aids in the interpretation or
establishment of ground-water background quality. Gschwend
and co-workers (personal communication) have observed that
turbidity diminished dramatically after prolonged pumping,
changing similarly, although possibly more slowly, than other
water quality parameters (e.g., O2, conductivity). An initial esti-
mate proposed for time of pumping necessary to collect water
from a formation is around two times the time required to get
plateau values for the above parameters.
No Filtration for Mobile Fraction Determination
Those samples intended to indicate the mobile substance load
should not be filtered. Steps to preserve their integrity, such as
acidification, should be performed as soon as possible.
Filtration for Specific Geochemical Information
Any filtration for estimates of dissolved subsurface species loads
should be performed in the field with no air contact and immedi-
ate preservation and storage. In-line pressure filtration is best
with as small a filter pore size as practically possible (e.g., 0.05,
0.10 micron). Using a smaller pore size filter will require longer
sample collection time, increasing the need for air exclusion
from the sample (Laxen and Chandler, 1982; Holmetal., 1988).
Polycarbonate membrane-type filters with uniform and sharp
size cutoffs are recommended to minimize particle loading on
the filter. Although membrane filters are more prone to clogging
than fiber-type filters, the uniform pore size, ease of cleaning,
and minimization of adsorptive losses from the sample tend to
improve the precision and accuracy in the analytical data. The
filter holder should be of material compatible with the metals of
interest. Holders made of steel are subject to corrosion and
may introduce non-formation metals to samples. Large diam-
eter filter holders (e.g., > 47 mm) are recommended to reduce
clogging and pore size reduction and for ease of filter pad re-
placement. The use of disposable in-line filters are suggested
for convenience if of sufficient quality. Prewashing of filters
should be routinely performed. Work by Jay (1985) shows that
virtually all filters require prewashing to avoid sample contami-
nation.
Quality assurance and quality control becomes increasingly
important when adopting the above recommendations. The use
-------
of field blanks and standards for field sampling is essential. Field
blanks and standards enable quantitative correction for bias due
to collection, storage and transport. Analysis of the filters them-
selves and their particulate load is suggested as a check on
mass balance and filtration effects on solid/solution separation
efficiency.
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