United States

Environmental Protection

Agency

EPA/540/4-89/001

March 1989

Office of Solid Waste

and Emergency

Response

Office of

Research and

Development

Superfund

Ground Water Issue

EPA

Robert S. Kerr Environmental

Research Laboratory

Ada, OK

Environmental Monitoring

Systems Laboratory

Las Vegas, NV

Superfund Technology Support Centers for Ground Water

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 issue identified by the Forum as a concern of Superfund decision-makers. Inconsistency in EPA Superfund cleanup practices 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 transport and ground-water sampling technology to examine this issue 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 Nicholous T. Loux, ERL-Athens.

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 reproGround Water Sampling for Metals Analyses

Robert W. Puls and Michael J. Barcelona

ducible in providing information on metals mobility in groundwater systems, nor was it appropriate for determination of truly

“dissolved” constituents in ground water. A dual sampling approach was recommended, with collection of both filtered and

unfiltered samples. If the purpose of the sampling is to determine possible mobile contaminant species, the unfiltered

samples should be given priority. This means that added emphasis is placed on appropriate well construction methods,

materials and ground-water sampling procedures. For accurate estimations of truly “dissolved” species concentrations, filtration 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 procedures.

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 contaminants may move through fractured and porous media not

only as dissolved species, but also as precipitated phases, polymeric species or adsorbed to inorganic or organic particles of

colloidal dimensions. Colloids are generally considered as particles 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 groundwater colloidal material caused the mobilization of some radionuclides in Gorleben ground waters. Saltelli et al. (1984) studied americium percolation in glauconitic sand columns and attributed the unretained fractions to migrating colloidal species.

* Micron = mm = 10-6 meter
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 radionuclides by colloidal clay particles must be considered in any contaminant transport model. Champlin and Eichholz (1968)

showed that the movement of radioactive sodium and ruthenium in sand beds was associated with particulate matter of

micron dimensions. Gschwend and Reynolds (1987) demonstrated that submicron ferrous phosphate colloids were suspended 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) concluded that particles in the range 0.1 to 0.5 mm were most mobile 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 natural colloidal material in the transport of hydrophobic contaminants. Carter and Suffet (1982) found that a significant fraction

of “dissolved”DDT in surface waters was bound to colloidal humic material. Takayanagi and Wong (1984) found over 70% of

the total inorganic colloidal particles.

Analytical methods used to determine “dissolved” metal concentrations have historically used 0.45 micron filters to separate 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 colloidal 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 magnitude 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 considered, including suspended particles acting as adsorbents for

contaminants. While not all suspended species may necessarily be sufficiently mobile or toxic to pose a health risk, a conservative approach is proposed at this time until more definitive

data are available. Contaminant transport models which account 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 geochemical speciation model is to obtain estimates of the free ion activities of the major and trace elements of interest. Since there are

relatively few easily performed analytical procedures for making 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 models 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 techniques such as ion selective electrodes, ion exchange and polarography 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 procedures 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
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 sampling techniques can permanently bias the usefulness and integrity of wells as sampling points. Several aspects of well construction and sampling procedures must be carefully considered 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 disturbance 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 sufficient structural integrity to prevent collapse of the intake structure. 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 literature 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 representativeness 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 adjacent formations by careful well development (Driscoll, 1986).

This guidance also applies to the removal of the low permeability “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 documented 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 encouraged.

It is also important to take care to follow manufacturer's guidelines on the hydration of cement or expanding cement as grouts

or seals. Excess water addition and grading of cement components 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 hydraulic conductivity over the useful life of the well. Redevelopment and replacement of the well should be considered if deterioration 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 sampling 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 issues in obtaining representative hydraulic and water quality information. 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 interval. Minimizing slot screen width however, often leads to greater

time and energy spent in well development. The need to document 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 periods 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 readings 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 interior walls of the well casing screen or at the bottom of the well.
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 devices, 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 constituents in the formation of interest.

A note of caution should be voiced to encourage repetitive sampling of monitoring wells prior to judging the representativeness

of determinations of hydraulic conductivity , water level readings 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 allowed 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 constituents 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 atmosphere. No filtration for mobile metals determination is recommended. If the unfiltered values exceed maximum contaminant level concentrations for ground-water quality, additional

analyses and re-evaluation of sampling artifacts are required.

It should be emphasized that extreme differences between unfiltered 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 additional analyses with other larger filters (e.g. >0.45 mm) may be

most appropriate given the current size estimates for upper limits for mobile particles.

Isolation of Sampling Zone

Isolation of the sampling zone is necessary to minimize the 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 packers can be used to achieve isolation of the sampling zone.

Pumping for Sample Collection

It is recommended that a positive displacement pump can be

used. Other types 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. Acknowledging that this may be impossible or impractical in some instances, 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 quiescent mode.

Additional research is needed in this area, particularly with respect to the appropriateness of this generic equation. An inexpensive 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 estimate 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 immediate 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; Holm et al., 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 diameter filter holders (e.g., > 47 mm) are recommended to reduce

clogging and pore size reduction and for ease of filter pad replacement. 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 contamination.

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 themselves and their particulate load is suggested as a check on

mass balance and filtration effects on solid/solution separation

efficiency.

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