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

-------
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

-------
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.

-------
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.

References

Avogadro, A. and G. De Marsily 1984. The Role of Colloids in
Nuclear Waste Disposal. In Scientific Basis for Nuclear Waste
Management, Gary L. McVay, Ed., pp. 495-505.

Backhus, D., P. R. Gschwend, M.D. Reynolds.  1986.  Sampling
Colloids in Groundwater, Abstract. EOS 67:954.

Baker, J.E., PD. Capel, and S.J. Eisenreich. 1986. Influence of
Colloids on Sediment-Water Partition  Coefficients of
Polychlorobiphenyl Congeners in Natural Waters. Environ. Sci.
Technol. 20(11):1136-1143.

Barcelona, M.J., G.K.  George, and M.R. Schock. 1988. Com-
parison of Water Samples from PTFE, PVC and SS Monitoring
Wells. Illinois State Water Survey Internal  Report Prepared for

USEPA-EMSL, Las Vegas, NV, Aquatic and Subsurface Moni-
toring Branch (CR #812165-02), 37 pp.

Barcelona, M.J., J.P Gibb, and R.A. Miller. 1983. A Guide to
the Selection of Materials for Monitoring Well Construction and
Groundwater Sampling.  Illinois State Water Survey Contract
Report #327 prepared for USEPA-RSKERL,  Ada, OK,  and
USEPA-EMSL, Las Vegas, NV. EPA-600/2-84-024,  78 pp.

Barcelona, M.J., J.P. Gibb, J.A. Helfrich, and E.E. Garske. 1985.
Practical Guide for Groundwater Sampling.  Illinois State Water
Survey Contract Report #374 prepared for USEPA-RSKERL,
Ada, OK and USEPA-EMSL,  Las Vegas,  NV.  EPA-600/2-85-
104,  94 pp.

Bergseth, H. 1983. Effect of Filter Type and Filtrate Treatment
on the Measured Content of Al and Other Ions in Groundwater.
Acta Agriculturae Scandinavica 33 (1983)  353-359.

Brobst, R.B. 1984. Effects of Two Selected Drilling Fluids on
Ground Water Sample Chemistry; Monitoring Wells, Their Place
in the Water Well Industry.  Educational Session, NWWA Na-
tional Meeting and Exposition, Las Vegas,  NC.

Carter,  C.W and I.H. Suffet.   1982.  Binding  of DDT to  Dis-
solved Humic Material.  Environ. Sci. Technol. 16(11 ):735-740.

Champ,  D.R., WF Merritt, and J.L. Young.  1982. Potential for
Rapid Transport of Pu in Groundwater as Demonstrated by Core
Column Studies. In Scientific Basis for Radioactive Waste Man-
agement. Vol. 5,  Elsevier Sci. Publ., NY.

Champlin, J.B.F.,  and  G.G. Eichholz.  1968. The Movement of
Radioactive Sodium and Ruthenium through a Simulated Aqui-
fer. Water Resour. Res. 4(1):147-158.
Danielsson, L.G.  1982. On the Use of Filters for Distinguishing
Between Dissolved and Particulate Fractions in Natural Waters.
Water Res. 16, 179-182.

Driscoll, FG. 1986. Ground Water and Wells. 2nd Ed., Johnson
Division, St. Paul,  MN, 1108 pp., pp. 497, 438-9, 722, 725-26.

Enfield, C.G. and G. Bengtsson. 1988.  MacromolecularTrans-
port of Hydrophobic Contaminants  in  Aqueous  Environments.
Ground Water 26(1 ):64-70.

Evans, L.G. and S.B. Ellingson.  1988.  The Formation of Ce-
ment Bleed-Water and Minimizing Its Effect on Water Quality
Samples.  In Proceedings of the Ground Water Geochemistry
Conference, pp. 377-389. Hyatt Regency, Denver, CO, Asso-
ciation of Ground Water Scientists and Engineers. NWWA-Water
Well Journal Publishing Company, Dublin, OH.

Faust, C.R. and J.W Mercer. 1984. Evaluation of Slug Tests in
Wells Containing a Finite Thickness Skin. Water Resour.  Res.
20(4):504-506.

Faust, C.R. and J.W. Mercer.  1985.  Reply Water Resources
21(9):1462.

Garske, E.E. and M.R. Schock.  1986.  An Inexpensive Flow-
Through Cell and Measurement System for Monitoring Selected
Chemical Parameters in Ground Water. Ground Water Monitor-
ing Review 6(3): 79-84.

Gschwend, P.M. and  M.D. Reynolds.   1987.  Monodisperse
Ferrous Phosphate Colloids in  an Anoxic Groundwater Plume.
J. of Contaminant  Hydrol.  1(1987):309-327.

Holm,  T.R., G.K. George  and M.J. Barcelona.  1988.  Oxygen
Transfer Through  Flexible Tubing and  its Effects on Ground
Water Sampling Results.  Ground  Water Monitoring Review.
8(3):83-89.

Jay, PC.  1985. Anion Contamination of Environmental Water
Samples Introduced by  Filter Media.  Analytical Chemistry
57(3):780-782.

Kennedy, V.C. and G.W Zellweger.  1974. Filter Pore-Size Ef-
fects on theAnalysis of Al, Fe, Mn, and Ti in Water. Water Resour.
Res.  10(4):785-790.

Kim, J.I., G.  Buckau, F Baumgartner, H.C.  Moon  and D. Lux.
1984.  Colloid Generation and theActinide Migration in Gorbelen
Groundwaters. In Scientific Basis for Nuclear Waste Manage-
ment, V 7, Gary L. McVay, Ed., Elsevier, NY, pp. 31-40.

Kovenya, S.V.,  M.K. Mel/EnikovaandA.S. Frid. 1972. Studyof
the Role of Mechanical Forces and Geometric Conditions in the
Movement of  Highly  Dispersed Particles in Soil Columns.
Pochvovedeniye. 10, 133-140.

Laxen,  D.P.H. and I.M. Chandler. 1982.  Comparison of Filtra-
tion Techniques for Size Distribution in Freshwaters. Analytical
Chemistry 54(8): 1350-1355.

-------
Means, J.C. and R. Wijayaratne.  1982.  Role of Natural Col-
loids in the  Transport of Hydrophobic Pollutants.  Science
215(19):968-970.

Moench, A.F. and PA. Hsieh.  1985. Comment on "Evaluation
of Slug Tests in Wells Containing a Finite Skin" by C.R.  Faust
and J.W. Mercer. Water Resour. Res. 21 (9): 1459-1461.

O'Melia, C.R.  1980.  Aquasols: The  Behavior  of Small Par-
ticles in Aquatic Systems. Environ. Sci. Technol. 14(9): 1052-
1060.

Reynolds,  M.D. Colloids  in Groundwater. 1985.  Masters The-
sis. Mass. Inst. of Tech.  Boston, MA.

Saltelli, A., A. Avogadro and G. Bidoglio.  1984. Americium Fil-
tration in Glauconitic Sand Columns. NuclearTechnol. 67, 245-
254.

Sheppard, J.C., M.J. Campbell and J.A. Kittrick.  1979. Reten-
tion of Neptunium, Americium and Curium by Diffusible Soil Par-
ticles. Environ. Sci. Technol.  13(6):680-684.

Sheppard, J.C., M.J. Campbell, T Cheng and J.A.  Kittrick.  1980.
Retention  of Radionuclides  by Mobile  Humic  Compounds.
Environ. Sci. Technol. 14(11):1349-1353.

Stumm, W and J.J. Morgan.  1981. Aquatic Chemistry.  John
Wiley and  Sons, Inc., NY.

Takayanagi,  K. and G.T.F. Wong. 1984. Organic and Colloidal
Selenium  in South Chesapeake  Bay and Adjacent Waters.
Marine Chem.  14:141-148.

Tillekeratne, S., T. Miwa and A.  Mizuike.  1986.  Determination
of Traces of Heavy Metals in Positively Charged Inorganic Col-
loids in Fresh Water. Mikrochimica Acta 6:289-296.

Wagemann,  R.  and G.J.  Brunskill.  1975.  The Effect of Filter
Pore-Size onAnalytical Concentrations of Some Trace Elements
in Filtrates of Natural Water.  Intern. J. Environ. Anal. Chem.
4:75-84.

Walker, S.E. 1983. Background Water Quality Monitoring: Well
Installation Trauma. In  Proceedings of the Third National Sym-
posium on Aquifer Restoration and Ground Water  Monitoring,
NWWA, Fawcett Center, Columbus, OH.  pp. 235-246.

Yao, K., M.T Habibian and C.R. O'Melia.  1971.  Water and
Waste Water Filtration:  Concepts and Applications.  Environ.
Sci. Technol. 5(11):1105-1112.

-------