Environ. Sci. Technol. 1997, 31, 1359-1364
In-Place Inactivation of Pb in
Pb-Contaminated Soils
WILLIAM R. BERTI* AND
SCOTT D. CUNNINGHAM
Environmental Biotechnology Program, DuPont Central
Research & Development, Glasgow Business Community 301,
P.O. Box 6101, Newark, Delaware 19714-6101
There has been increasing attention to the use of soil
amendments and green plants to remediate surface soils
contaminated with Pb. We call one form of this technique
in-place inactivation. In-place inactivation reduces the
hazards associated with contaminated soils through the
use of chemicals that change the molecule species of the
Pb to stabilize the soil Pb chemically and physically in
situ. We have seen significant changes in soil Pb chemistry,
Pb leached from soil, and Pb measured by a physiologically
based extraction test (PBET) after incorporating
inexpensive and readily available materials to three soils.
The soils have total Pb concentrations that range from
1200 to 3500 mg kg-1. The leachable soil Pb was significantly
reduced in all cases from as high as 30 mg Pb L-1 to below
the regulatory limit of 5 mg Pb L-1 after soil treatment.
The PBET mimics the mammalian gastric-intestinal tract
solutions. In the simulated intestinal phase of the PBET,
Pb in solution was reduced by 72% in one of the soils treated
with a high Fe-containing industrial byproduct. These
results help to illustrate the utility of incorporating soil
amendments to reduce hazards associated with Pb-contaminated soils.
Introduction
Current soil remediation often relies either on a “capping
strategy” or an excavation of the contaminated soil and burial
in a landfill. Some highly contaminated soils are solidified
and stabilized with portland cement prior to disposal. Cost
of these types of site remediation typically ranges from about
USD15 m-2 for a 60-cm-thick soil cap to USD730 m-2 for
excavating to 60 cm, stabilizing, and off-site disposal. An
agricultural-based alternative to current remediation methods
may be appropriate at many contaminated sites. Cost for
this remediation strategy has been estimated at USD6 m-2
for incorporation of materials 60 cm deep (USD is U.S. dollars,
1996 estimates; Quinton, G., DuPont Corporate Remediation
Group, personal communication, 1996).
This alternative uses green plants and soil amendments
to remediate contaminated soils through the biological,
chemical, and physical ‘in-place inactivation’ of heavy metals,
including Pb. This stabilization technique depends on
changing the contaminant chemistry by adding and incorporating soil amendments. The soil amendments must be
able to effectively change trace elemental chemistry while
having a neutral to positive effect on plant growth. Measuring
these changes in soil chemistry can be somewhat problematic.
Our current ability to speciate trace elements in complex
matrices such as soil is limited because they are found in
* Corresponding author e-mail address: bertiwr@a1.esvax.umc.
dupont.com; telephone: (302) 451-9224; fax: (302) 451-9138.
S0013-936X(96)00577-9 CCC: $14.00
 1997 American Chemical Society
relatively low concentrations and in a variety of physicochemical forms (1, 2).
One speciation technique, sequential chemical extractions,
separates forms of trace elements into operationally distinct
fractions, such as water-soluble, exchangeable, carbonate,
oxyhydroxide, organic, and precipitated. This is accomplished
by using progressively harsher solutions to dissolve trace
elements from the soil matrix. We have developed and are
using a sequential chemical extraction technique for our Pbcontaminated soils based on our own work (3) and that of
others (4, 5). Our method differs from that of others by using
AgNO3 in the first solution. Silver nitrate was chosen because
Ag+ has an ionic potential and Misono softness similar to
that of Pb (6). Silver ions should therefore be effective for
exchanging with Pb2+ that occurs on surfaces as inner-sphere
complexes. Additionally, because Ag+ is highly reactive with
anions, the formation of Ag precipitates should reduce the
redistribution of Pb to solid forms in this fraction.
Sequential chemical extractions should help provide an
indication of the hazard the contaminated soil poses to the
environment due to similarities between extraction solutions
and environmental conditions. The inherent hazard posed
by Pb contained in a soil matrix is a function of its relative
mobility and bioavailability. Not all soil Pb is equally
bioavailable or mobile. It forms associations with numerous
organic and inorganic phases in soils. The nature of its
chemical distribution within the soil, and thus the inherent
hazard posed by the matrix, is often a function of soil pH,
mineralogy, texture, organic matter content, source and
quantity of Pb in the soil, and time. Several of these factors
can be modified by soil amendments. Sequential chemical
extraction techniques can be used to measure these modifications. They fill a need few other techniques currently
offer when applied to the examination of soil-chemical
changes that occur on the same or similar soils to which
different treatments have been imposed (7).
The human health and environmental hazard posed by
Pb-contaminated soils is primarily through direct ingestion
of the contaminated soil. The hazard to children by direct
soil ingestion is generally acknowledged as the hazard of
greatest concern. Ruby et al. (8, 9) have developed a
physiologically based extraction test (PBET) that determines
soil Pb bioaccessibility, a chemically surrogate measurement
for soil Pb bioavailability in animals as a result of soil ingestion.
This surrogate measurement for soil Pb bioavailability fills a
need in research and development of remediation technologies for a test that is an appropriate measure of the soil Pb
hazard and that is quick and inexpensive as compared to the
animal models that are normally used to assess bioavailability.
Bioaccessibility is defined as the solubility of soil Pb in
simulated stomach and intestinal solutions of the PBET
relative to the total Pb in the soil. Soil Pb bioaccessibility
determined in both the simulated stomach and intestinal
phases of the PBET was shown to be well correlated with soil
Pb bioavailability using a Sprague-Dawley rat model (9). The
simulated stomach phase of the PBET, however, was more
highly correlated to the rat model than the intestinal phase
of the PBET. Our own research and that conducted at
DuPont’s Haskell Laboratory indicated that soil Pb bioaccessibility determined in the simulated stomach phase of
the PBET correlated well with the results of absolute soil Pb
bioavailability using microswine as the animal model (unpublished results).
This paper builds on our earlier work that grew out of
phytoremediation of Pb-contaminated soils (3), further
examining the affect of materials such as phosphate fertilizer
and iron oxyhydroxides to change soil Pb chemistry and
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reduce the hazard of soil Pb, as measured by sequential
chemical extractions, the toxicity characteristic leaching
procedure, and a physiologically based extraction test.
Experimental Section
Soils were collected from the surface to 60 cm of three former
industrial areas, screened through a 9.5-mm sieve, and
homogenized. Total soil Pb (using a nitric acid/perchloric
acid open vessel microwave digestion technique), Pb leachability [modified from the toxicity characteristic leaching
procedure, TCLP (10)], and percent gravel (particles > 2 mm)
by sieving were performed on air-dry samples. Soil tests [e.g.,
soil pH, extractable P, Ca, Mg, K; organic matter; and texture
(11)] and sequential chemical extractions were performed
on air-dry material passing a 2-mm sieve. The PBET [modified
from Ruby et al. (9)] was performed on air-dried soil material
smaller than 250 µm.
We tested a wide range of soil amendments including KH2PO4, agricultural limestone, gypsum, sulfur, iron oxyhydroxides [i.e., iron-rich material (IRM), a mineral byproduct formed
in the production of TiO2, previously found to be suitable as
a component of soil-less media or manufactured topsoil (12)],
and various sources of organic carbon, including ground
alfalfa, sphagnum peat moss, biosolids from industrial and
mining processes [e.g., humate from TiO2 mining, which has
also been tested as a soil amendment in plant nutrition and
growth (Mollerup, D. A., DuPont White Pigments and Mineral
Products, personal communications, 1994)], and composted
leaves (i.e., natural humus from Twin Oaks Mulch, Quarryville,
PA). Of all amendments tested, only amendments whose
addition resulted in significantly reduced leachable Pb (as
measured by the TCLP) at rates that are compatible with
plant growth are included in this report. A standard
engineering stabilization method (portland cement) was also
used at rates high enough to reduce the TCLP Pb values of
the resulting soil materials to below the regulatory limit of 5
mg Pb L-1.
Applications were on a dry-weight basis. Soil amendments
were mixed by hand into 100 g of soil, the mixture was placed
into 500-mL glass jars, and water was added to attain a
moisture content of about 30%, which is in a range considered
optimum for microbial activity (13). The jars with soil were
covered with laboratory film (i.e., Parafilm “M”) to allow gas
exchange and to minimize moisture loss. The samples were
placed in a dark incubation chamber at about 30 ( 3 °C.
Samples were taken after mixing as well as several times over
the course of the study, up to 30 weeks. The samples were
air-dried immediately after taking.
Sequential chemical extractions were performed on 1-g
samples using 40 g of the following six solutions and
conditions: (1) 0.1 M Ca(NO3)2 + 0.05 M AgNO3 for 16 h
(exchangeable), (2) 1 M NaCHCOO at pH 5 for 5 h (carbonates), (3) 0.1 M NH2OH‚HCl + 0.1 M HNO3 for 30 min
(manganese oxides), (4) 0.1 M Na4P2O7 for 24 h (organic), (5)
0.4 M NH2OH‚HCl in 25% v/v CH3COOH and mix periodically
for 6 h in a boiling water bath (iron oxides), and (6)
concentrated HNO3/HClO4 using the method for total (residual). Solutions were centrifuged at about 4000g for 20
min and decanted. Soils were washed with 40 g of 0.025 M
Ca(NO3)2 for 5 min after extracting fractions 1-5. Washings
were discarded.
A Pb leach test was scaled down from the standard TCLP
procedure by using 10.0 g of soil and 200 mL of an extraction
fluid, which was 0.0992 M CH3COOH and 0.0643 M NaOH at
a pH of 4.9. The mixture was shaken end-over-end at 30 rpm
for 18 h and then vacuum filtered through a glass fiber filter
with a nominal pore size of 0.7 µm. The pH of the filtrate was
measured and the solution preserved until analysis by
acidifying with HNO3 to pH <2.
A simulated soil ingestion procedure was essentially the
same as the PBET procedure of Ruby et al. (9). A 0.800-g soil
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TABLE 1. Characterization of Untreated Soils
soil parametera
kg-1
total Pb, mg
soil pH
buffer pH
Pb
Kb
Mgb
Cab
organic matter, %
gravel, %
texture of <2-mm
fraction
soil 1
soil 2
soil 3
1200
8.0
8.00
15
68
150+
150+
0.3
>50
sandy loam
2500
4.6
6.90
16
23
23
14
3.0
<20
sandy loam
3500
6.0
7.69
12
189
150+
150+
6.6 ( 0.4
<5
loamy sand
a Total Pb determined in HNO /HClO acid microwave digest. Soil
3
4
pH measured in 1:1 soil/water slurry; buffer pH using Adams-Evans
technique; P, K, Mg, and Ca using Mehlick 1; organic matter using
Walkley-Black soluble salts by measuring electrical conductivity of 1:1
soil/water extract; particle size by hydrometer method (9); and gravel
by sieving. b Plant fertility index value where 0-10 is very low, 11-25
is low, 26-50 is medium, 51-100 is high, and 101-150+ is excessive
(23).
sample and 80 mL of solution were mixed in a 100-mL glass
spinner flask with a stir bar. The flask had an external water
jacket connected to a circulating water bath and kept at a
constant temperature of 37 °C. Two-milliliter samples were
taken at 30 and 60 min of the simulated stomach phase of
the test controlled at a pH of 2.5 ( 0.1. The simulated
intestinal phase of the test, controlled at pH 7.0 ( 0.1, was
sampled at 60 and 120 min. Sampling was done without
replacing the sample solution with an equal-size aliquot. The
final volume in the flask was determined at the end of the
test. Pb bioaccessibility was determined by dividing the Pb
concentration measured in the supernatant by the total Pb
determined separately for each sample.
Lead was determined in all solutions using inductively
coupled plasma-atomic emission spectrometry (ICP-AES).
Standard soil reference materials from the U.S. Department
of Commerce National Institute of Science and Technology
were used to assess accuracy and precision of methods.
Results and Discussion
Characterization data of the three Pb-contaminated soils that
were used for laboratory experiments are presented in Table
1. These soils represent a range of total Pb contamination
that is higher than the current limit for residential land use
in the United States [about 400 mg of Pb (kg-1 of soil)-1]. The
Pb in soil 1 resulted from manufacturing the antiknock
gasoline additives tetraethyl lead and tetramethyl lead (i.e.,
TEL and TML) from about 1920 to 1989. Analytical testing,
however, indicated that the sample did not contain TEL, TML,
or their known intermediate breakdown products (data not
presented). Soil 2 was a blasting cap destruction area. The
source of Pb in this soil is most likely from lead azide [Pb(N3)2] used in the blasting caps. The Pb in soil 3 came from
the manufacturing of explosives. Lead azide was probably
the main source of Pb in this sample also.
The soils in this study represent not only a range of Pb
concentrations but also a range of chemical and physical
characteristics (Table 1). The soils vary in pH, texture, and
plant macronutrients. They also have low to moderately high
organic matter content. Soils 1 and 2 are poor plant growth
media. Soil 1 is a poor material for plant growth because of
its alkaline pH, high gravel, low organic matter content, low
plant-available P, and excessive levels of plant-available Mg
and Ca. Soil 2 is poor because of its acid pH and low levels
of plant-available nutrients. Both of these soils require plant
fertilizers and other soil amendments to quickly establish a
robust plant cover. Soil 3 is more easily amended for plant
growth. It has a near-neutral pH and moderate concentrations of most plant nutrients. Of the three soils in this study,
FIGURE 1. Sequential chemical extraction of Pb from untreated
soils 1-3. Fraction number: 1, exchangeable; 2, carbonates; 3,
manganese oxides: 4, organic; 5, iron oxides; 6, residual.
only this one had abundant plant growth in its native state.
It also contained the highest concentration of total Pb (Table
1).
The sequential chemical extractions (Figures 1 and 2)
resulted in a “fingerprint” of the soil Pb, which we believe
gives information on its potential for leaching, plant uptake,
and mammalian bioavailability through soil ingestion. Soils
in which the Pb resides mostly in the first fractions are
potentially of greatest hazard when compared to soils in which
the Pb resides primarily in later fractions, especially the last.
Lead in the first two fractions is extractable by relatively mild
solutions and would be most available to the effects of water
leaching through the soil, plant uptake into roots, and
bioavailability through soil ingestion. Lead resistant to
extraction until the last two fractions, which include boiling
nitric-perchloric acids, would be available only under the
harshest chemical conditions rarely found in the environment
or only over geological time-frames.
The sequential chemical extractions of the three soils
suggests that Pb resides in different chemical forms (Figure
1). In all three untreated soils, the majority of the Pb was
extracted in the first two fractions. In soil 1, more than 50%
of the Pb, however, resided in the second fraction alone. Threequarters of the Pb in soil 2 was extracted from the first two
fractions. In soil 3, almost 70% of the total Pb resided in
fractions 1 and 2. The relative availability of the Pb in these
samples, as determined by the sequential chemical extractions, would suggest that these soils present a potential hazard
to human health and the environment through direct soil
ingestion and leaching to groundwater.
Lead in soil 1, with a total concentration of 1200 mg of Pb
kg-1, was primarily in fractions 2, 5, and 6. We would not
expect the Pb in this soil to be readily available for plant
uptake. Lead forms in this soil as well as in the other two,
however, will be easily extractable in an acid solution, such
as that used in the TCLP. Also, the sequential chemical
extraction results suggest that forms of Pb in all three soils
are available for dissolution in the acid portions of the GI
tract of mammals, which may result in Pb poisoning.
It may be possible to significantly reduce the hazard
presented by the soils by in-place inactivation. In-place
inactivation builds on the excellent work of many previous
research groups, who have demonstrated that soil amendments can change soil Pb chemistry, reducing the soil Pb
hazard (14-18). Application of these amendments should
cause the Pb to shift from forms with high relative availability
(fractions 1 and 2) to those with low or no relative availability
(fractions 3-6). Amendments are chosen to be compatible
with plant growth and microbial activity so that soil productivity is maintained or increased. Also, amendments
should be locally available, relatively low in cost, easy to apply
and incorporate, benign to the people using them, and not
cause further environmental degradation (e.g., nitrates in
biosolids at rates excessive for plant N requirements). To
this end, we selected eight materials and incorporated them
at varying rates in the three soils. These materials were chosen
because of their known positive effects on plant growth at
certain application rates. They were also chosen for their
potential abilities to inactivate soil Pb in forms not available
to plant growth, soil ingestion, and leaching and to reduce
the characteristic hazard of a material to acceptable levels.
The sequential chemical extraction was significantly
altered as a result of adding materials, especially IRM and P
(Figure 2). Lead extracted in the first fraction was reduced,
and a greater percentage was extracted in subsequent
fractions. Others (14, 18) have shown that the addition of P
to soil containing Pb results in the formation of lead phosphate
with low water solubility. The iron oxides in the IRM probably
affect soil Pb chemistry through surface adsorption (19) and
solubility mechanisms. Humate and natural humus applications seemed to have less overall effect on changing the
results of the sequential chemical extraction when compared
with IRM and P. Sequential chemical extractions of samples
amended with humate and natural humus showed reduced
Pb extracted in the first fraction and increased Pb in
subsequent fractions. Organic materials, such as humate and
natural humus, most likely worked in ways similar to that of
IRM through surface adsorption (19) and solubility mechanisms.
Certain soil amendments at relatively low rates, such as
KH2PO4, dramatically reduced the leachable Pb (Figure 3A).
Phosphate added as low as 0.1% P was able to reduce leachable
Pb to below 5 mg of Pb L-1 (which is the regulatory limit of
the TCLP) in soils 1 and 3. Potassium phosphate added at
0.5% P reduced leachable Pb in all three soils to less than 5.0
mg of Pb L-1. IRM applications of at least 6% were needed
to reduce the leachable Pb to below 5 mg of Pb L-1 in any of
the soils (Figure 3B). In two soils, IRM alone at rates up to
10% (the highest level in the study) significantly reduced
leachable Pb as compared to the untreated soils. IRM at 10%
was not sufficient, however, to reduce the leachable Pb below
5 mg of Pb L-1 for soils 2 and 3. Other materials, such as
CaSO4, CaCO3, and biosolids, when applied at rates as high
as 10% also were not as effective as P for reducing leachable
Pb (see Figure 3C for natural humus; other data not presented).
Furthermore, some of these materials at high levels are
probably incompatible with optimal plant growth.
Portland cement is often used to immobilize metals in a
solid matrix, such as soils, prior to landfilling. For the three
soils, empirically derived additions of 11.6%, 11.6%, and 12.2%
portland cement resulted in leachable Pb concentrations of
0.6, 1.6, and 1.0 mg of Pb L-1, respectively. Portland cement,
however, significantly increased the pH of the leachate
solution from 4.9 to 6.9 or higher. The other amendments
shown (i.e., IRM, KH2PO4, and natural humus) had little effect
on the pH of the leaching solution (data not shown). Portland
cement probably resulted in the production of lead carbonates
(Figure 2A-C), which are not highly soluble in a leachate
with a pH of 6.9 or higher.
The results of the PBET showed that treating the soil with
IRM, P, and portland cement may be effective in reducing Pb
bioaccessibility in these soils when compared to the untreated
soils (Figure 4). This can be seen for soil 1 in which Pb
bioaccessibility was reduced in the stomach phase (pH 2.5)
from about 40% [standard error of the mean (SE) ) 3%, n )
6] of the total for the untreated soil when averaged over both
time points to about 11% (SE ) 1%, n ) 6) of total after
treatment with 10% IRM. Phosphorus (added at the rate of
0.5% P as KH2PO4) was also highly effective in reducing Pb
bioaccessibility in the stomach phase of this soil to about
20% (SE ) 2%, n ) 6) of total.
The IRM and portland cement soil treatments were also
effective in reducing Pb bioaccessibility in the simulated
intestinal phase of soil 1 (Figure 4A). Lead absorption into
the blood of animals occurs primarily in the small intestines
(20), the chemistry of which we attempted to simulate in the
intestinal phase of the PBET. As shown by Ruby et al. (9), the
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B
A
C
FIGURE 2. Sequential chemical extraction of Pb after treating (A) soil 1, (B) soil 2, and (C) soil 3 with portland cement; 2, 4, 8 and 10%
IRM; 0.01, 0.05, and 0.5% P; 4 and 10% humate; and 4 or 6 and 10% natural humus. Fraction number: 1, exchangeable; 2, carbonates; 3,
manganese oxides; 4, organic; 5, iron oxides; 6, residual (see Figure 1 for legend).
bioaccessibility of Pb in the simulated stomach phase,
however, correlated better than Pb bioaccessibility in the
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simulated intestinal phase with a Sprague-Dawley rat model.
Also, because Pb bioaccessibility is usually higher in the acid
FIGURE 3. Leachable Pb from soil 1 (9), soil 2 (b), and soil 3 (2)
untreated and treated with (A) P (as KH2PO4), (B) IRM, and (C) natural
humus; error bars indicate standard error of the mean, n g 2.
solution of the simulated stomach phase, it is a more
conservative estimate of the Pb bioavailability potential than
the simulated intestinal phase.
The P and IRM treatments were also effective for reducing
Pb bioaccessibility in soil 2 (Figure 4B). The IRM and P
treatments had the lowest overall Pb bioaccessibility in the
simulated stomach solution, although the IRM treatment was
initially the highest. Furthermore, Pb bioaccessibility in the
soil treated with IRM or P was significantly lower than
untreated or portland cement-treated soil in the simulated
intestinal phase.
The effect of soil amendments on Pb bioaccessibility was
highly variable in soil 3, which had the highest total Pb
concentration of the three soils (Figure 4C). Pb bioaccessibility in the simulated stomach phase of soil 3 was highest
in the 10% IRM treatment and lowest in 0.5% phosphorus
FIGURE 4. Pb bioaccessibility of (A) soil 1, (B) soil 2, and (C) soil
3 untreated (9) and treated with 10% IRM (b), 0.5% P (as KH2PO4)
(2), and 11.6% (soils 1 and 2) or 12.2% (soil 3) portland cement (();
error bars indicate standard error of the mean, n ) 3. Samples were
collected at 0.5 and 1 h after initiation of the stomach phase at pH
2.5. The pH of the stomach phase solution was subsequently raised
to pH 7 and sampled at 1 and 3 h after initiation of the intestinal
phase.
treatment. The P treatment was significantly lower (p < 0.05)
as compared to untreated soil and the other two treatments.
For the simulated intestinal phase of this soil, the Pb
bioaccessibility of the portland cement treatment was significantly greater than the other treatments shown. This result
suggests that, in the short term, these stabilization techniques
will most likely be empirically derived. Additional work,
however, is necessary to help predict a mix of amendments
that are needed to reach an acceptable Pb bioavailable or
bioaccessible end point for all soils.
Remediation criteria for Pb in soils are set primarily to
protect children, who may incidentally ingest small quantities
of soil. It is thought that exposure to Pb can increase the risk
of irreversible neurobehavioral damage at concentrations as
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low as 10 µg of Pb dL-1 of blood (21). The integrated exposure
uptake biokinetic model for lead in children (IEUBK) was
designed to estimate Pb uptake from Pb in soil and dust that
is ingested or inhaled (22). The default value for the
bioavailability of Pb in soil that is ingested is 30% of the total
Pb, meaning that 30% of the total Pb in the ingested soil
enters the blood. In this study, lead bioaccessibility varied
from 6 to 42% of the total in the three untreated soils used.
This variability in bioaccessbility indicates the variability of
soil Pb bioavailability in animal models (8) and that Pb
bioavailability may differ significantly from 30%. The default
value of 30% may underestimate or overestimate the risk
posed by a Pb contaminated soil. More importantly, the data
indicate that if the PBET can be used to help generate sitespecific data on Pb bioavailability, it may also be useful in
helping to determine site-specific soil treatment strategies
that reduce the risk of Pb poisoning due to soil ingestion.
These strategies may include cost-effective soil treatments
that can help lower the bioavailability of Pb in soil by changing
the solubility of Pb in the GI tract.
Reducing metal availability and maximizing plant growth
through in-place inactivation may prove to be an effective
method of in situ soil Pb remediation on industrial, urban,
and mining sites. In addition, these stabilization techniques
can occur as part of a treatment train with other phytoremediation methods now under development, the most
intriguing of which may be “biomining” the available fraction
of metal pollutants with plants.
Acknowledgments
The authors wish to recognize Robert Cox, Steve Germani,
Marty Holmes, Stacey Pepe, and Erin O’Reilly for their
technical support. We also wish to acknowledge Rufus
Chaney, Jim Ryan, and Mike Ruby for their invaluable
discussions and insights.
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(v0.99d); U.S. Environmental Protection Agency, U.S. EPA Office
of Emergency and Remedial Response: Research Triangle Park,
NC, 1994; EPA-540/R-94/040.
(23) Sims, J. T. Soil test notes; University of Delaware: Newark, 1990.
Received for review July 3, 1996. Revised manuscript received
December 10, 1996. Accepted January 8, 1997.X
ES960577+
X
Abstract published in Advance ACS Abstracts, March 15, 1997.