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Session 3: Arsenic in Soil:
Occurrence, Geochemistry and Remediation
Arsenic Distribution in Massachusetts Orchard Soils
Peter Veneman,
University of Massachusetts, Amherst, MA
Arsenic
Background and Cleanup Goals in Soil: Should They be
Related?
Terri Bowers,
Gradient Corp., Cambridge, MA
Vertical Migration of Arsenic in Soil after Arsenical
Pesticide Application
David J. Folkes,
Enviro Group, Denver, CO
Approaches to
Characterizing Solid Phase Arsenic Speciation in Soils
Robert Ford; U.S.
EPA NRMRL
Techniques to Evaluate the Mobility and Treatment of
Arsenic in Soils
Roger L. Olsen, Camp
Dresser & McKee, Inc., Denver, CO
Use of Chemical Amendments to Stabilize Arsenic in
Soils
Todd Martin,
Exponent, Boulder, CO
Phytoremediation of Arsenic in Soils
Michael Blaylock,
Edenspace Systems Corporation, Reston, VA
Arsenic
Background and Cleanup Goals in Soil: Should They Be
Related?
Teresa S. Bowers, Gradient Corporation
By now, almost everyone is aware that standard risk
assessment approaches suggest that safe levels of arsenic
in both soil and water fall within the range of natural
background in many areas of the US. This observation has
led to at least two outcomes: one is to protest that the
risk assessment procedures are overly conservative, and
the other is to look for evidence that background levels
are lower than currently reported. These are not
unexpected responses to the notion that our natural
environment could be affecting our health. An alternative
response is to take a closer look at how cleanup goals
should be applied. Here we examine background soil arsenic
levels and arsenic cleanup goals established for sites in
South Carolina and New York. These sites have large
background data sets with considerable variability, and
include samples representing both natural and
anthropogenic arsenic sources. The cleanup goals, derived
considering both cancer and noncancer endpoints, fall
within the range of background. However, individual
sampling locations with soil arsenic levels above a
cleanup goal based on chronic health endpoints do not
necessarily represent a health threat as they are not
representative of average long-term exposure. As an
additional constraint, soil arsenic levels at individual
locations can be compared to soil criteria based on acute
health endpoints, to ensure that an individual location
does not pose an acute risk. This approach, together with
consideration of other background exposures to arsenic,
can lead to an informed decision about the level of
cleanup needed for arsenic in soils that does not result
in excessive cleanup of areas representing natural
background conditions. The potential application of these
concepts is widespread. For example, current risk
assessment procedures yield a noncancer preliminary
remediation goal for arsenic of 23 mg/kg, while national
statistics on non-anthropogenically affected soil arsenic
levels suggest that one to two million households
nationwide may have soil arsenic levels in their yards
above this level.
Vertical
Migration of Arsenic in Soil after Arsenical Pesticide
Application
David J. Folkes, EnviroGroup Limited and Robert A.
Litle, ASARCO Incorporated
Extensive investigations in an older neighborhood in
Denver, Colorado indicate that high concentrations of
arsenic due to arsenical pesticide applications in the
1950’s and 1960’s still persist in the surface soils
of residential lawns, despite over 30 years of exposure to
rainfall, irrigation, and lawn care. SESOIL modeling using
site-specific partition coefficients and meteorological
data, assuming 35 years of leaching, predicted arsenic
concentration profiles that matched the observed profile
at a typical property, supporting the pesticide source
theory and demonstrating the usefulness of relatively
simple migration modeling for dating and confirming
sources in forensic studies. A comparison of lead and
arsenic ratios (both components of the pesticide) at a
large number of properties shows that the degree of
leaching varies from property to property, but falls
within predicted ranges. Further, ratio
"vectors" provide a useful tool for mapping the
leaching history of metals in soil and distinguishing
various sources of contamination from background levels.
Approaches
to Characterizing Solid Phase Arsenic Speciation in Soils
Robert G. Ford, U. S.
Environmental Protection Agency, Yuji Arai and Donald L.
Sparks, University of Delaware
The partitioning of arsenic to soil solids is an important
process controlling the stabilization of arsenic wastes
and mobility of arsenic in the environment. Identification
of the physicochemical characteristics of the partitioning
mechanism(s) is important for treatment optimization and
assessment of the stability of arsenic-bearing solids.
Arsenic may reside in the solid phase as a discrete
precipitate, a minor constituent coprecipitated within the
structure of a separate phase, or as a sorbed ion bound to
surface sites of a separate phase. Each of these
partitioning mechanisms possess a unique stability and,
thus, solid phase speciation may be required as part of
site assessment. The implementation of an analytical
approach to identify arsenic speciation in a soil sample
is a challenging process. The accuracy of the analytical
finding is dependent on the method of sample
collection/preservation and the tools used to identify the
mechanism of arsenic partitioning. The analytical protocol
must be designed to address the redox sensitivity of
arsenic and the solid phase(s) to which arsenic may be
partitioned. Tools to evaluate the mechanism of arsenic
solid phase partitioning range in complexity from
relatively simple chemical extractions to advanced
spectroscopic techniques. Potential strengths and pitfalls
of these techniques will be reviewed, and due
consideration will be given to approaches that can be
practically applied by environmental practitioners.
Notice: The U.S. Environmental Protection Agency through
its Office of Research and Development funded and managed
the research described here through in-house efforts. It
has not been subjected to Agency review and therefore does
not necessarily reflect the views of the Agency, and no
official endorsement should be inferred.
Techniques
to Evaluate the Mobility and Treatment of Arsenic in Soils
Roger L. Olsen, Kent S.
Whiting1 and Richard W. Chappell1,
Camp Dresser and McKee Inc.
A variety of methods are
available to evaluate the mobility and fate of arsenic in
soils and the effectiveness of treatment of arsenic
contaminated soils. The forms and species of arsenic in
both the solid phase (soil or treated soil) and the
aqueous phase (leachate generated through an extraction
test) must be adequately characterized. Evaluation of
solid phases: The chemical compositions or forms of
arsenic in solid materials are typically evaluated using
indirect methods such as sequential leaching procedures or
elemental analyses. As opposed to these indirect methods,
electronmicroprobe (EM) techniques can determine the form,
size, associations and morphology of individual particles
of arsenic-containing materials. The following examples of
arsenic in soils will be provided: coprecipitates with
iron oxyhydroxides, adsorbed to iron containing minerals,
minerals (eg, scorodite) and discrete secondary compounds
(eg, arsenic trioxide). Evaluation of soil/water
interactions: When evaluating the mobility and treatment
of arsenic in soils, the leachability of arsenic and
potential impact on groundwater or surface water is a
critical determination. These evaluations can be performed
using adsorption and desorption column tests and batch
tests. Standardized batch tests include TCLP and SPLP,
which typically do not accurately represent field
conditions. Comparison of isotherms and leachate
concentrations from column and batch tests will be
provided. Analysis of aqueous phases: Once the arsenic is
in the aqueous phase (by means of extraction or direct
collection of groundwater/surface water), various
analytical techniques exist to determine the species of
arsenic present. Example methods to be discussed include
separation followed by hydride generation AAS (SW-846
Method 1632), ion chromatography, separation using anion
exchange columns/filters and liquid chromatography
followed by AAS.
Use
of Chemical Amendments to Stabilize Arsenic in Soils
Todd A. Martin and Michael
V. Ruby1, Exponent, Steve Hilts, Cominco, Ltd.
The use of chemical
amendments to stabilize arsenic in soils has been the
topic of bench- and field-scale studies at a number of
sites. These studies have relied primarily on iron- and
manganese-based amendments to attenuate arsenic through
adsorption and co-precipitation of arsenic. In general,
this remedial approach has been used to reduce the
leachability of arsenic, and hence to provide protection
of surface and groundwater, as well as to reduce the
bioavailability of arsenic to human receptors. This
presentation will discuss the use of chemical amendments
to stabilize arsenic in soils, as illustrated by a number
of bench- and field-scale studies. Bench-scale studies
have demonstrated that both iron- and manganese-based
amendments can be highly effective in reducing the
leachability and mobility of arsenic in soils. At
historical smelter sites, it is uncommon to find arsenic
alone in soils, and thus, soil remedial approaches may
have to deal with other metals in addition to arsenic. At
the lead smelter in Trail, B.C., soils have been affected
by both lead and arsenic. The large areal extent of
contamination and the desire to preserve existing land
uses makes remediation of the soils via conventional
practices, such as excavation and disposal or in situ
stabilization, impracticable. As a result, the use of in
situ chemical amendments was evaluated as an
alternative that would reduce the leachability and
bioavailability of both arsenic and lead from soils.
Initial bench-scale work demonstrated that addition of
phosphate-based amendments, which are used to stabilize
lead in soil, served to increase the leachability of
arsenic from soil. Based on these findings, a mixed
phosphate-iron amendment was selected for evaluation.
Results indicated that iron addition at rates of 0.5–5%
(w/w, iron as hydrous ferric oxide) resulted in a
reduction in both the leachability and bioaccessibility of
arsenic relative to the unamended soils.
Phytoremediation
of Arsenic in Soil
Michael J. Blaylock,
Jianwei W. Huang and Mark P. Elless, Edenspace Systems
Corporation
The discovery of metal
hyperaccumulating properties in select plants has led to
their application in removing heavy metals from
contaminated soil. This process, termed phytoextraction,
can be used to remove heavy metals from soil, allowing
preservation of topsoil and eliminating costly excavation
and disposal.
Current phytoextraction
efforts are developing novel approaches for removing
contaminants from the environment. Through screening and
selection, several metal-accumulating lines of crop plants
have been identified. These plants, in combination with
soil and foliar amendments that enhance metal uptake, have
been successful in remediating metal contaminated soils.
Recent developments have expanded phytoextraction
applications to arsenic contaminated soils as well as the
development of techniques suitable for treatment of lead
contaminated soils in residential areas.
The Brake fern (Pteris
vittata) has been shown to accumulate and tolerate high
concentrations of arsenic in its foliage when grown on
arsenic contaminated substrates. Initial studies have
shown the ability to reduce arsenic levels in water to
less than 10 µg/L (ppb) through the use of the Brake
fern.
Remediation of residential
soils with lead contamination is often difficult to
achieve in a cost effective manner. Phytoremediation
practices designed for large contiguous areas are often
not suitable for a residential setting. Several turfgrass
varieties have been identified that have the ability to
accumulate lead from contaminated soils and may be useful
in developing a low-maintenance remediation program for
homeowners and residents with lead contaminated soil.
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