PB95-963135
EPA/ESD/R07-92/079
April 1995
EPA Superfund
Explanation of Significant Difference
for the Record of Decision:
E.L du Pont de Nemours,
County Road X23, Lee County, IA
12/27/1991
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Explanation of Significant Differences
z.
Zn~roduc~ion
On May 28, 1991, a Record of Decision (ROD) was issued for
the E.I. du Pont de Nemours County Road X23 Site in Lee County,
Iowa.
The ROD .presented the remedy selected by the u.S.
Environmental Protection Aqency (EPA) for this site, which call.ed
for stabilization/so~idification of contaminated soil at each of
the two subsites.
As a result of information which has been
developed since the issuance of the ROD, EPA has determined that
.siqnificant chanqes to the remedy are necessary.
This
Explanation of Significant Differences (ESD) describes and
summarizes the basis for these chanqes.
The EPA serves as the lead aqency for site activities, with
support from the Iowa Department of Natural Resources (IDNR).
This ESD is issued in accordance with the requirements of the
Comprehensive Environmental Response, Compensation, and Liability
Act of 1980 (CERCLA), as amended by the Superfund Amendments and
Reauthorization Act of 1986 (SARA).
CERCLA provides that after
the adoption of a final remedial action plari, if any settlement
or consent decree under section 106 and section 122 is entered
into and differs in any significant respects from such final
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plan, the lead agency shall publish an explanation of the
siqnificant differences and the reason such changes were made.
This ESD will explain these changes and the reason for them.
In accordance with the National contingency Plan, 40 C.F.R. ~
300.435 (c) (2) (i) (A), this ESD and the information supporting it
are part of the administrative record file and are available for
public coment.
II.
site Historv and contamination Problems
The County Road X23 site is in rural Lee county, Iowa,
approximately 3.5 miles south from West Point, Iowa.
The site
consists of two subsites, the McCarl subsite and the Baier
subsite.
The McCarl subsite is located in the South East quarter
of the South West quarter of section 22, Township 68 North, Range
5 West.
The Baier subsite is located in the North East quarter
of the South West quarter of section 28, Township 68 North, Range
5 West.
The two subsites are located approximately three-fourths
of a mile apart.
The land adjacent to the sites is used for
agricultural or residential purposes.
Other adjacent land is
undeveloped and wooded.
The nearest residences are approximately
500 feet from the McCarl subsite.
During the period of 1949 to 1953, a contractor hired by
DuPont deposited paint wastes at both the Baier and McCarl
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subsites.
The Baier subsite was the primary disposal subsite,
while the McCarl subsite was used in inclement weather when the
Baier subsite was inaccessible.
An estimated 48,000 to 72,000
55-gallon (equivalent volume) drums of waste were disposed at the
two subsites.
The paint waste was placed into trenches and
reportedly burned, and an estimated 4,000 to 7,000 tons of ash
may still remain on the subsites.
The Baier subsite was listed as a potentially uncontrolled
hazardous waste site in the Eckhardt Subcommittee Report in
November 1979.
A hazard ranking system (HRS) package for the site
. .
documented a score of 46.01.
The site was proposed for the
National Priorities List' (NPL) in the June 1988, 7th update and
became final on the NPL in August of 1990.
The selected response action of stabilization/solidification
addressed soil contamination as the principal threat to human
heal. th and the environment.
Based on investigations of the site
during 1989 and conclusions found in the Remedial Investiqation
(RI), the source of contamination at the site was identified as
paint waste disposed at the site by DuPont.
Of particular
concern was the effect of heavy metal contaminants in soil if the
site were to be used for residential or agricultural purposes.
Because the RI concluded that no completed exposure pathway
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existed for the groundwater, the "no action" alternative was
selected for the qroundwater.
This conclusion was based on the
low permeability of the qroundwater unit and the presence of a
confining unit below the water-bearing unit.
Monitoring of the
qroundwater is being continued as a precaution.
The nature and extent of contamination of qroundwater and
soil at both subsites is summarized below.
This summary is based
primarily on data generated by the work performed by DuPont in
May through Auqust of 1989 and in data discussed by the RI.
Detailed information regarding the nature and extent of
contamination can be found in the Final RI Report (January 16,
1991).
Baier Subsi'te
The approximate areal. and vertical extent of disposed waste
material was estimated by probing with a hand auger during the
May 1989 investigation.
Depths to the contact between the waste
and native soil ranged from approximately two inches below qrade
to greater than 36 inches. below qrade.
samples collected from
the waste material showed high levels of lead, cadmium, chromium,
selenium, and zinc.
Concentrations of metals contaminants were
highest in the waste material itself, with lesser, but still
elevated, concentrations limited to the upper two to three feet
below the interface between the waste and native soil.
Elevated
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concentrations of several volatile organic compounds were also
detected in the waste material.
Evidence of migration of
volatile and semi-volatile organic contamination was noted as
deep as 18 feet below the waste/native soil contact.
In general,
the vertical extent of contamination at tqe Baier subsite is much
deeper than the contamination found at the McCarl subsite.
The
RI focused on characterizing the horizontal and vertical extent
of contamination outside of the waste disposal areas.
Generally,
this investigation showed that contamination is limited to within
50 feet of the waste disposal areas.
contamination in these
areas is generally restricted to the upper four feet of soil.
Sample analyses indicated elevated concentrations of metals
(only) in several drainage ditches leading from the subsite.
contamination is limited to within 50 feet of the ditch heads.
Heavy metals contamination in the soil represents the principal
threat at the Baier subsite based on the risk to sensitive
populations, the risk posed through possible ingestion of
vegetables qrown in contaminated soil, and the risk to possible
future residents.
AnalYtical results for groundwater samples collected to date
indicates that contamination is limited to the upper-most water-
bearing zone and has not migrated to the deeper water-bearing
uni t .
Due to extremely low yield and low permeability of the
upper water-bearing unit, and the confining layer separating the
water-bearing units, it was determined that a completed exposure
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pathway for groundwater did not exist at the site for humans or
environmentally sensitive receptors.
Therefore, no action was
selected as the appropriate remedial alternative to address
groundwater contamination.
The Baier subsite is located on an upland. adjacent to
several ravines that are areas of high erosion and slope
instability.
A portion of the waste or contaminated soil at the.
subsite is on or at the top of these slopes.
XaCar1 Subsite
Visual observations made during auquring and test pit
excavations at the McCarl subsite in May 1989 showed that the
amounts of disposed paint wastes are much smaller than at the
Baier subsite.
While some paint cans, ash-like material, and
sludge castings were observed at the subsite, there did not
appear to be concentrated, siqnificant areas of waste disposal as
were observed at the Baier subsite.
50il samples collected
during pre-remedial and RI field investigations indicated
concentrations of cadmium, chromium, selenium, lead, and zinc
above naturally occurring background ranges.
Generally, elevated
concentrations are limited to the upper four feet of soil.
Sampling of drainage paths leading from the subsite indicated
that elevated concentrations of heavy metals were not migrating
off of the site in siqnificant concentrations.
Sampling also
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indicated very limited volatile and semi-volatile organic
contamination of soil at the subsite.
The volatile organic
contamination is generally limited to the upper six feet of soil.
Since the contamination is so shallow, this would allow the
material to be easily excavated.
similar to the Baier subsite,
heavy metals contamination in the soil represented the principal
threat at the McCarl subsite based on the risk to sensitive
populations, the risk posed through possible ingestion of
vegetables grown in contaminated soil, and the risk to possible
future residents.
The McCarl subsite encompasses approximately
three acres and would provide no room to mobilize eqUipment
without removing vegetation and grading adjacent areas.
The hydroqeology at the McCarl subsi te is similar to that
described for the Baier subsite.
As at the Baier subsi te,
elevated levels of contaminants do not appear to be present in
the lower water-bearing zone.
Due to the extremely low yield and
low permeability of the upper water-bearing unit and the
confining layer separating the water-bearing units, a completed
exposure pathway for groundwater also did not exist at this
subsite.
Therefore, no action was selected as the appropriate
remedial alternative to address groundwater contamination.
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. .
xxx. Selec~ed Remedv
In the ROD dated May 28, 1991, EPA selected a final remedy
for remediation of soil and groundwater contamination at .the
site.
The major components of the selected remedy include:
o
Removal of surface debris not amenable to solidification, to
be disposed of in an EPA-approved hazardous waste landfill;
o
stabilization/solidification of contaminated soil at each of
the two subsites;
o
Construction of soil covers at each subsite to prevent human
or environmental contact;
o
Introduction of vegetation to prevent erosion of the soil
cover;
o
Inspection and evaluation of the site every 5 years;
o
No action for the qroundwater;
o
Long-term qroundwater monitoring to insure that no
unacceptable conditions occur in the future.
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:IV.
Siani~icaDt Difference aDd Basis for the Difference
As previously indicated, EPA proposes to make certain
changes to the selected remedy.
The proposed changes will serve
to facilitate the long-term management of the remedial action,
provide more protection of human health and the environment as
well as improve the cost-effectiveness of the remedial action.
The changes are not fundamental in that the selected remedy and
the changes to such remedy both utilize stabilization/
solidification.
The changes are merely process related in that,
while the selected remedy provided for in situ mixing using
devices such as vertical augers the proposed changes would
require excavation of the contaminated soil, mixing in an above
ground mixer, such as a pug mill, and replacement of the
stabilized material.
The significant modifications to the
selected remedy include the following:
(1)
The excavation and removal of soil from the McCarl subsite
that is contaminated at concentrations above the action
levels specified in the ROD. . Instead of being solidified at
the McCarl subsite, the soil would be moved to the Baier
subsite for consolidation and subsequent solidification and
stabilization.
The operation of removing contaminated soil
from the McCarl subsite and transporting it to the Baier
subsite will be considered part of the surficial removal
process, which will take place prior to any construction of
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".
. the stabilization/solidification remedy.
There are many
advantages to transferring the soil to the Baier subsite.
The McCarl subsite will be completely available for
unrestricted future use.
A source for potential groundwater
degradation will be eliminated.
Long term operation and
management of the DuPont County Road X23 site will be
improved by consolidating soils at one subsite for
subsequent stabilization/solidification.
Also, reduction in
remedial costs due to lower mobilization costs associated
with the stabilization/solidification equipment will be
achieved.
Finally, excavating and removing the soil from
the McCarl subsite will have substantially less impact on
the vegetation arid land surface around the McCarl subsite
given the larger equipment necessary to implement the
in-situ stabilization/solidification, which would require
more space, is no longer required.
(2)
The use of an above ground mixing operation, such as a pug
mill, to perform the selected remedy, as opposed to in-situ
technology as prescribed by the ROD.
This type of operation
has several advantages over in-situ technology.
It would
provide better protection of human health and the
environment through more homogeneous mixing to improve the
durability of the matrix.
The mixing process produces a
more homogenous matrix, because the material is being mixed
inside a closed system as opposed to loosely around a
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vertical auger as would be done in the in-situ remedy.
Also, material from across the site is being mixed which
results in blending that will allow the stabilizing agents
to function more efficiently.
Further, this process reduces
erosion and slope instability: the stabilized material from
the mixing operation is placed into excavated areas of lower
slope than the high slope areas along the ravines adjacent
to the subsite where the contaminated material is located
currently.
The above-ground technology is also more cost-
effective in areas of shallow soil "contamination because of
less need for moving of necessary equipment.
The use of
above-ground mixers is considered more efficient than augers
for depths less than about five feet.
v.
Affirmation of tbe statutory Determinations
Considering the new information that has been developed and
the changes that have been made to the selected remedy, EPA and
IDNR believe that the remedy remains protective of human health
and the environment, complies with federal and state requirements
that are applicable or relevant and appropriate to this remedial
action, and is cost effective.
In addition, the remedy, as
revised, utilizes permanent solutions and alternative treatment
technologies to the maximum extent practicable for this site.
Information has been developed since the ROD which indicates that
the revised remedy will increase protectiveness of human health
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and environment, both short term and long term, and decrease
costs compared to the original remedy.
vz.
Role of CommUDi~y in the Process
The EPA solicits input from the community on the cleanup
methods proposed for the response action.
This ESD, along with
other documents which formed the basis for the changes in the
remedy, can be found in the administrative record file.
The EPA
encourages the public to review these documents to gain a more
comprehensive understanding of the Site and ongoing activities at
the Site..
The administrative record file is available at the
Idol Rashid Memorial Public Library.
Please submit written comments on this ESD to:
Hattie Thomas
Office of Public Affairs
Environmental Protection Agency
Region VII
726 Minnesota Avenue
Kansas City, Kansas 66101
If you have any questions or need additional information on
the Site, please contact:
u.S.
Paul Roemerman
U.S. Environmental Protection Agency
Region VII .
Waste Management Division
Superfund Branch
726 Minnesota Avenue
Kansas City, Kansas 66101
(913) 551-7694
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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
REGION VII
726 MINNESOTA AVENUE
KANSAS CITY, KANSAS 66101
XEHORANDUH
SUBJECT:
Explanation of Significant Differences for the
DuPont County Road X23 superfund Site, Lee county, Iowa
FROM:
Robert Morby
Chief, Superfund Branch
TO:
David Wagoner
Director, Waste Management Division
Whis Explanation of Significant Differences notifies the
public of the decision to implement onsite, above ground
stabilization/solidification of contaminated soil instead of
in-situ stabilization/solidification.
The major components of this remedy include consolidation of
the contaminated soil into one location, mixing of the
contaminated soil with a stabilization/solidification reagent,
placing the treated soil into a monolith, and covering the
monolith with a low permeability cover. ..
This Explanation of Significant Differences has been
coordinated with the Office of Regional Counsel, the Office of
Public Affairs, the Congressional and Intergovernmental Liaison,
the Agency for Toxic Substances and Disease Registry and the Iowa
Department of Natural Resources.
On December 27, 1991, the remedy selection authority for,
the DuPont County Road X23 site was delegated to the Regional
Administrator by Don R. Clay, Assistant Administrator. I
recommend approval.
Attachment
Agree
~rJ~lic.J,'>rs 'pili;;;., ~
(::> I"'~e~"
Disagree
RECYCLE :.\
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