PB95-963105
                              EPA/ESD/R09-91/123
                              January 1995
EPA Superfund
      Explanation of Significant Difference
      for the Record of Decision:
       Phoenix-Goodyear Airport
       (First ESD), AZ
       1/24/1991

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PHOENIX-GOODYEAR AIRPORT SUPERFUND SITE
GOODYEAR, ARIZONA
~
EXPLANATION OF SIGNIFICANT DIFFERENCES
u.S. Environmental Protection Agency
Region IX - s~n Francisco, California
January 1991
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Phoenix-Goodyear Airport Superfund site Final Remedy
EXPLANATION OF SIGNIFICANT DIFFERENCES
January 1991
I.
INTRODUCTION
On
September 26,
1989,
the United
States
. \
EnvJ.ronmental 1
Protection Agency (EPA) signed a Record of Decision (ROD) for the
final
remedy
at the Phoenix-Goodyear Airport
(PGA)
Site
in
Goodyear, Arizona.
The State of Arizona concurred with the remedy
selected in the 1989 ROD.
The purpose of this Explanation of
Significant
Differences. (ESD)
is to
explain the
significant
differences between
the final remedy originally selected in the
1989 ROD and the final remedy which will be implemented at the
Site.
These changes are not fundamental alterations of the remedy
described in the 1989 ROD.
Under Section 117 of the comprehensive Environmental Response,
Compensation,
and Liability Act of
1980,
as amendeq... by the
Superfund Amendment and Reauthorization Act of 1986 (CERCLA), and
pursuant to 40 C.F.R; Section 300.435(c) (2) (i) (55 Fed.Reg. 8666,
8852 (March 8, 1990», EPA is required to publish an Explanation
of Significant Difference when significant (but not fundamental)
changes are made to a final remedial action plan as described in
a ROD.'
This document provides a brief background of the Si te, a
'If the changes made after the ROD was signed had
fundamentally altered the nature of the selected remedy, then a ROD
amendment would have been -prepared. 40 C.F.R. Section
300.435(c) (2) (ii) (1990).
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summary of the remedy selected in the ROD, a description of how the
changes affect the remedy originally selected by EPA in the 1989
ROD, and an explanation of why EPA is making these changes to the
ROD.
, EPA is issuing this, ESD' in order to take into account
\
information received after the ROD was signed in September 1989, \
and to clarify some ambiguities in the selected remedy.
This ESD:
(1)
revises the cleanup level for methyl ethyl ketone (MER)
in groundwater from 170 parts per billion (ppb) to 350 ppbi
..
(2)
sets the cleanup level for acetone in groundwater at 700
ppbi
(3)
clarifies the target area for the soil remedy in the
northern portion of the Site and the criteria for establishing the
cleanup levels;
(4)
clarifies the role of soil excavation as a remedy option
should'the selected soil remedy, soil vapor extraction, at the
northern portion of the site prove ineffectivei and
(5)
revises the selected remedy for an off-site agricultural
well referred to as the "Phillips Well" from well-head treatment
to routine water quality monitoring.
The ESD and supporting documentation will become part of the
PGA Administrative Record.
Copies of the Administrative Record
have been placed at the following location:
.'
City of Avondale Public
328 West Western Avenue
Avondale, Arizona 85323
(602) 932-9415
Library
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EPA provided a fifteen (15) day comment period for the State
of Arizona in accordance with 40 C.F.R. Section 300.S1S(h}(3}.
State of Arizona comments are summarized in this ESD and will be
included in the PGA Admninistrative Record file. Pursuant to 40'
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C.F.R. Section 300.43S(c}(2} (i), a public comment period is not,
,
required fo~ an ESD.
II.
BACKGROUND
The following provides a brief backqround of the PGA site and
a short summary of the remedy selected in the ROD.
Additional
background information can be found in the September 26, 1989 ROD
and in the PGA Administrative Record.
A.
Site Backqround and Description
The PGA site is. located primarily in Goodyear,
Arizona,
approximately seventeen (17) miles west of Phoenix in the western
part of the Salt River Valley (See Attachment 1).
A qroundwater
flow divides the Site along Yuma Road into northern and southern
study areas.
In 1981, the Arizona Department of Health Services
(ADHS) discovered that groundwater in the area of the Site was
contaminated with solvents and chromium.
EPA and ADHS conducted
additional sampling of wells in 1982 and 1983 which revealed 18
wells contaminated with trichloroethylene (TCE).
As a result, EPA
added the PGA Site to the National Priorities List in september
1983, originally listed as the "Phoenix-Litchfield Airport Area
Superfund Site".
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Other hazardous substances found at the PGA Site
include methyl ethyl ketone (HEX), 1,1,l-trichloroethane (TCA),
acetone, and other volatile organic compounds (VOCs).
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Most of the groundwater and soil contamination in the southern
portion of the Site is
located within an area
of the Site
designated as "Section 16".
Contaminated shallow groundwater
(Subunit A) a~ the southern portion of the Site is being addressed..
by a separate remedy referred to' as the "Section 16 operable unit".
A Record of .Decision for the Section 16 operable unit was signed \\
on September 29, 1987.
The designated remedy of a pump and treat
system for Subunit A groundwater has been operating since December
1989.
The 1989 ROD addresses the final remedy for this. site as
described below and incorporates the remedy selected for the
Section 16 operable unit~
The current
land uses
on and near the Site
consist of
agricultural,
industrial,.~and residential
uses.
Groundwater
currently used for drinking water meets federal and state drinking
water standards.
As mUnicipal water supplies .in the area of the
Site are solely dependent on groundwater, future population growth
in the area could require use of groundwater in contaminated areas
and may result in potential exposure to hazardous substances.
B.
Remedv Selected in the 1989 ROD
The ROD for the final remedy at the PGA site was signed by the
EPA Regional Administrator on September 26, 1989.
In ~ddition to
the
remedial
action
described below,
the
final
remedy
also
incorporates the section 16 operable unit.
For the southern half of the Site, the remedy consists of
extraction and treatment of Subunit B/C groundwater and soil vapor
extraction for the vadose zone. . The groundwater remedial action
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requires a pump and treat system using air stripping to remove VOCs
from the groundwater.
~he ROD states that groundwater remedial
action shall continue to use twenty existing wells for extraction
. and
requires. the
addition
of
new wells
for
extraction
and'
treatment.
A central plant- will be constructed to treat the water.
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~he one remaining \
from all but one of the new extraction wells.
extraction well is commonly referred to as the "Phillips Well"
(after the owner of the property on ~hich the well is located).
The 1989 ROD requires treatment at the wellhead for the Phillips
Well because it exceeded ARARs for TCE and is located a significant
distance from the proposed location of ' the central treatment plant.
The ROD requires that groundwater be provided to current users of
the existing twenty extraction wells, with the treated water from
the central treatment plant available to the City of Goodyear for
municipal use.
A soil vapor extraction (SVE) system was selected for the
contaminated vadose zone.
The SVE system will be implemented in
an area identified as Target Area 2 in Figure 5-2 of the ROD.
The
total present worth post of the extraction and treatment facilities
for the groundwater remedy for the southern portion of the Site is
estimated at $9,160,000.
The total present worth cost of the soil
remedy for the southern portion is estimated to be from $3,904,000
for a phased implementation,
implementation.
to $5,370,000 for 'a full scale
The remedial action selected for the northern portion of the
site is similar to that chosen for the south and includes a
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Subunit A and Subunit B/C groundwater remedy and a soil remedy.
The groundwater remedy consists of a pump and treat system using
air stripping, followed by liquid phase granular activated carbon
to remove VOCs from the groundwater.
required for the groundwater remedy.
Air emission controls are
The ROD requires that the
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treated water from Subunit A be.reinjected, and the treated water \
from Subunit B/C be available for incorporation into the community
water supply.
The soil remedy consists of a SVE system with air
emission controls to be implemented in the target area.
The ROD
identifies the target area as that area where VOCs were detected
in soil samples and the area where soil gas samples exhibited VOCs
greater than J. ug/J..
The ROD provides that this area may be
expand~d or reduced, as necessary, to include removal of. 99 percent
of the contaminants.
In addition, the ROD states that excavation
and treatment may be required to remove residual contamination
where soil vapor extraction is not effective.
The estimated
present worth cost of the groundwater remedy for the northern
portion of the site is $J.4,027,000.
cost of the SVE system is $3,136,000.
The estimated present worth
ARARs for the PGA site are identified in Table 2-5 of the ROD
(See Attachment 2).
The ARARs for the operable unit are identified
in Table 2-5 and in Table 1 of the 1987 ROD.
. III.
DESCRIPTION OF SIGNIFICANT DIFFERENCES
This ESD clarifies and modifies portions of EPA's September
1989 ROD.
To the extent that this ESD differs from the ROD, the
ESD supersedes the ROD.
The significant differences contained in
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this ESD are described below.
A.
CleanuD Level for Methvl Ethvl Ketone in aroundwater
Table 2-5 of the ROD shows the ARARs for the PGA Site.
The
cleanup level. for MEK in groundwater is identified as 170 parts per'
billion (ppb) based on the levels set by Federal Ambient Water,
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Quality Criteria (AWQC) and by the Arizona Department of \
Environmental Quality (ADEQ) Action. Level.
After the ROD was
signed, EPA discovered that an AWQC level has not been set for MEK
and that the ADEQ Action Level is not "promulgated", a necessary
requirement for a state ARAR2.
As a result, neither of these two
standards is applicable nor is relevant and appropriate; the MEK
level was identified as an ARAR in error.
In fact, no ARARs are
in effect for MEK in groundwater.
EPA
has
reviewed
additional
information and has determined that the correct cleanup level for
ME!<
in
groundwater
at
the
PGA Site
is
3.50
ppb.
Section
300.400(g) (3) of the NCP allows EPA to use advisories, criteria,
or guidance developed by EPA, other federal agencies, or states
which may be useful in developing CERCLA remedies.
This category
of information is referred to as "to be considered" (TBC) and can
be used to set cleanup levels
when ARARs do not exist. The
preamble to the NCP states as follows:
"when an MCLG or MCL does not exist for a particular
contaminant, EPA intends that the lead or support agency
. use EPA-developed toxici ty information such as cancer
potency factors and reference doses for noncarcin~enic
-e~fects when developing preliminary remediation goals."
2 This requirement is found in Section 300.400(g) (4) of the
National Contingency Plan, issued on March 8, 1990, which was
proposed at the time EPA signed the ROD for the PGA Superfund Site.
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In accordance with the NCP, EPA has used TBC guidance and
criteria to establish the cleanup level for HER at 350 ppb.
EPA
has reached this cleanup level by using the reference dose for MEK
and applying the procedure for calculating.MCLGs as established by'
the
proposed National
primarY
and
Secondary
Drinking
Water
Regulations.. 54 Fed. Reg. 22062 (May 22, 1989).
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The MCLGs set \
cleanup levels for current or potential drinking water sources.
As discussed in the 1989 ROD, all ARARs at the PGA site are set at
drinking water standards (See pages 2-21 and 2-23 of the 1989 ROD) .
The MCLG calculation for HEX is as follows:
MEK MCLG = (Rfd-MEK) x (70 ka adult)
2 liters water consumed per day
x
RSCF = 350 ppb
where:
Rfd-MEK, the MEK toxicological risk reference dose
found in the EPA's Integrated Risk Information
system (IRIS) database, equals 50 micrograms per
kilogram per day; and,
RSCF, the relative source contribution factor,
the percent of exposure to MEK that may be
attributed to drinking water at or near the
Site is 20 percent.
The PGA Site Risk Assessment did not address the potential
routes of exposure for MEK3. . Therefore, in accordance with the
proposed
National
Primary
and
Secondary
Drinking
Water
Regulations', EPA has determined that the most protective RCSF (20
percent) is appropriate for the PGA Site. The proposed regulations
3 The PGA Site Risk Assessment is located in Appendix R,
Volume 5 of the PGA Remedial Investigation/Feasibility Study, dated
June 1989.

, Although not yet final, EPA uses the procedures contained in
these regulations (54 Fed. Reg. 22062, dated May 22, 1989) to
calculate MCLGs.
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recommend that when calculating the HCLGs, the following approach
be taken:
"[w]hen data did not exist, EPA then estimated drinking
water's contribution at 20 percent of total exposure.
This value was considered protective and conservative and
accounts, for the range of actual (but unknown) exposures
from different sources." 54 Fed. Reg. 22069.
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Moreover, EPA has determined that the use of a 20 percent RSCF

is also appropriate based on the follosing information:
(1)
groundwater is the primary source of drinking water in
Arizona and potential exposure to VOCs via drinking water is highly
probable; and
(2)
persons working on-Site and nearby residents have
potential
exposure
to VOCs
from
inhalation
due
to
possible
emissions
of VOCs during 'Clean-up activities on-site and from
nearby industry.
B.
Cleanup Level 'for Acetone in Groundwater
Table 2-5 of the ROD did not contain a cleanup level for
acetone
because
acetone
has
not
yet
been
detected
in
the
groundwater at the PGA site.
Yet, acetone has been detected in
soil
in the north~rn portion of the Site.
Because acetone
migration to groundwater is possible, EPA has determined that a
cleanup level for acetone in groundwater should be added to Table
2-5 of the ROD.
In addition, a cleanup level for acetone in
qroundwater is needed to determine the cleanup level for acetone
in the soil upon applying the EPA-approved contaminant transport
model.
The cleanup level for acetone in the soil
will be
determined based upon a decision-tree described in the 1989 ROD and
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will be related to the cleanup level for acetone in groundwater.
In the manner described below,EPA has determined that the
appropriate cleanup level for acetone in the groundwater is
700 ppb.
As. there are no ARARs in effect for acetone, EPA has.
calculated the MCLG as described above for. MEK.
The MCLG \
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calculation "for acetone is as follows:
Acetone MCLG = (Rfd-Acetone) x (70 ka adult)
2 liters water consumed per day
x
RSCF = 700 ppb
where:
Rfd-Acetone is 100 micrograms per kilogram per day;
and,
RSCF is 20 percent.

As with MEK, EPA applied a 20 percent RSCF due to a lack of
information regarding the routes of exposure for acetone in the
Risk Assessment for the" PGA Site.
C.
Phillips Well
As previously discussed .above, the 1989 ROD required treatment
for the Phillips well ~t the well head. EPA selected this remedy
based on EPA sampling of the well prior to 1989, which indicated
an
average concentration
of trichloroethylene
(TCE)
of
10.3
micrograms per liter (ug/l) at this well5.
The ARAR for TCE is
5.0
ug/l.
Representatives
of
EPA,
Arizona
Department
of
Environmental Quality (ADEQ), and the Goodyear Tire & Rubber
Company (GTRC) sampled the Phillips well again on July 24, 1990.
. Samples were taken from both the wellhead of this agricultural well.
and at the first points of discharge to irrigation canals.
The
5microqrams per liter is a measurement equivalent to parts per
billion.
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results of the July 24 samplings showed that average concentrations
of TCE at the wellhead and at both points of discharge to the
irrigation canals fall beloW the 5.0 ppb cleanup level.
No other
VOCs were detected at the Phillips well above ARABs.
The results'
of the July 24, 1990 samplings o~f the Phillips well are as follows: ~
Location
TCE Concentration (DDb)
EPA ADEO GTRC
wellhead
discharge to canal (#1)
discharge to canal (#2)
3.1
3.2
3.5
5.4
5~6
5.3
5.6
5.3
4.6
The Phillips Well lies west and down gradient of the airport
portion of the site.
With the exception of the Phillips well, all
other groundwater data indicating VOC levels above ARARs were
located on or near the airport property.
Based on these sampling
results, EPA has determined that treatment at the wellhead for the
Phillips well is not required at this time.
Although treatment is
not required, EPA continues to require routine testing for VOC
contamination at the Phillips well as part of :the remedial action
at the PGA Site.
This ESD does not alter EPA authority to reimpose
the requirement for wellhead treatment at the Phillips Well should
future monitoring indicate that the concentration of any VOC has
exceeded the cleanup level identified in Table 2-5 (See Attachment
2).
EPA's decision to reimpose wellhead treatment will be based
~n the agency's review of water-quality sampling results for the
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Phillips well.
D.
Soil Remedv Target Area in the Northern Portion of the Site
EPA has received and reviewed information after the ROD was
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signed which indicates that the soil remedy target area in the
northern portion of the PGA Site is not described clearly.
On page
four of the ~989 ROD, the soil remedy target area is described as
"that area where VOCs were detected in soil samples and the area
where soil qas samples quantified VOCS qreater than ~ ug/l. The
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area may be -expanded or reduced to include removal of 99 percent \
of the contaminant". EPA intends for these statements to identify
the soil remedy target area for the northern portion of the PGA
site to consist of target areas Band C defined by all four, circles
in Figure 5-7 of the ROD.
E.
Use of Soil Excavation and Treatment in the northern portion
of the PGA Site.
EPA has received and reviewed information after the ~989 ROD,
was signed requesting a clarification of the use of soil excavation
and treatment if soil vapor extraction is not effective to meet the
required soil cleanup levels.
The 1989 ROD states on page four
that "excavation and treatment may be required to remove residual
contamination where soil vapor extraction is not effective."
EPA
interprets this to mean that excavation and treatment of soil is
one, but not the only, remedial alternative EPA will consider for
the soil in the northern portion of the site if .soil vapor
extraction is ineffective.
IV.
SUPPORT AGENCY COMMENTS
The
Arizona
Department
of
Environmental
Quality
(ADEQ)
reviewed
this
ESD
and
has
concurred
with
all
changes
and
clarifications herein.
ADEQ commented that the ESD should state
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that the Phillips well is an agricultural well and not a domestic
well.
This comment has been incorporated into this ESD.
v.
STATUTORY DETERMINATIONS
Considering the new information that has" been developed and'
the changes that have been made to the selected remedy, EPA
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believes 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 revised remedy uses
permanent solutions and alternative treatment technologies to the
maximum extent practicable for this Site.
The changes and
clarifications contained in this ESD are significant but"do not
fundamentally change the remedy.
VI.
PUBLIC PARTICIPATION ACTIVITIES
EPA has presented "these changes to the remedy in the form of
an Explanation of Significant Differences becaus~ the changes are
of a significant, but not a fundamental, nature.
EPA provided the
State of Arizona with a fifteen (15) day comment period on this
ESD.
In accordan~e with Section 117 (c)
of CERCLA, 42 U.S.C.
Section 9617(c), EPA will publish a notice in the Arizona Republic
newspaper which describes this ESD and its availability for review.
This
ESD
and
all
documents
which
support
the
changes
and
clarifications herein are contained in the Administrative Record
for the PGA Site.
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Date
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Tlble 2-5
LECALLY APPLICABLE
STAT! AND PEDERAL REQUIREHF.NTS AND OTHER CRITERIA
FOR GROUNDWATER
(Concentrltlons In Ps/l)
COIIIDOUnct

1,I-Dlchloroeth,lene
1.2-Dlchloroproplne
Chlorofol'lll
Toluene
Trlchloroeth,lene
Trlchlorofluoro.eth.ne
Carbon Tetrlchlorlde
"~th,lene Chloride
~th,l Eth,l letone
.,Ienee
, Ant IJIIon,
Auenle
18 r hili
Ber, I U...
Cad1wt-
ChrOl8IU8
Lud
Hercury
Nickel
SelenlU8
SUnr
Ztnc
""8111,
Appltuble .
. . SDWA..
HCL
Other Crlter"

. . AWOC--DrtnUna Weter OnlY
ToxldtY C.ncer 1004'> Rhlt
1
0.033
0.19
100
5
15,000
2.8
s
  170
 1.46 
50  o.oon
I .000  
  0.0039
10 10 
50 50 
50 50 
2 .0 
 15.4 
10 10 
50 50 
 5.000 
Hotel'
ADEQ . Arlzon. Dep'rtMent of En.lronmentel Quellt,.
AWQC . Ambient W.ter Qu.llt, Crttert., IdJu8ted for conswmptlon of
drlnklns ..ter onl" ftsh In8estlon compone~t removed CU.S. EPA. 1986).
AWQC (IO~) . The Ambient Wster Qu.llt, Criteria relultln! In a 10~ excee.
llfet~ cancer rl8k CU.S. EPA, 1986).
Het . Ma.~ Cont8mtnant te.el.
MeLC . "a.l- ConU.dn.nt I.e.el Coat.
SOWA . S.f. Drlnltlns Water Act, 40 erR 141, No.~ber IS, 1985.

U.S. I'A. 1981. tiiS Databaee.
Propoied HeLl. 'eder.l Resister, Ma, 22, 1989.
Source.
.r
ADEQ,
Action Lent
Water
1
I
3
340
5
1
5
I
170
440
. ..
Propoled'
MeL
Clelnup
Level
1
I
100
340
5
1
S
I
170
440
1.46
50 ,
1,000,.
0.0039 i
10 '.,'
50 ".
50
2 .
15.4 ,
10
50
5.000
,
200
10,000
',000
5,000
5
100
5
50
.

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