STATE OF ARIZONA
SILVER CREEKSUBDMSION
TUCSON, PIMA COUNTY, ARIZONA

JUNE 3, 2005

u.s. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Public Health Service
Agency for Toxic Substancesand DiseaseRegistry
Division of Health Assessmentand Consultation
Atlanta, Georgia 30333

Health Consultation: A Note of Explanation

An ATSDR health consultation is a verbal or written response from ATSDR to a specific
request for information about health risks related to a specific site, a chemical release, or
the presence of hazardous material. In order to prevent or mitigate exposures, a
consultation may lead to specific actions, such as restricting use of or replacing water
supplies; intensifying environmental sampling; restricting site access; or removing the
contaminatedmaterial.

hI addition, consultationsmay recommendadditional public health actions, such as
conductinghealthsurveillanceactivitiesto evaluateexposureor trendsin adversehealth
outcomes;conductingbiological indicatorsof exposurestudiesto assessexposure;and
providing health education for health care providers and community members. This
concludesthe health consultationprocessfor this site, unlessadditional information is
obtainedby ATSDRwhich, in the Agency'sopinion,indicatesa needto revise or append
the conclusionspreviouslyissued.

You May ContactA TSDR TOLL FREE at
1-888-42ATSDR
or
Visit our HomePageat: http://www.atsdr.cdc.gov

HEALTH CONSULTATION

STATE OF ARIZONA
SILVER CREEK SUBDIVISION
TUCSON, PllvIA COUNTY, ARIZONA

Prepared by:

u.s. Departmentof HealthandHumanServices
Agencyfor Toxic Substances
andDiseaseRegistry
Division of HealthAssessment
and Consultation

Purpose
The Arizona Department of Health Services (ADHS) completed this health consultation
at the request of the Arizona Department of Environmental Quality (ADEQ). This
consult evaluateswhether soil vapors from volatile organic compoundsin the subsurface
near the Silver Creek Subdivision in Tucson, Arizona are present at levels that may cause
adversehealth effects.

Background
The Silver Creek subdivision is located approximately 4 miles northwest of downtown
Tucson. Approximately 700 residentslive in the 288 home subdivision. A gasoline
releasefrom a ruptured high-pressurepipeline occurred in the Silvercroft Wash near the
Silver Creek subdivision on July 20, 2003. Over 50,000 gallons of gasoline have been
recovered from the subsurface. Five homes under construction in the areawere
demolished following the pipeline rupture becauseof gasoline contamination. The
homes in this portion of the subdivision have not beenrebuilt.
Levine Fricke (LFR) on behalf of pipeline owner Kinder Morgan Energy Partners
(KMEP) perfomled a soil vapor survey as part of the Silvercroft Wash Fuel Releasesite
assessmentand remediation project. In addition to total petroleumhydrocarbons (TPH)
and their related compounds (benzene,toluene, ethylbenzene,and xylenes [BTEX],
methyl-t-butyl ether [MTBED, LFR sampling detectednon-fuel related chlorinated
solvents, such as tetrachloroethene (PCE) and tricWoroethene(TCE) in soil vapor and
groundwatersamples beneaththe study area. As the study focused only on fuel-related
volatile organic compounds, it did not fully evaluatethe extent of the non-fuel related
volatile organic compounds. The Arizona Departmentof Environmental Quality
conductedadditional soil vapor sampling to obtain analytical soil vapor data to
independentlyverify and expand upon soil vapor data collected by Levine Fricke.

ExposurePathways
Five elementsareconsideredin the evaluationof exposure
pathways:
A sourceof contamination
Transportthroughan environmentalmedium
A point of exposure
Exposureroute
A receptorpopulation.
The Arizona Department of Health Servicescategorizesan exposurepathway either as
"completed" or as a "potential" exposurepathway if the pathway cannotbe eliminated.
In completed exposurepathways, all five elementsexist, and exposureto a contaminant
has occurred in the past, is presently occurring, or will occur in-the future.
In potential exposurepathways, at leastone of the five elementsis missing but could
exist. Potential pathways indicate that exposureto a contaminantcould have occurred in

1

. ..
..

the past, or may occur in the future. A potential exposurepathway may be eliminated if
one of the five elements is missing and is unlikely ever to be present.
Vapor intrusion and inhalation of those vapors is a potentially complete exposure
pathway. Soil vapor intrusion refers to the migration of volatile chemicals from the
subsurfaceinto overlying buildings. Volatile chemicals in contaminated soils, buried
wastes and/or contaminated groundwater can emit vapors that may migrate through
subsurfacesoils and into indoor air spacesof overlying buildings similar to the seepage
of radon gas into homes. Residentsare likely to be exposedto the soil vapors that migrate
into homes in the Silver Creek neighborhood resulting in a completed exposurepathway.
ADEQ Flux Chamber Sampling -Silver Creek Subdivision
The Arizona Department of Environmental Quality conductedadditional soil vapor
sampling to evaluate the potential public health impact to the residents of the Silver
Creek subdivision from vapors detectedpreviously during investigation of Silvercroft
WashFuel Release.
The Arizona Department of Environmental Quality collected soil vapor samples by
placing a flux chamber over open soil and on concrete. The data set available for this
health consultation is limited to the results of environmental.samplescollected at 60 flux
chamberssamplescollected at a fixed point in time. One of the flux chamberswas set up
directly adjacentto the pipeline where no homes are presentand was excluded from the
database. In addition, there were 2 background samplesthat were collected but excluded
from the databasebecausethey are not specific to the neighborhood. Data from a total of
57 flux chamberswere used in this health consultation.
An extensive list of target contaminants,including volatile organic compounds not
associatedwith gasoline was evaluated.The results of the evaluation found elevated
concentrationsof compounds that are consistent with materials that have been previously
detectedat and near known sources in the area as well as compounds that may be
associatedwith the gasoline pipeline release.
The results from the flux chamberssampleswere used to estimate indoor air
concentrationsusing standardbox models (SECOR 2005). The models provide a
conservativeestimate of predicted indoor air concentrations.This formula takes into
accountthe mobility of the contaminantsand use factorsdesignedto result in the highest
(and thus most protective) concentrationestimatesof contaminants in the air. The
following expressionsdisplay the model equations and assumptionsused to calculate
predicted indoor air concentration at both the open soil and concrete slab sampling
locations:

2

Open Soil Flux Chamber Expression:
Cair =

Fj x SA x CL x CF

AERxV
Where:
Cair

-

Fi

SA
CL

CF
AER
V

hldoor air concentration in micrograms per cubic meter (~g/m3)
Site specific hlfiltration flux, in micrograms per feet a minute (~g/ft-min)
Surface Area Perimeter, 144square meters (m2)
Crack level, estimatedto be 0.01 (unitless) (ASTM 1997)
Conversion factor 60 minutes per hour;
Air exchangerate, assumedat 0.25 hour-1(USEPA 2003)
Average volume of residential structure, assumed351 cubic meters (m3)
estimated for a home measuring 12 meters (m) by 12 meters (m) and a ceiling
height of2.44 meters (m)

Concrete Slab Flux ChamberExpression:
Cair= Fi x P x CFl~CF2.

AERxV
Where:
Cair
Fj

P
CF1
CF2
AER

V

-

Indoor air concentration in micrograms per cubic meter (J.1g/m3)
Site specific Infiltration flux, in micrograms per feet a minute (J.1g/ft-min)
Perimeter, 48 meters (m)
Conversion factor 1, 0.3 feet per minute (ft/m)
Conversion factor 2, 60 minutes per "hour
Air exchangerate, assumedat 0.25 hour-1(USEPA 2003)
Average volume of residential structure, assumed351 cubic meters (m3) for a
home measuring 12 meters (m) by 12 meters (m) and a ceiling height of 2.44
meters (m).

Modeling results for each flux chamberare available in the documententitled Soil Vapor
Sampling Report, Silver CreekSubdivision. (SECOR 2005)

Risk Analysis
Contaminantsof Concern (CaCs) were evaluated for systemic toxicity and
carcinogenicity. Systemic toxicity refers to the potential for a compound to causeeither
symptomatic or asymptomatic health problems in humans. Carcinogenicity refers to the
potential for compoundsto causecancerin humans.
Systemic Toxicity
The Arizona Department of Health Servicesselectedcontaminantsof concernby
comparing the averagepredicted indoor air concentrationsto the Agency for Toxic
Substancesand Disease Registry (ATSDR) ComparisonValues (CVs) and the United
StatesEnvironmental Protection Agency (USEPA) ReferenceConcentrations(RfC).
The flux chambersampleswere used to predict indoor air concentrationsby applying
conservativeassumptionsthat provide an upper-bound estimate of actual indoor air
concentrations. These modeledresults representconditions observedon the days samples
3

I

were collected and do not predict potential variations in contaminantconcentrations that
may occur over time.

The Arizona Departmentof HealthServicesselectedthe contaminantsof concernby
averagingthepredictedindoor air concentrationsand comparingthe resulting
concentrations
to screeninglevels. Compoundswere selectedasa contaminantof
concernif thepredictedaverageconcentrationexceededa screeninglevel.
The averageindoor air concentrations, rather than the maximum indoor air
concentrations,were chosen for comparison with the screeninglevels to optimize the
limited datasetavailable from this single sampling event, to provide a broader statistical
baseto make comparisons,and to provide a better benchmark for drawing conclusions
for the entire neighborhood.
The primary screeninglevels that were used to select contaIninantsof concern are called
Air ComparisonValues (CVs). These screeninglevels are concentrations in air that are
unlikely to pose a health threat. Where the air comparison values were not available for a
specific compound, the modeled results were compared to the United States
Environmental Protection Agency's ReferenceConcentration (RfC) or preliminary
remediation goals (pRGs). The air reference concentrations and the preliminary
remediation goals are applicable to both indoor and outdoor air and are based on a
residential exposurescenario using standardexposure factors. These screening levels
may also be used as a health protective indoor air target for determining soil gas
screeninglevels for the evaluation of the subsurface vapor intrusion pathway (ATSDR

2005).
Arizona Ambient Air Quality Guidelines were us,edas screeninglevels when no
comparisonvalues,reference concentrations or preliminary remediation goals were
available. Theseguidelines are developed by the Arizona Department of Health Services,
and are protective of human health, including children, over a lifetime. Chemical
concentrationsin air that exceed any of these guidelines may not necessarilyrepresenta
health risk (AAAQG 1999).
Predicted indoor air concentrations that exceeda screening criteria do not necessarily
pose a health threat, but require further evaluation.

Table 1. Predicted Indoor Air ConcentrationsCompared to ScreeningValues
Average
dOc'"":
""Rre I..~~~d

Frequency"
Compound

Health
c'

"fn~oQi~jr~

Air
Concentration

Eff~ct

"Concentration ,Screening
Detect~d
, ", ,
..
Value"
(n=57')
(fi§Jfu3)
,

Chemica]of

~v~rageAbove Concern.?
Scr~~ning
VariJe?

meter(l.J;9!m~)"
c"

Acetone

1

0.51

Acrylonitrile

7

0.06

Allyl chloride

6

0.06

Benzene

32

Benzyl chloride

8
0

0.47
0.11
0

Bromodichloromethane

4

~

13000~
i

22I
1;j

37.r
O2
na5

-

No
No

No
No

No
No
No
No

No
No
No
No

Average
Predicted:

Compound

Freguency Indoor Air
Detected Concentration
.
(n=57)

(micrograms

per cubic
meter

3
5
0
24
18
19
0
5
6
27

4
0
5
0
5
6
4
7
47
0
5

0
0

22
1

1
0
0

11
0
5
5

14

12
0

4
7
17
8
57
1
0
20
3

I

Health
Effect!

Air

Concentration
ScreeningI
ValueI

Average Above

Screening

Chemical of
Concern?

Value?

(~g/m3)

(!-1g/m3)

0.16
0.03
0
0.06
0.24
0.02
0
0.08
0.03
0.04
0.12
0

1./

~

na
50002

~-1951

~
~
na
21

na

0.25

0.07
0.12
0.83
0.06
0.13
92
0.12
0.18
9.06
0.14

5

No

No
No
No
No
No

No
No
No
No
No
No
No
No
No

No
No

No
No

No
No
No
No
No
No
No
No
No
No
No
No
No
No

No
No
No
No
No
No
No
No
No
No
No
No
No
No

No
No

No
No
No
No

1~

na1000Q2

0.21
0
0.18
0.14
0.15
0.08
0.03
0.02
0.06
0.06
2.70
0.08
0.08
0
0
0.03
0
0.03
0.16
0.11
0.04

No
No
No
No
No

na

-92
-~
6001

~

na

6001

-~
-.:?:Q~
4.13-~
na
na
na-21

3001

No
No
No
No
No

310003

No

No

na

No
No
No

No
No

No
No
No
No
No
No
No
No

No
No

No

na
202
10002

35002
0.863
403
15003

No

:~~r~No
No

na

na
30002
1104

No
No
No

No

-~
~

I

Frequency
.'
.'

Compound

Detected
(n=57)

AverageI
PredictedI
Indoor Air

Concentration
(micrograms
per cubic

meter (~g'm3)
Methyl ten butyl ether

6

Methylstyrene

6

Napthalene
Octane

18
21

Propionitrile

7
7
2
6
7

Trimethylbenzene

36

Trimethylbenzene
.vinyl acetate

22
14

0.21

Tetrachloroethane
Tetrachloroethane

Tetrachloroethene
1,2,4
1,1,2
1,1,1

1,2,3

_J!2,4

~

0-

0.46
0.10
0.07
0.08
0.10
0.15
0.08
0.86
1.48
0.04
0.03
0.03
0.06
0.11
0.19
0.11

Styrene
1,1,1,2
1,1 ,2,2

11.31
0.10

Toluene
Trichlorobenzene
Trichloroethane
Trichloroethane

10

Trichloroethene

1

Trichlorofluoromethane

30

I rlCnloropropane

6
5
0

3

Health
Effect

Air

Concentration...Chemical of
Average Above

Screening
Value
3

(/-lg/m )

No
No

424
3.51

.:!J.§~
2401

~

401

~

3204

Na
7001

na
562002~~

6.23
6.23

~-

0

0

na

Xylene

5

0.62

4351

Xylene

5

0.19

~

No
No
No
No
No
No
No

2581

~-

Vinyl chloride

Screening
Value?

No
No
No
No
No

Concern?

No
No
No
No
No
No
No
No
No
No
No
No
No

No
No
No
No
No

No
No
No
No
No
No

No
No

No
No

No

No

I ATSDR EMEG
2EPA Reference Concentration
3EPA Region 9 Preliminary Remediation Goal
4Arizona Ambient Air Quality Value
5No screening value available (na)
6 Simple asphyxiant value is the Lower Explosive Limit

As shown in Table 1, no chemicals of concernwere selected for further analysis,
indicating that for all of the compounds,predicted indoor air concentrationsin the
neighborhood do not pose a systemic health hazard.

Carcinogenicity
Carcinogepic risk is calculated as the incremental probability of an individual developing
cancerover a lifetime (70 years), due to exposureto a carcinogenic compound. This is
also referred to as incremental or excesslifetime cancerrisk (ELCR) and representsthe
increasedrisk of developing cancerabQvethe background rate, which is estimated to be
about33%.
While the criteria for selecting the contaminantsof concern for cancerare the same as
with the systemic health effects, predicted indoor air concentrationsare compared to a
different set of air comparison values. The guidelines used for this selection are also from
the Agency for Toxic Substancesand DiseaseRegeistry's air comparisonvalues, but are
6

values used to screencontaminants that are suspectedof causing cancer.These guidelines
are known as cancerrisk evaluation guides (CREGs).
If there are no cancerrisk evaluation guides for contaminants, then the U.S.
Environmental Protection Agency's CancerPreliminary Remediation Goal is used to
detennine if more evaluation is warranted. The contaminantsof concernwere selectedby
comparing the averagepredicted indoor air concentrationsto the air comparisonvalues.
Contaminantsof concern were selectedfor further analysis if the averagepredicted
indoor air concentrationexceededits screeninglevel. Table 2 displays a list of all
suspectedcarcinogensobserved in the sampling and their screeningvalue.

Table2. PredictedIndoor Air ConcentrationsComparedto CancerScreeningValues

Compound

Air
Ayerage
Concentration
rrequ~~cy Predicted CancerScreenina
Chemical
Average
Air
Detected' Indoor
,Concentration!
of
IConcern?

(11=511

I 3
,Hgm )
I

!c'"

7

0.06
0.06
0.47
0
0.16
0
0.02
0.08
0.03
0
0.12
0.15
0.02
2.70
0.08
0.03
0.03
0.83
0.46
0.10
0.15
0.03
0.04
0.03
0

6
32
0
3
0
1-9
5:
6
0
4
7
5
22
1
11
5
7
18
2
6
7
5
1
0

Above

Value

1
I

(""91m3)

0.01'

31
0.11
0.112

M1
Q'Q31

-ow
2]2
0]41

b:oar
0.0034

~

Q]'4;l
-31

--ow
~

~

0.31

-~

~

ow~
~
o:o1r

~

Screening

Value?
Yes
No
Yes
No
No
No
No
No
No
No
Yes
No
No
No
No
No
No
Yes
Yes
No
Yes
No
No
Yes
No

A TSDR Cancer Risk Evaluation Guideline
2USEr A Region 9 Preliminary Remediation Goal

There were 7 contaminantswith an averagepredicted indoor air concentrationthat
exceededthe screeninglevels. Table 3 summarizesthese7 contaminants.
7

Yes
No
Yes
No
No
No
No
No
No
No
Yes
No
No
No
No
No
No
Yes
Yes
No
Yes
No
No
Yes
No

Table 3. Suspectedcarcinogeniccontaminantsdetectedabovethe screeninglevels

lAveragePredictedI

Compound

Frequency
Detectedi'
(n=57)

7
32

4
7

1..2

18
6
Trichloroethene

I

rh~OorA!r

Cpncentration
(pglm3)
.,

0.06
0.47
0.12

0.83
0.46
0.15
0.03

Cancer
Screening
Value
(~g/m3)

0:01'

Yes

Qm1

Yes
Yes
0.0034
Yes
~"'0m2-Q"i)2T
Yes
Yes
Yes
0:0172

~

Yes
Yes
Yes
Yes
Yes
Yes
Yes

'ATSDR Cancer Risk Evaluation Guideline
2USEPA Region 9 Prelirnmary Remediation Goal

Using the list of contaminants of concern,estimatesof estimated lifetime cancer risk
were developed by evaluating potential exposurepathways, estimating exposure
concentrationsand intake, and combining exposure estimateswith toxicology
information (USEPA 1991).
The dose-responserelationship is consideredto be linear under the low dose conditions
usually encounteredin environmental exposures. Under this assumption,the US
Environmental Protection Agency slope factor (SF) for a compound is a constant, and
risk is directly related to intake. Therefore, the linear low-dose cancer risk is:
Risk = Cancer Inhalation Unit Risk (l/J.lglm3) x Concentration (J.lglm3)

where:

Risk
Unit Risk
Concentration

a unitless probability of an individual developing cancer;
Dose averaged over 70 years (1/J.l.glm3)
(USEPA 2005); and
Predicted indoor air concentration (J.l.glm3).

Table 4 summarizesestimatedlifetime cancer risk using averagepredicted indoor air
concentrations. Cancerinhalation unit risk is the upper-bound excesslifetime cancerrisk
estimatedto result from continuous exposureto a compound at a concentration1 ~g/m3
in air. The unit risk values are developedby the US Environmental Protection Agency
after careful and detailed analysesof data regarding the potential cancerpotency of a

compound.

8
AcrylonitrileBenzeneDibromoethaneHexachlorobutadieneNapthaleneTetrachloroethane
~

;::U!

Table4. EstimatedExcessCancerRisk

otal Excess Lifetime Cancer Risk

0.00007

Micrograms per cubic meter of air
2 Source: (USEPA 2005)
3 Average Air Concentration X Unit Risk

The estimatedupper-boundexcesscancerrisk estimate of 0.00007 or seven-in-onehundred-thousandrepresentsthe increasedrisk of developing cancer. This estimatewas
calculated by multiplying averagepredicted contaminant concentrationsby the Unit Risk.
There is general(although not unanimous)consensusanlong the scientific and regulatory
communities on what level of estimatedexcesscancer risk is acceptable. An increased
lifetime cancerrisk of one in one million or less is generally considerednegligible.
According to the United StatesEnvironmental Protection Agency National Contingency
Plan and subsequentguidance,an estimateof excess cancerrisk betweenone in a million
to less and one in ten thousandis within a range of acceptablerisk (USEPA 1990, 1991).
Risks greaterthan one in ten thousanddo not necessarilypose a significant cancerrisk,
but require additional in-depth analysis in order to draw conclusions about potential
cancerrisk.
The upper-boundestimatedrisk from indoor infiltration of contaminantsis within the
range of acceptablerisk and posesno apparentpublic health hazard to neighborhood
residents.

Limitations
There are many sourcesof uncertainty in every risk analysis. The objective of this health
consultationis to determine whether soil vapors from compounds in the subsurfacenear
the Silver Creek Subdivision in Tucson, Arizona are present at levels that may cause
adversehealth effects. This health consultation is a screeninglevel analysis of health
risks, meaning that the report usesa conservative (or upper-bound)analysis.
Severalconservativeassumptionswere made in this analysis. Infiltration into
neighborhoodhomeswas assumedto be a complete exposurepathway even though this
pathway may not actually be complete. Compounds that were detectedin flux chambers
at levels less than the reporting limits (called J Flagged Data -meaning that they were
qualitatively but not quantitatively accuratelyidentified) were still included in the
databaseto ensurethat no compoundswere left out of the analysis. The screeninglevels
that were usedto selectcontaminantsof concernhave a large margin of safety. The
9

.

exposureassumptionsusedto developthe screening levels assumecontinuous 30-year
exposureaveragedover a 70-year lifetime. Finally, the US Environmental Protection
Agency unit risk values usedto calculate cancer risk are upper-bound estimatesthat
almost certainly overestimaterisk.
\
The average air concentrationsused in this health consultation are estimatesbased on
flux chamber air sampling data collected at the site. The results were used to estimate
indoor air concentrations. The air concentrations used in this health consultation'
representenvironmental conditions at one point in time. Ideally, for where vapor
intrusion is a concern, pennanentsub-surfacemonitoring points for sample collection
would be used to evaluate the long-tenn behavior of soil vapors. In addition, it may be
necessaryto collect soil gassamples at different time intervals to compensatefor the
effects of weatherevents, suchas recentrainfall or barometric fluctuations (DTSC 2004).

Child Health Issues
The Agency for Toxic Substancesand Disease Registry recognizes that the unique
vulnerabilities of infants and children demand special emphasis in communities faced
with contaminantsin air. Children's developing body systemscan sustain permanent
damageif toxic exposuresoccur during critical growth stages.Children breathe a greater
volume of air relative to body weight, resulting in higher burden of pollutants.
Furthermore,children, eventhose without pre-existing illness or chronic conditions, are
susceptibleto air pollution becausetheir lungs are still developing, and they often engage
in vigorous outdoor activities, making them more sensitive to pollution than healthy
adults. All calculations and health analysesin this report take into considerationthe
unique vulnerability of children.
-'

Conclusion
The predicted indoor air concentrations in Silver Creek neighborhood suggestthat
the subsurfacecontaminantspose no apparent public health hazard.

Recommendations
The Arizona Department of Health Serviceshas no recommendationsat this time.

10

11

References
AAAQG 1999. Arizona Annual Air Quality Guidelines. Arizona Departmentof Health
Services.May, 1999.
ASTM 1997. American Society for Testing and Materials. StandardGuide for Risk-based
Corrective Action Applied at PetroleumReleaseSites. E 1739-95. 1997

ATSDR1995. Agencyfor Toxic Substances
andDiseaseRegistry,ToxFAQsfor
Hexachlorobutadiene.
September
1995.
ATSDR 2005. Agency for Toxic Substancesand DiseaseRegistry, Air ComparisonValues
January2005

DTSC.2004. VaporIntrusionGuidanceDocument.Stateof California.DTSC/CAL
EPA. December15,20-4.
NIST/SEMATECH e-Handbookof Statistical Methods,
ht!Q://www.itl.nist.gov/div898/handbook/. March 3, 2005.
USEPA 1990. USEPA National Oil and HazardousSubstancesPollution Contingency
Plan (NCP), 40 CFR Part 300, July 16, 1982 (47 FR 31180) and comprehensiverevision
March 8, 1990 (55 FR 8666) http://www.epa.gov/superfund/sites/nDl/f900830.htm
USEPA 1991. Risk AssessmentGuidance for Superfund (RAGS), Supplemental
GuidanceStandardDefault Exposure Factors (OSWER Directive, 9285.6-03) dated
March 25, 1991.
USEPA 1992. Technology Transfer Network Air Toxics Website. Hexachlorobutadeine.
httQ://www.eDa.gov/ttnatw01/hlthef/hexa-but.html.March 3, 2005.
USEPA 2003. User Guide for Johnsonand Ettinger Model.
USEPA 2005. Integrated Risk Infoffilation System. Online service at:
htiD://www .epa.gov/iris/
SECOR2005. Soil Vapor Sampling Report. Silver Creek Subdivision, Silverbell Landfill
Water Quality Assurance.February2005.
Preparers of Report
Arizona Department of Health Services, Office of Environmental Health

KristinaSchaller,EnvironmentalHealthScientist
DonHerrington,PrincipleInvestigator
Will Humble,BureauChief

Certification
This Silver Creek Subdivision Public Health Consultationwas prepared by the Arizona
Departmentof Health Services under a cooperative agreementwith the federal Agency
for Toxic Substancesand DiseaseRegistry (ATSDR). It was completed in accordance
with approvedmethodologies and proceduresexisting at the time the health consultation
was initiated. Editorial review was completed by the Cooperative Agreementpartner.

;:JJ/J/l~ ~~ss1f~ {tP
Technical Project Officer, CAT, SPAB, DHAC