Study Method and Procedures
Background
The City of Chester is located approximately 15 miles
southwest of Philadelphia along the Delaware River. According to
the 1990 United States Census, 41,856 persons reside in Chester,
which has an area of 4.8 square miles. Surrounding communities
also examined in development of this report include Eddystone,
Trainer, Marcus Hook, and Linwood. Major surface transportation
routes transect Chester including Interstate 95, and US Route 13,
which parallels Interstate 95 to the east. US Route 322 bisects
Chester from northwest to southeast.
Drinking water for the City of Chester is supplied by the
Chester Water Authority (CWA) and Philadelphia Suburban Water
Company (PSWC).
Large sources of surface water in the City of Chester
include Chester Creek and the Delaware River. All streams in the
Chester vicinity ultimately drain into the Delaware River in a
branching pattern. The Delaware River is a protected waterway
for the maintenance and propagation of fish species that are
indigenous to a warm-water habitat.
The hydrogeologic conditions that exist beneath the study
area are highly dynamic in nature. Water levels are influenced
by tides and high rates of infiltration from storms.
Methodolooy
A key element in the project scope called for environmental
risks to be quantitated wherever possible, and supplemented with
qualitative information.
Chemical data were gathered from existing sources. The
scope of this project did not include collection of new data
specifically designed for a Chester risk assessment. Instead the
workgroup performed an examination of available data which
yielded the following observations:
The data had-been collected for different programs and
different agencies. These data were not originally designed to
support a quantitative risk assessment of the Chester area.
The databases were of varying quality, and certain
chemicals and media had not been tested. However, with the
limited data available, it was possible for many data sets to be
used to generate estimated risks.
Modeling of air data from point sources preceded the air
risk assessment, such that point source air risks are based on
projected data rather than data actually collected in the field.
The lead (Pb) data, area sources of volatile organic compound
(VOC) emissions, Resource Conservation and Recovery Act (RCRA)
site information, and Toxic Release Inventory (TRI) data did not
involve the types of environmental data conducive to quantitative
-risk assessment.
In a risk assessment, the hazards posed by chemicals
detected by chemical analysis are evaluated. Potential risks may
exist when chemicals are present in the air, water and soils and
sensitive receptors (i.e. humans, wildlife, and plantlife) are
present which have access to the chemicals. This constitutes a
complete exposure pathway.
To evaluate risks, several steps are taken. First, the data
are assessed for usability and comparability. Data may then
undergo statistical manipulations for use in the quantitative
risk assessment. An initial screening step occurs during data
evaluation for the purpose of narrowing down the list of
chemicals that are quantitatively assessed. Using conservative
assumptions, the chemical concentrations that would correspond to
the lower end of the target screening risk range1 are
calculated. These concentrations are called risk-based
concentrations(RBCs), and are compared to the site data during
the data evaluation stage to rule out chemicals that will not
contribute significantly to risks at the site.
Exposure pathways are then determined. The receptors that
may be exposed are also chosen. Both current and future land
uses must be considered. Using site-specific or default
assumptions, estimated exposure doses are calculated for each
receptor.
Once the amount of exposure each receptor receives has been
calculated, that amount or dose is compared with values designed
to assess the safety or toxicity of a chemical. This step, which
is called risk characterization, helps the risk assessor
determine the likelihood of adverse effects occurring for that
exposure scenario.
Finally, the uncertainty of the risk analysis is described,
either quantitatively, qualitatively, or both. This step helps
give a more complete picture of environmental risks, and helps
risk managers weigh their options in addressing potential
hazards
The data were examined in order to determine chemicals of
potential concern (COPCs). COPCs are defined as those substances
that are potentially related to the risk source being studied and
whose data are of sufficient quality for use in the risk
assessment. It is appropriate to select COPCs for each medium of
concern.
Data were often screened using RBCs. RBCs were used to
determine whether, if included in the risk assessment, the
chemical would be likely to contribute significantly to the risk.
UNCERTAINTY ANALYSIS
Uncertainty associated with the assessment of risk may be
associated with exposure estimation, toxicity assessment, and in
risk characterization. The policy of the USEPA is to be
protective of human health and the environment. In accordance
with this policy, exposure estimates and the parameters used in
the characterization of the exposures are of a conservative
nature whenever possible. These conservative parameters are
designed to ensure that all estimates are protective and that all
sensitive subpopulations are considered. Some of these exposure
parameters may be overestimates of the actual exposures
experienced by receptors.
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1 target screening risk range: within the EPA Superfund program defines
acceptable cancer risks as those which do not exceed the established range of
lE-06 to lE-04. This range corresponds to an additional cancer risk of 1 in
one million(lE-06) to 1 in lO,OOO(lE-04) from exposure to a given chemical.
The lower, more conservative -- and more protective -- end of this range is
lE-06.
For non-cancer-causing chemicals, the ratio between the calculated potential
dose and the dose known to be safe should not exceed one.
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Last modified: 11 November 1997
http://www.ejnet.org/chester/epa-sum1.html