EPA Methods

Study Method and Procedures


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.


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 plant life) 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 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.

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 l0,000(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.

Last modified: 11 November 1997