The second key step of the process is the exposure pathway analysis. This step includes identification of potential exposure pathways, identification of potential receptor populations and exposure scenarios, development of the CSM, and quantification of exposure. This section discusses the human health exposure assessment. The approach to the ecological exposure assessment is described in Section 7.

A preliminary CSM for the CSSA facility has been developed (see Figure 5.1). The CSM will be revised as necessary based on any additional information acquired as the investigation progresses. Additional CSMs may also be developed for individual sites within CSSA if appropriate. These CSMs will be used to determine if the default exposure assumptions used by the TNRCC are appropriate for CSSA.

5.1 - Characterization of Exposure Setting

To characterize the exposure settings, all features related to the sites and surrounding areas were reviewed. Potentially complete exposure pathways were identified based on the following elements:

A source and mechanism of chemical release from a source (e.g., contaminated soil);

An environmental transport medium (e.g., volatilization from soil to air);

An exposure point (e.g., a SWMU site);

A human or ecological receptor at the exposure point (e.g., a facility worker); and

A route of exposure through which the receptor may be exposed (e.g., inhalation of volatile organics).

A potential exposure pathway is not considered complete unless each of these elements is present for the specific exposure area. The potential receptor evaluation considered criteria such as:

Current and future land use on or near the site;

Zoning status and/or deed restrictions of the site and adjacent properties;

Current and future access to the installation, the exposure areas, and the affected media;

Past, present, and planned site activities;

Extent that the sites are developed and vegetated; and

Potential for soils to be disturbed and produce volatile emissions and fugitive dusts (e.g., soil-invasive activities such as excavation).

The identification of current and future receptors relative to the site evaluated the distance, direction, and description of use of on-site and adjacent property: the closest occupied building; closest off-site property from the source area; closest downgradient property from the source area; and closest downwind property from the source area. The nearest current on-base or off-base residential land from the source area and the nearest on-base or off-base parcels which are zoned for residential use were also identified. The presence of sensitive subpopulations (e.g., children) was also determined and described.

In addition, it is noted that if remediation has been performed at a site, the comparison will be based on the most recent chemistry data. An assessment of risk based on contaminant concentrations prior to remediation will not be performed.

5.2 - Development of the Conceptual Site Models

The CSMs are conceptual views of the exposure pathway analyses which identify and illustrate the contamination sources, affected media, release mechanisms, migration pathways, exposure routes, potential receptors, land-use assumptions, and potential for exposure. Potential pathway completion, or lack of completion, is also presented. Current and future site exposure pathways are depicted. CSMs are developed for both the human health exposure pathways (Section 5) and the ecological exposure pathways (Section 7).

5.3 - Identification of Potential Receptor Scenarios

To perform the evaluation, it is necessary to determine if the default exposure scenarios defined by the TNRCC are appropriate for CSSA. Exposure scenarios take into consideration the current and future land use of the site. CSSA currently includes residential areas. The closest off-site residential areas to CSSA are just outside the CSSA boundary to the west and south.

The residential scenario will be evaluated for all sites at CSSA.

At CSSA, permanent living quarters are located on the west side of the facility. There are seven residences currently housing 25 people, including children. No schools or day care centers are located on the facility. Residents are required to stay within the residential area, thus they will not have direct access to SWMUs or AOCs (Silva, 2000). The nearest off-site residences are located just west and south of the CSSA boundary.

For risk assessment purposes, residential exposure scenarios typically assume that residents may be exposed to contaminants through direct contact with contaminated soil or potable drinking water at their place of residence. Since access to SWMUs and AOCs is controlled, direct exposure of residents to contaminated soil is not expected to occur.

CSSA provides its own drinking water for both on-base residential and office areas for approximately 100 facility employees. The source of water for CSSA is on-site ground water wells. Nearby residential areas also obtain drinking water from wells located near the CSSA boundary. Thus, both on-site and off-site residential drinking pathways are potentially complete. In addition, the use of private wells as drinking water sources by off-base residents cannot be ruled out.

Hunting is allowed at CSSA, and game animals could potentially travel through SWMUs and AOCs, and have access to watering facilities supplied by on-site groundwater wells. Thus, complete exposure pathways exist through consumption of game animals. Residents may contact surface water and sediment in on and off-base portions of the three creeks flowing through CSSA through recreational activities (e.g., wading).

The commercial/industrial worker scenario assumes that workers are employed full-time in the exposure areas. Therefore, the default input parameters reflect conservative assumptions regarding exposure. The commercial/industrial worker can be present in both indoor and outdoor locations. Affected surface soils are assumed to be located in unpaved areas that are contacted by worker populations on an occasional basis and, if applicable, beneath buildings that would allow for volatilization into the building.

CSSA workers could potentially be exposed to soil via incidental ingestion, dermal contact, and inhalation of volatiles from soil in work areas. There are also potentially complete pathways for direct exposure of indoor workers to groundwater that would be used for drinking or other routine purposes. In addition, chemicals in shallow groundwater may volatilize to work areas.

5.4 - xposure Areas

Exposure areas within CSSA will be designated after consideration of the exposure pathways, the nature and extent of contamination, the types of contaminated media and COPCs resulting from site-related activities, and the past, present, and future site activities.

Sediment and surface water exposure will be considered for Leon, Salado, and Cibolo Creeks, including where they cross the facility and possible downstream impacts.

 [Next Section]