5.13 Data evaluation and data gaps

Existing data for concentrations of contaminants in the medium of concern—surface soil, subsurface soil, groundwater, and air—are evaluated. This evaluation will determine if data are adequate to estimate exposure point concentrations and to evaluate contaminant migration and toxicity. Existing data will also be evaluated to determine if data are of ade­quate quality for use in a risk assessment according to the methods specified in the U.S. EPA guidance, Data Usability in Risk Assessments.

If additional environmental data are necessary to complete the BLRA, an SAP is devel­oped. In addition to chemical data, collection of water quality data (i.e., hardness) may be specified in the SAP because the toxicity and mobility of metals in surface water is hard­ness dependent.

At this time an evaluation of existing data indicates that on-site groundwater data are available. Current groundwater data and new empirical data are evaluated for background and down-gradient information. The arithmetic means and 95% UCLs of the mean are cal­culated for COPCs. For the groundwater pathways a well survey is conducted within 1 mile of the site to verify the locations and uses of all wells.

If recontouring of the soil surface has occurred to direct storm water runoff toward col­lection points, additional surface soil samples (0-2 in.) may be necessary for the analysis of COPCs in the soil ingestion and air pathways.

Data collection points are designed to identify hot spots and to calculate average con­centrations over the entire site and in the areas of concern. Soil pH must also be measured because soil pH influences metals transport.

Surface water data for up-gradient and down-gradient points and sample collection points for this data are identified. No surface water or sediment data from on-site

drainages, or drainage pathways from the site, may be available to provide information on the extent of contaminant migration. Sediment data from the surface water pathway pro­vide information on whether the rivers or standing bodies of water subject to runoff are another source of contamination to other surface waters. Collect surface water and sedi­ment samples from these areas.

The level of generation of dust at the site is required information. This information may be collected via real-time particulate measurements and/or by collecting samples for lab­oratory analysis. The level of dust generation is assumed to vary significantly with time and climatic data. The collection of dust generation data is planned carefully so that dust generation is neither underestimated or overestimated.

5.14 Uncertainties

Uncertainties may relate to several factors, such as toxicity information or exposure assessments.

5.14.1 Uncertainties Related to Toxicity Information

The Rffls for COPCs are a major source of uncertainty in a BLRA. A chronic RfD is an estimate of the daily exposure unlikely to present an appreciable risk of deleterious effects during a lifetime. Uncertainty factors are applied to selected exposure levels identified in animal or human studies to derive the RfD. To avoid overestimating the RfD, identified exposure levels are divided by these uncertainty factors. An uncertainty factor of 10 is used to account for variations in human sensitivity when using data from valid human studies involving long-term exposure of average, healthy subjects. When extrapolating from observations of toxicity in animals to predicted toxicity in humans, additional uncertainty factors of 10 are applied.

Uncertainties are also present when identifying whether a compound is a likely human carcinogen and at what level of exposure an increased risk of cancer may exist. Uncertainties in quantifying the exposure levels that may result in elevated carcinogenic risk for specific compounds are corrected for by using the 95% UCL of the slope relating exposure to the prob­ability of developing cancer. The actual slope may be greater, but is unlikely to be greater.

The lack of an RfD and a cancer SF for lead will introduce uncertainty into the BLRA if lead is a COPC.