RISK-BASED SCREENING LEVEL (RBSL) SPREADSHEET FOR UPSTREAM SITES

Lynn Spence
Spence Environmental Engineering
4536 Mirador Drive
Pleasanton, CA 94566
Voice: 925-846-4721
E-mail: lynnspence@bprisc.com

Harley Hopkins
American Petroleum Institute
Washington, DC

Sara McMillen*
Renae Magaw
ChevronTexaco Energy Research and Technology Company
Richmond, CA

Michele Amaral BBL
Petaluma, CA

A Risk-Based Screening Level spreadsheet has been developed for calculating human health risk-based screening levels (RBSLs) in soil for total petroleum hydrocarbons (TPH) and the individual constituents, benzene, toluene, ethylbenzene and xylenes (BTEX) at upstream sites. The TPH RBSL is a single concentration in soil that is protective of human health for the entire TPH mixture with the exception of the BTEX constituents which should be evaluated separately (and concurrently). The spreadsheet consists of databases containing default input parameters, equations used to calculate the RBSLs and tables showing the results of the analysis.

Four different soil RBSLs are calculated in the spreadsheet for both residential and non-residential receptors. These four different RBSLs are calculated for: (1) surface soil, (2) subsurface soil to outdoor air, (3) subsurface or surface soil to protect ingestion of groundwater directly beneath impacted soil, and (4) subsurface or surface soil to protect ingestion of groundwater downgradient from area of impacted soil.

The RBSL in surface soil assumes that ingestion of soil, dermal contact with soil and inhalation of volatile and particulate emissions from soil can occur simultaneously. Both RBSLs in soil that protect for ingestion of groundwater use a "completely mixed model" to predict transport in the unsaturated zone. The fourth RBSL assumes that the receptor is located downgradient from the source. In addition to the "completely mixed model" in the unsaturated zone, it uses a simple groundwater fate and transport model to predict concentrations downgradient from the source area.

The spreadsheet was developed for exploration and production sites, however, it is applicable to other sites where BTEX and/or TPH are the main contaminants of concern.