IMPACT OF WEATHERING PROCESSES ON
TPH RISK-BASED SCREENING LEVELS
Sara J. McMillen*
Nishi Nijahwan
Kirk T. O'Reilly
ChevronTexaco Energy Technology Company
100 Chevron Way
Richmond, CA 94802
Voice: 510-242-3485
Fax: 510-242-1954
E-mail: sajm@chevrontexaco.com
In 1997, the Total Petroleum Hydrocarbon Criteria Working Group (TPHCWG) published a methodology for analyzing and assessing the potential risk of hydrocarbons in soil and groundwater to human health. In risk assessment, the first step is to calculate risk-based screening levels (RBSLs) for soil or water using very conservative, generic exposure, fate and transport assumptions. The TPHCWG approach to calculating screening levels for petroleum mixtures using an analytical method that quantifies 13 fractions of hydrocarbons has been adopted by several states. Some states have published "Tier 1" RBSLs for specific, fresh petroleum mixtures such as mineral oil, diesel, and crude oils. The impacts of weathering processes in soil, such as volatilization and biodegradation, on hydrocarbon RBSLs are not well defined. In this paper, the compositional changes that occur in petroleum in soil over time due to weathering processes have been quantified for both laboratory and field collected samples. Risk-based screening levels are then calculated for fresh petroleum mixtures and their weathered/ biodegraded counterparts. A discussion of how various weathering events can influence TPH RBSLs is presented along with the implications for establishing site closure criteria for bioremediation technologies, such as landfarming.