Risk assessment for polycyclic aromatic hydrocarbon NAPLs using component fractions

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are important environmental contaminants because of their suspected carcinogenicity. PAH contamination is often found in the form of nonaqueous phase liquids (NAPLs), such as coal tar. These materials are very complex mixtures with the constituent compounds exhibiting a wide range of chemical, physical, and toxicological properties. This makes it difficult to characterize a PAH NAPL and predict its behavior. Lumped metrics, such as total petroleum hydrocarbons (TPH), are often used to specify remediation goals at PAH NAPL sites, but these metrics do not capture the variation in NAPL composition overtime and as such do not consider changes in risk over time. This paper presents an alternative method for characterizing PAH NAPLs that is based on the fraction approach developed by the Total Petroleum Hydrocarbon Criteria Working Group (TPHCWG). The fraction approach characterizes the PAH NAPL as a collection of U.S. EPA Priority Pollutants plus hydrocarbon fractions. This approach allows prediction of the NAPL compositional dynamics while retaining some of the simplified analysis of a lumped parameter approach. The fractions and priority pollutants are modeled as NAPL components for performing risks assessments and assessing the potential of remediation strategies. This makes the fraction approach amenable for use in a Risk-Based Corrective Action (RBCA) framework. In this work, the TPHCWG fraction approach was modified for description of NAPL Constituents that are solids in their pure states. This was accomplished by assigning an equivalent solid-liquid reference fugacity ratio for each fraction. Thirty-year simulations with two model coal tars are presented, comparing the risk calculations for the fraction approach to those accounting for the complete NAPL composition. These simulations show that the model using the fraction approach predicts similar NAPL compositional dynamics and PAH carcinogenic risk as the model describing all the compounds. Use of the fraction approach at PAH NAPL sites will allow a more accurate accounting of the risk variation than the TPH approach. Additionally, it will emphasize development and use of remediation methods that address risk reduction, not just the reduction in total contaminant mass.