Determination of aqueous concentrations of polycyclic aromatic hydrocarbons (PAHs) in an urban stream

The concentrations of freely dissolved polycyclic aromatic hydrocarbons (PAHs) in an urban stream at high-flow and base-flow were estimated by an equilibrium partitioning model (EPM), and by use of lipid-filled semipermeable membrane devices (SPMDs), then compared to direct measurements made on bulk (unfiltered) water samples. The SPMD method was slightly more effective in detecting smaller three- and four-ring PAHs and the EPM method was slightly more effective in detecting five and six-ring PAHs. Although the SPMDs sequestered slightly fewer of the large PAHs, they sampled more compounds of a size range likely to be bioavailable to aquatic organisms. Estimates of the concentrations of freely dissolved PAHs for the EPM and the SPMD methods were similar when compared under the same flow regimes. The SPMD method also provided a time-integrated average measure of freely dissolved PAHs that cannot be easily duplicated with conventional sampling procedures. Concentrations of freely dissolved PAHs (EPM method) as well as concentrations of total PAHs at high-flow averaged about 20 times greater than at base-flow. This suggests the potential for immediate toxicological impacts on stream biota is greater at high-flow because of increased concentrations of bioavailable PAHs. For individual PAHs, differences between bulk and freely dissolved concentrations were most evident at high-flow for high Kow compounds strongly partitioned to suspended solids. Thus, in hazard assessments for aquatic biota it is important to employ measures of freely dissolved PAHs during storm water runoff events.