Challenges in applying high energy water accommodated fractions in oil toxicity testing

The high energy water accommodated fraction (HEWAF) protocol has been increasingly applied to prepare test media for assessing the aquatic hazard of oils. The relevance of this method was evaluated by characterizing mixing vessel hydrodynamics and comparing results to mixing energy scenarios encountered in the environment. The HEWAF system greatly exceeded environmental mixing energies in open water (e.g., blowout, breaking waves) by more than 100-fold producing high concentrations of oil droplets, with a predicted median size of <10 μm as corroborated by empirical measurements from earlier studies. Biomimetic extraction (BE) measurements were performed using solid phase microextraction fibers to characterize oil bioavailability in HEWAF exposures relative to a low energy WAF (LEWAF) with one fresh and two weathered crude oils over a range of dilutions. In contrast to LEWAF exposures, BE measurements for HEWAFs indicated significant droplet fouling precluding reliable estimation of dissolved oil exposures. Toxicity was modeled and compared to oil toxicity test results reported in the literature for the test oils and HEWAF and LEWAF systems using sum dissolved oil toxic units (TUs) and TPAH₅₀ concentrations. Results highlight the utility of using dissolved oil TUs as a unifying exposure metric and divergent predictions between oils and test systems when TPAH₅₀ is adopted. Given the unrealistic nature of high energy mixing and associated potential for laboratory artifacts associated with microdroplet exposures, particularly for weathered oils, we recommend future oil toxicity testing relies on traditional, less energetic protocols that focus oil hazard assessment on dissolved and chemically dispersed oil exposures.