Recommendations for advancing media preparation methods used to assess aquatic hazards of oils and spill response agents

Abstract

Laboratory preparation of aqueous test media is a critical step in developing toxicity information needed for oil spill response decision-making. Multiple methods have been used to prepare physically and chemically dispersed oils which influence test outcome, interpretation, and utility for hazard assessment and modeling. This paper aims to review media preparation strategies, highlight advantages and limitations, provide recommendations for improvement, and promote the standardization of methods to better inform assessment and modeling. A benefit of media preparation methods for oil that rely on low to moderate mixing energy coupled with a variable dilution design is that the dissolved oil composition of the water accommodation fraction (WAF) stock is consistent across diluted treatments. Further, analyses that support exposure confirmation maybe reduced and reflect dissolved oil exposures that are bioavailable and amenable to toxicity modeling. Variable loading tests provide a range of dissolved oil compositions that require analytical verification at each oil loading. Regardless of test design, a preliminary study is recommended to optimize WAF mixing and settling times to achieve equilibrium between oil and test media. Variable dilution tests involving chemical dispersants (CEWAF) or high energy mixing (HEWAF) can increase dissolved oil exposures in treatment dilutions due to droplet dissolution when compared to WAFs. In contrast, HEWAF/CEWAFs generated using variable oil loadings are expected to provide dissolved oil exposures more comparable to WAFs. Preparation methods that provide droplet oil exposures should be environmentally relevant and informed by oil droplet concentrations, compositions, sizes, and exposure durations characteristic of field spill scenarios. Oil droplet generators and passive dosing techniques offer advantages for delivering controlled constant or dynamic dissolved exposures and larger volumes of test media for toxicity testing. Adoption of proposed guidance for improving media preparation methods will provide greater comparability and utility of toxicity testing in oil spill response and assessment.

Keywords: Media preparation; Oil toxicity testing; Passive dosing; Water accommodated fraction.

Fig. 1.

Overview of methods used for aqueous test media preparation

Fig. 2.

Comparison of physically (WAFs) and chemically (CEWAFs) dispersed oils during (A) mixing and (B) settling after 4 hrs. Note the different behavior of floating oil and increased droplet concentration in CEWAFs than WAFs for both oils.

Fig. 3.

VDROP simulations showing influence of mixing speed on median oil droplet size for two oils in 4 L aspirator bottle with a 3.8 cm length stir bar during WAF preparation. Solid lines denote the median droplet size (d50) while lower and upper shaded area show the predicted droplet sizes corresponding to 10^th (d₁₀) and 90^th (d₉₀) percentiles of the droplet distributions.

Fig. 4.

The rising time for spherical droplets to travel 10 cm in the water column due to buoyancy based on Stokes Law. Different lines highlight the influence of oil density on droplet rise times.

Fig. 5.

Schematic of the continuous oil dosing system described by Nordtug et al. (2011).