Correspondence Comment on “Effect of Methyl tert-Butyl Ether on the Bioconcentration and Photoinduced Toxicity of Fluoranthene in Fathead Minnow Larvae (Pimephales promelas)” The authors of a recently reported laboratory aquatic toxicity investigation (1) broadly concluded “... that, because of enhanced bioconcentration factors, the photoinduced toxicity of fluoranthene and potentially other PAHs will be greater in aquatic environments where reformulated gasoline containing MTBE is utilized in motorized watercraft”. This conclusion was based upon reported laboratory coexposures of 4-day-old fathead minnow larvae (Pimephales promelas) to MTBE (at 0 or 40 µg/L) and fluoranthene (at 0, 5, or 15 µg/L) concentrations under simulated sunlight conditions. The coexposures increased the photoinduced toxicity of fluoranthene, indicated by reductions in the median lethal times (LT50), as compared to fluoranthene-only exposures at 5 and 15 µg/L concentrations. Cho et al. (1) specifically attributed the increased fluoranthene phototoxicity to enhanced bioconcentration and hypothesized that such phenomena may include other PAHs where MTBE-containing gasolines are used in motorized watercraft. We question whether such an unqualified conclusion is warranted given documented, ambient concentrations of both MTBE and fluoranthene in surface waters that are significantly lower than the test concentrations used in these laboratory toxicity tests. While we agree with the authors that there is a need to examine the toxicity of mixtures of co-occurring chemicals in aquatic environments, it is equally important to examine such interactions at realistic and prevailing ambient concentrations. In the Introduction to the paper, Cho et al. (1) appropriately document the ranges of ambient concentrations of MTBE reported from surface waters and acknowledge, if only by reference, that MTBE concentrations greater than approximately 10 µg/L are not commonly reported (2, 3). Published reports indicate that, even under the authors’ assumed exposure conditions, such elevated concentrations seem to be temporally and spatially limited to the vicinities of marinas during periods of abundant watercraft activity (i.e., warm season, weekends, and holidays; 2). In a related, previous publication of MTBE/fluoranthene coexposures, a test concentration of 1 µg of MTBE/L was selected and reported by one of the current project authors (4). We believe that unitary micrograms of MTBE per liter concentrations are more realistic and relevant aquatic toxicity test conditions than the 40 µg MTBE/L concentration selected for the subject study. An equally important issue concerns ambient concentrations of fluoranthene in surface waters, even in the presence of burned and unburned fuel. The aqueous solubility of neat fluoranthene is variously reported as 120-300 µg/L, and its concentration in gasolines is reported as ranging from 0.7 to 7.5 mg/L (5 and refinery product data). Given the high percentage of unburned exhaust gasoline from carbureted two-cycle engines cited by Cho et al. (1), it is expected that fluroanthene concentrations would be similar to those in
10.1021/es030483h CCC: $25.00 Published on Web 08/22/2003
2003 American Chemical Society
raw gasolines (i.e., unitary mg/L). The most liberal Raoult’s law (mole fraction solubility) calculation of fluoranthene concentrations in water, at equilibrium with raw gasoline or unburned fuel, indicates that such concentrations must be ,1 µg/L. Indeed, a summary of ambient concentrations of fluoranthene reported from primary literature sources indicates a range of only 10s-100s ng/L in surface waters where it is detected (5). A relevant and site-specific study of total phototoxic PAHs (sum of anthracene, fluoranthene, pyrene, benz[a]anthracene, benzo[b]fluoranthene, and benzo[a]pyrene concentrations) in Lake Tahoe in proximity to elevated levels of boating activity documented “peak” concentrations of only 10-70 ng/L (6). It is our view, therefore, that coexposures of MTBE at 40 µg/L and fluoranthene, even at 5 µg/L, are likely to be rare and unusual events even in water bodies where motorized watercraft are utilized. If the broad conclusion of Cho et al. (1) is to be tested under relevant ambient conditions, it is our view that test concentrations should be
