Assessing The Toxicity of a Petroleum-based Hydraulic Oil to Aquatic Organisms and the Photo-induced Toxicity of Polycyclic Aromatic Hydrocarbons in Two Amphibian Species

Abstract

A hydraulic oil, UNIVIS™ HVI 13, is used extensively along pipeline networks in Canada and presents an environmental concern with reports of leaking fluid from actuator valves at compressor stations and staining of the surrounding soil. Given the thousands of stations and (in some cases) their geographic locations, there is significant potential for leaked oil to impact nearby water bodies via spring runoff or groundwater migration. The hazard of UNIVIS HVI 13 to aquatic organisms was uncertain; therefore, the acute toxicity of water-extractable constituents of both fresh (UNIVIS-F) and aged (UNIVIS-A) UNIVIS HVI 13 was assessed by exposing a range of standard test organisms to prepared water accommodated fractions (WAF). Bioassays were conducted using Vibrio fischeri (Microtox® test), Daphnia magna (invertebrate), Lemna minor (plant), as well as acute exposures to early life stage fish (Pimephales promelas), and amphibians (Xenopus laevis and Lithobates sylvaticus). Chemical analyses of 1:1 loadings of WAF from UNIVIS-F and UNIVIS-A were performed to confirm concentrations of hydrocarbons and metals. Effective concentration (EC) values were determined for each toxicity test and expressed as g L-1 oil-to-water, derived from serial dilution of 1:1 (880 g L-1) UNIVIS HVI 13 WAF preparation. Based on EC50 values, D. magna was the most sensitive organism to UNIVIS HVI 13 with EC50 values of 114 g L-1 for UNIVIS-F WAF and 66 µg L-1 for F1 hydrocarbon fraction. Results indicated that X. laevis is more sensitive to UNIVIS-A WAF (EC50 value = 372 g L-1) compared to UNIVIS-F WAF (EC50 value > 880 g L-1) but D. magna is more sensitive to UNIVIS-F (EC50 value = 114 g L-1) compared to UNIVIS-F WAF (EC50 value > 362 g L-1). Overall, the acute toxicity data indicated that the water-soluble constituents of UNIVIS HVI 13 pose minimal hazard to aquatic organisms. Due to the presence of polycyclic aromatic hydrocarbons (PAHs) in the environmentally relevant PAH mixture UNIVIS HVI 13, a second experiment was performed to test whether ultraviolet (UV) light enhances the toxicity of UNIVIS HVI WAF. PAHs are ubiquitous environmental contaminants produced mainly from the incomplete combustion of organic materials. Generally, the acute toxicity of PAHs to aquatic organisms is low within the range of their water solubility limits; however, in the presence of ecologically relevant intensities of UV radiation, the acute toxicity of PAHs and petroleum-products containing a mixture of PAHs can increase substantially. The second experiment was designed to determine whether: (a) early-life stage amphibians exhibit photo-induced toxicity to PAHs and a PAH mixture; (b) two amphibian species, Xenopus laevis and Lithobates sylvaticus (wood frog) demonstrate differences in sensitivity; and (c) differences in sensitivity are correlated with species-specific uptake. For each species, 96 h toxicity tests were performed where newly hatched tadpoles of each species (stage-matched) were exposed to benzo(a)pyrene, anthracene, naphthalene or the water accommodated fraction (WAF) of an environmentally relevant hydraulic oil (UNIVIS HVI 13) for 8 h, after which they were transferred to clean water and exposed to UV light for 12 h then allowed to develop without chemical or UV until the end of the 96 h test. Mortality, morphometrics, body burden and deformities were assessed and compared between species. For both larval Xenopus and wood frog, exposure to anthracene or benzo(a)pyrene (8 h) followed by UV light (12 h) resulted in a significant increase in mortality compared to exposure to either PAH alone. Exposure to naphthalene or WAF of UNIVIS HVI 13 and UV light did not result in increased in mortality in larval amphibians at all concentrations tested. Larval Xenopus exhibited decreased length and increased deformities when exposed to UV light alone regardless of chemical treatment. Wood frogs exposed to 2 and 20 μg L-1 anthracene, and 100 μg L-1 benzo(a)pyrene had a significantly higher body burden compared to Xenopus exposed to the same concentrations. The LC50 value for Xenopus exposed to anthracene and UV light was 5 μg L-1, while the LC50 value for wood frogs exposed to anthracene and UV light was 124 μg L-1 leading us to conclude that for anthracene, toxicity thresholds based on Xenopus would be protective for the wood frog. The LC50 values for benzo(a)pyrene and UV light were more comparable between species with 45 μg L-1 for Xenopus and 17 μg L-1 for wood frogs. Based on the significant mortality to low concentrations of anthracene, and the increased sensitivity to UV treatment alone, Xenopus allows for conservative comparisons to the wood frog when assessing the photo-induced toxicity of PAHs and complex mixtures containing PAHs. Overall, the acute toxicity of UNIVIS HVI 13 was minimal to aquatic organisms and the low concentrations of PAHs with phototoxic potential within this environmentally relevant PAH mixture is likely the reason we did not observe photo-induced toxicity in the two amphibian species tested. However, photo-induced toxicity is still an important consideration in the hazard assessment of PAH mixtures, evidenced by the interactive effects of single PAH exposure and UV on tadpole mortality and development. This research provides new data on the acute phototoxic effects of PAHs in two amphibian species and indicates that species differences in response are not driven by differences in PAH uptake.