The Aquatic Toxicity of Dissolved Hydrocarbon Gases and the Effect of High Pressure Conditions on Narcosis

Releases of oil and gas to aquatic environments may result in exposure of aquatic organisms to petroleum hydrocarbons. Methods for evaluating potential toxicity arising from exposure to the liquid hydrocarbons are well developed. The target lipid model (TLM) and toxic unit approach assume that each hydrocarbon component acts via narcosis and the toxicity of each constituent is additive. In the case of possible releases from oil and gas operations in the deep sea, dissolved hydrocarbon gases may be present. The TLM has not been validated for hydrocarbon gases. A complicating factor is the marked increase in the aqueous solubility of hydrocarbon gases as hydrostatic pressure increases with increasing depth. Thus, the exposure of aquatic organisms to dissolved phase gases is also expected to increase with increasing depth. Further, elevated pressure has been shown to reverse narcotic effects on biota (including bacteria, mammals and aquatic organisms). Pressures required for a reversal of effects in the laboratory are rather high (circa 100 – 150 atm), but commensurate with water depths and pressures that exist in the vicinity of some well heads (commonly at 3,000 to 10,000 feet, or a total pressure of 100 – 300 atm). The present study describes how the TLM has been extended to quantitatively address both the aquatic hazard posed by dissolved gases and the role of pressure in mitigating the narcotic effects of dissolved gaseous and liquid hydrocarbons. Results indicate that the TLM can be applied to dissolved gases based on the octanol-water partition coefficient of the gas and an adjustment factor to the critical target lipid body burden can be used to account for the effect of elevated pressure on toxicity with increasing depth. This study provides an improved framework for predicting the effects of dispersed oil and gas released in the deep sea environment.