Study of ecological risk assessment of PAHs and phenols in produced water after partitioning in the water phase

Abstract

The impact of produced water from oil and gas operations is not only a function of its chemical composition but also of the receiving environment (e.g. marine versus freshwater organisms, high energy versus low energy water etc.). The resulting toxicity of produced waters is related to chemical compositions, and varies widely from nontoxic (LC₅₀>100% whole effluent) to moderately toxic (LC₅₀<l% whole effluent). The impact of produced water tends to be chronic rather than acute and therefore determining the agents in the produced water with the greatest impact has proved difficult, particularly in offshore operations where dilution is rapid. However, the polycyclic aromatic hydrocarbon (PAH) fraction in the oil present in the produced water has been proposed as toxic agent. -- In general, regulations prohibit the discharge of produced water containing more than 40 mg/L of oil. The purpose of this paper is to determine the effect of PAHs and phenols in produced water that tend to partition in the water phase once discharged to the ocean, as these compounds will be more readily bioavailable and therefore toxic. Experiments with produced waters from the Hibernia offshore platform and Terra Nova offshore platforms have been performed at Memorial University. The produced water contains dissolved and dispersed oil. In these experiments, the relative amount of PAH and phenol which partition into the water phase after the dispersed oil was separated, and was measured. The results were then used to determine what the hazard quotient (HQ) is for each of the identified PAHs and phenols in the water phase. A hazard index (HI) for PAHs and phenols, which is the summation of all hazard quotients, was then calculated. The HI gives an overview of the worst-case estimated hazard of PAHs and phenols to the marine environment. It was found that there was a strong relation between dispersed oil and the amount of naphthalenes and 4-6 rings PAHs as well as phenols but there was no relation between the amount of dispersed oil in produced water and 2-3 rings PAHs. According to the results of risk assessment for PAHs and phenols, there is no significant hazard from either PAHs or phenols on marine organisms. Also, this study showed the importance of dilution in reducing hazards of produced water in marine organisms. However, it was found there was no significant cancer risk from 4-6 ring PAHs in human.