Abstract
Produced water (PW) from offshore oil production platforms represents the largest discharge of effluent into the offshore environment and is known to contain a complex mixture of dissolved organic compounds and the residues of added production chemicals. The compounds routinely monitored in PW include oil-in-water, BTEX, PAHs (including alkylated naphthalenes, phenanthrenes and dibenzothiophenes), phenol and alkylated phenols. These compounds and the added chemicals are used to calculate the environmental impact factor (EIF), which accounts for both PW composition and release rate, and is used to identify which oil-deriving compounds or production chemicals contribute most to the environmental risk. However, there is a concern from the Norwegian Environment Agency (NEA) that the current input to the EIF calculations may not include all of the relevant compounds responsible for observed toxicity. In the current study, PW was collected from five oil platforms on the Norwegian continental shelf representing fields of different operational ages and crudes with different physical and chemical properties. PW samples were subjected to extraction followed by isolation of non-polar and polar fractions using solid phase extraction recovering 80% of the total GC amenable material. A thorough chemical characterization of the fractions and total extracts was performed, but here only the results from GC and GCMS analysis, including BTEX, PAHs and phenols, are discussed. The total PW extract, the polar and the non-polar fractions were subject to acute toxicity tests using Acartia tonsa nauplii. The LC50 values for the total PW extracts ranged between 0.05 and 1.77 ppm (based on total GC amenable fraction analysis). For three of the PW, the observed toxicity was mainly attributed to the polar fraction. Interestingly, the toxicity of the last two PW was mainly attributed to the non-polar fraction. This study demonstrates that PW toxicity may be associated with compounds that are currently poorly characterized and suggests that in many cases PW toxicity is not directly correlated with the GC quantifiable compounds used for regulating discharges today. NEA and the oil industry are now investigating the whole effluent toxicity (WET) of PW. Our study will provide valuable input to this work. Further studies should be conducted with a wider array of PWs from a range of sources to determine if alternative methods of characterization are needed for regulation of PW discharges.
