Abstract
Regulation of produced water (PW) discharges on the Norwegian continental shelf is based on a maximum oil-in-water limit of 30 mg L¬1. However, the use of conventional oil quantification methods based on traditional GC is limited when it comes to polar compounds that originate from produced crude oils. Furthermore, the use of treatment or production chemicals might also contribute to the overall PW toxicity. As a result, there may be discrepancies between measured concentrations of organic compounds and the total PW components that contribute to toxicity. In the current study, PW was collected at the “point of release” from four oil platforms on the Norwegian continental shelf. PWs were selected from oil fields of different operational ages, which produce oils exhibiting different physical and chemical properties. Samples were subjected to extraction with dichloromethane, followed by fractionation into apolar and polar fractions using solid phase extraction, recovering 80 % of the total GC amenable material in these fractions. The total extracts and fractions were thoroughly characterized using GC-¬MS, GCxGC-¬MS, LC¬Orbitrap-MS, and by direct infusion FT¬ICR-MS. The total PW extract, as well as the apolar and polar fractions were subject to acute toxicity tests using nauplii of the marine copepod Acartia tonsa.
LC50 values for the total PW extracts ranged between 0.05-¬0.98 mg L¬1 (based on total GC amenable fraction analysis). For three of the PWs, the toxicity was mainly attributed to the polar fractions, with LC50 values ranging between 0.17-¬0.57 mg L¬1. Interestingly, toxicity was mainly attributed to the apolar fraction of the fourth PW, with an LC50 of 0.05 mg L¬1. For the PWs where toxicity mostly related to the polar fraction, this fraction spanned from 16-¬55% of the total PW (GC amenable fraction analysis). For the PW where toxicity mostly related to the apolar fraction this was 35%. This study demonstrates that PW toxicity may be associated with compounds that are currently poorly characterized. Polar fractions may contain compounds not amenable to GC, or that contribute to the GC¬-based quantification of oil in water. This suggests that PW toxicity is not directly correlated with the GC quantifiable compounds that are used for regulating discharges today. Further studies should be pursued 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.
LC50 values for the total PW extracts ranged between 0.05-¬0.98 mg L¬1 (based on total GC amenable fraction analysis). For three of the PWs, the toxicity was mainly attributed to the polar fractions, with LC50 values ranging between 0.17-¬0.57 mg L¬1. Interestingly, toxicity was mainly attributed to the apolar fraction of the fourth PW, with an LC50 of 0.05 mg L¬1. For the PWs where toxicity mostly related to the polar fraction, this fraction spanned from 16-¬55% of the total PW (GC amenable fraction analysis). For the PW where toxicity mostly related to the apolar fraction this was 35%. This study demonstrates that PW toxicity may be associated with compounds that are currently poorly characterized. Polar fractions may contain compounds not amenable to GC, or that contribute to the GC¬-based quantification of oil in water. This suggests that PW toxicity is not directly correlated with the GC quantifiable compounds that are used for regulating discharges today. Further studies should be pursued 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.
