Evaluating the toxicity of produced water from carbonate reservoirs in the North sea using oyster Embryo development tests

Produced water (PW) is the largest waste stream from offshore oil and gas production and a major contributor to marine pollution. In the North East Atlantic, approximately 287 million m³ of PW-containing hydrocarbons, suspended particulate matter (SPM), metals, and production chemicals-is discharged annually, including ∼18 million m³ from Danish operations. While current OSPAR regulations and risk-based approaches (RBA) rely on oil-in-water limits and acute tests PNECs, they often overlook chronic and sublethal effects, particularly from unregulated naturally occurring compounds. Early life stages of marine invertebrates, such as the Pacific oyster Magallana gigas, are highly sensitive to such contaminants and serve as effective bioindicators. This study applies the M. gigas embryo bioassay to PW samples from multiple Danish North Sea fields, using manipulations that remove SPM and purge volatile compounds to help identify toxicity drivers. The results showed that most of PW samples inhibited larval development, with significant differences among fields and sampling times, even in identically treated samples. PW from two out of four fields exhibited notably higher toxicity, potentially linked to elevated SPM content, metals, and production practices such as waterflooding. Evidence suggests that SPM may act as a carrier for pollutants, slightly increasing toxicity. Temporal variation in PW toxicity further indicates that more frequent monitoring is needed to capture fluctuations in discharge composition and associated risks. Given that carbonate reservoirs dominate global oil and gas production and M. gigas is established in the North Sea, these findings have both regional and global relevance. They underscore the importance of integrating compositional data with field-specific toxicity responses to guide targeted management strategies and support the industry's goal of "zero harmful discharge" by 2050.