Mercury and Metalloid Occurrence in Produced Water: Analytical Approaches and Environmental Risks

Produced water (PW) represents the most significant waste stream generated during offshore oil and gas extraction. It contains a highly complex mixture of both inorganic and organic constituents. In this work, we have quantified totalmercury (THg), arsenic (As), selenium (Se), and antimony (Sb) in PWs from four mature North Sea chalk oilfields (sampled in 2022–2024). In addition, results from cold vapor/hydride generation atomic fluorescence spectrometry (CV/HG-AFS) were compared with inductively coupled plasma mass spectrometry (ICP-MS). Field-average THg measured by CV-AFS (0.267 μg L–1) exceeded both the North Atlantic background and PNEC thresholds in the PW samples considered; Se by HG-AFS also frequently exceeded these thresholds. Instantaneous discharge risk (PEC/PNEC) was >1 for THg in three fields (up to 14) and for Se across most fields, suggesting localized ecological impacts even at low-volume discharge sites. THg showed inverse correlations with outlet temperature and oil-in-water content, illustrating how current discharge controls may not mitigate THg risk. Elevated daily variability of THg (RSD of up to ∼40%) underscores the need for optimized sampling and more frequent monitoring. We recommend CV-AFS as the primary THg reporting technique and propose that metal monitoring complements oil-in-water regulations to improve risk management of offshore PW discharges.