Chemical Characterization and Ecotoxicological Effects of Metal(loid)s in Produced Water

The oil and gas production is still crucial in our society. From pharmaceuticals to fuels, petrol-derived products support our current lifestyle. However, oil and gas production facilities directly affect the surrounding environment besides emitting greenhouse gases. This specific industry's primary waste is the water extracted together with crude materials. The waste is produced water (PW), a complex saline byproduct containing hydrocarbons, metals, metalloids, and production chemicals, which is usually discharged in significant amounts both on- and offshore. As offshore oilfields mature, the volume of PW is expected to grow, raising concerns about its environmental impact when discharged into marine ecosystems. The North Sea oil production began in the late 1970s, but the release of PW into the water column remains the best available practice. In addition, other direct discharges, like drilling fluids and cuttings, might have impacted the marine ecosystems since the beginning of extraction activities. In addition, the only current regulation focuses on the oil-in-water content of PW before discharge, and this does not consider the great variety of pollutants that disperse and dissolve in the seawater. Currently, environmental reporting relies on PW compositional data to assess environmental impact using dispersion models (such as DREAM). However, this approach overlooks the analytical quality of less abundant, yet highly toxic pollutants. This includes toxic metals. Additionally, despite their persistence and bioaccumulation potential, limited research has been conducted on the occurrence of metals and metalloids in PW. Also, official reports lack details on the analytical methods used to characterize this complex wastewater and often approximate concentrations. PW from the Danish sector may have toxic effects that remain unknown for certain marine zooplankton species, including oyster embryos, which play a vital role in the entire food chain. Likewise, the effects of the various offshore discharges on the sediment contamination and benthos communities health are underreported for this area.

Therefore, this study investigates the elemental composition of PW from oilfields in the Danish North Sea offshore sector and matches it with reservoir rock composition. Additionally, this work evaluates the occurrence and mobility of potentially toxic elements (PTEs) in sediments near platforms and analyzes benthic fish tissue to look for metal(loid)s contamination.

By employing advanced analytical techniques such as Sector Field Inductively Coupled Plasma Mass Spectrometry (ICP-SFMS), Cold Vapor Atomic Fluorescence Spectroscopy (CV-AFS), and a proper sample pre-treatment, the presented study provides a comprehensive characterization of the environmental risks connected to metal(loid)s from offshore production facilities.

The results reveal significant variability in PW composition, reflecting differences in reservoir characteristics, production additives, and operational conditions. A total of 35 elements were analyzed, with mercury (Hg) and arsenic (As) identified as key contaminants of concern in PW and flat fish tissue. Geochemical anomalies in sediments near discharge points indicate that offshore discharges influence the mobility and accumulation of elements, such as Ba, Zn, and As, in marine sediments.

Ecotoxicological testing using marine copepods and oyster embryos demonstrates the acute and sub-lethal effects of PW exposure, with suspended particulate matter (SPM) and volatile organic compounds contributing significantly to toxicity. These findings underscore the limitations of current regulatory frameworks, neglecting the broader range of contaminants present in PW discharges, including the ones associated with SPM. Finally, the presented study encourages improved analytical protocols, expanded ecotoxicological assessments, and enhanced regulatory measures to mitigate the environmental risks associated with PW and offshore discharges.