Offshore iron sand extraction in New Zealand: Potential trace metal exposure of benthic and pelagic biota

Plans to exploit an offshore source of iron sand in South Taranaki Bight (STB), New Zealand, caused concerns that such exploitation may expose benthic and pelagic biota to elevated trace metal concentrations. We conducted dilute-acid extractions and standard elutriate tests to investigate the potential of this exploitation to (1) create a new seafloor with elevated trace metal content, (2) mobilise trace metals during iron sand extraction and, (3) enrich the returning process seawater, which feeds iron sand through mills, with trace metals. We found that recruits of freshly uncovered sediment may encounter higher-than-natural concentrations of cadmium, nickel and chromium (but not of copper, lead, and zinc) and propose to investigate the bioavailability of these metals. Elutriate test with raw and milled iron sand revealed that, for nickel and copper, dilution of the process seawater may be required to meet the local water quality guideline. We argue that this dilution can be achieved by adjustment of the mass and seawater balance of the offshore extraction process. - Highlights: • Planned offshore iron sand mining in NZ caused environmental concerns. • We studied if this mining can expose marine biota to harmful levels of six metals. • Benthic recruits may encounter harmful concentrations of Cd, Ni, and Cr. • The process seawater must be diluted to meet the guidelines for Ni and Cu. • Dilution is possible by adjustment of the extraction's mass and water balances.