Oil spills can increase levels of toxic arsenic in the ocean, creating an additional long-term threat to the marine ecosystem, according to a team from Imperial College London.

Arsenic occurs naturally in the ocean, but sediments on the sea floor filter it out of seawater, which keeps the levels of naturally occurring arsenic low. However, arsenic is also flushed into the ocean in wastewater from oil rigs and from accidental oil spills and leakages from underground oil reservoirs.

Now, a team of researchers from Imperial College London have discovered that such oil spills and leakages clog up sediments on the ocean floor with oil, which prevents the sediments from bonding with arsenic and burying it safely underground with subsequent layers of sediment.

The scientists say this shutdown of the natural filtration system causes arsenic levels in seawater to rise, which means that it can enter the marine ecosystem, where it becomes more concentrated and poisonous the further it moves up the food chain.

The scientists say their work demonstrates how the chemistry of sediments in the Gulf of Mexico may be affected by the current oil leak there.

Prof Mark Sephton, from the Department of Earth Science and Engineering at Imperial College London, said: ’We can’t accurately measure how much arsenic is in the Gulf at the moment because the spill is ongoing. However, the real danger lies in arsenic’s ability to accumulate, which means that each subsequent spill raises the levels of this pollutant in seawater. Our study is a timely reminder that oil spills could create a toxic ticking time bomb, which could threaten the fabric of the marine ecosystem in the future.’

Wimolporn Wainipee, a postgraduate from the Department of Earth Science and Engineering at Imperial College London, said: ’We carried out our study before the leak in the Gulf of Mexico occurred, but it gives us a big insight into a potential new environmental danger in the region. Thousands of gallons of oil are leaked into the world’s oceans every year from big spills, offshore drilling and routine maintenance of rigs, which means many places may be at risk from rising arsenic levels, which could in the long run affect aquatic life, plants and the people who rely on the oceans for their livelihoods.’

For their research, the team analysed a mineral called goethite, one of the most abundant ocean sediments in the world, which is an iron-bearing oxide.

The team carried out experiments in the laboratory that mimicked conditions in the ocean, to see how the goethite binds to arsenic under natural conditions. They discovered that seawater alters the chemistry of goethite, where low pH levels in the water create a positive change on the surface of goethite sediments, making them attractive to the negatively charged arsenic.

However, the scientists discovered that, when they added oil, it created a physical barrier, covering the goethite sediments, which prevented the arsenic in the oil from binding to them. The team also found that the oil changed the chemistry of the sediments, which weakened the attraction between the goethite and arsenic.

In the future, the researchers plan to analyse other minerals such as clays and carbonates that are sediments on the ocean floor. Sediment content varies from ocean to ocean and the researchers will analyse how oil affects their ability to bind to arsenic after a spill.

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