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Australian researchers have developed a new method for spotting plastic rubbish on our beaches and successfully field tested it on a remote stretch of coastline.

The satellite imagery tool developed by RMIT University scientists picks up differences in how sand, water and plastics reflect light, allowing plastics to be spotted on shorelines from more than 600km above.

Satellite technology is already used to track the massive amounts of plastic floating around our oceans—from relatively small drifts containing thousands of plastic bottles, bags and fishing nets, up to gigantic floating trash islands like the Great Pacific Garbage Patch, which is three times the size of France.

However, the satellite technology used to spot plastic floating in water doesn't work so well in spotting plastic lying on beaches, where it can easily blend in with the sand.

This latest advance, in the Marine Pollution Bulletin, means we now have an effective way to spot plastic on beaches, where it can be more easily accessed and removed by clean-up operations.

Oceans of plastic

We're currently sending well over 10 million tonnes of plastic trash into our oceans every year. It's estimated that by 2030 that figure could reach 60 million. Study lead author, Dr. Jenna Guffogg, said plastic on beaches can have severe impacts on wildlife and their habitats, just as it does in open waters.

"Plastics can be mistaken for food, larger animals become entangled and smaller ones, like hermit crabs, become trapped inside items such as plastic containers," she said.

"Remote island beaches have some of the highest recorded densities of plastics in the world, and we're also seeing increasing volumes of plastics and derelict fishing gear on the remote shorelines of northern Australia."

Guffogg said if these plastics are not removed, they inevitably fragment further into micro and nano plastics.

"While the impacts of these ocean plastics on the environment, fishing and tourism are well documented, methods for measuring the exact scale of the issue or targeting clean-up operations, sometimes most needed in remote locations, have been held back by technological limitations," she said.

How it works

The team's Beached Plastic Debris Index is what's known as a spectral index, essentially a mathematical formula that sorts patterns of reflected light collected by satellites as they pass over an area, to reveal what you're most interested in seeing from the image.

The remote sensing team at RMIT have developed similar tools for monitoring forests and mapping bushfires from space. The Beached Plastic Debris Index is tailored to mapping plastic debris in beach environments using high-definition data from the WorldView-3 satellite, which orbits the earth in line with the sun at an altitude of 617 km.

To test its performance, 14 plastic targets of around two square meters each were placed on a beach in southern Gippsland, Victoria. Each target was made of a different type of plastic and was smaller than the satellite's pixel size of about 3 m².

The satellite images using the new index were compared with three existing indices, two of which were designed for detecting plastics on land and one for detecting plastics in aquatic settings. The Beached Plastic Debris Index (BPDI) outperformed all three, with the others either struggling to differentiate plastic-contaminated pixels on the beach or tending to mis-classify shadow and water as plastic.

Study co-author, Dr. Mariela Soto-Berelov, said the enhanced separability of the BPDI against water, in addition to sand, suggests its utility is superior in environments where water pixels and plastic-contaminated pixels are likely to coexist.

"This is incredibly exciting, as up to now we have not had a tool for detecting plastics in coastal environments from space," she said. "The beauty of satellite imagery is that it can capture large and remote areas at regular intervals.

"Detection is a key step needed for understanding where plastic debris is accumulating and planning clean-up operations, which aligns with several Sustainable Development Goals, such as Protecting Seas and Oceans."

Soto-Berelov said the next step is to test the BPDI's utility in real life scenarios.

"We're looking to partner with organizations on the next step of this research; this is a chance to help us protect delicate beaches from plastic waste," she said.

Guffogg led this study as part of her joint Ph.D. research at RMIT in Australia and University of Twente in the Netherlands, and now works in the geospatial mapping industry.

She was supported by an Australian Government Research Training Program (RTP) scholarship. The Worldview-3 imagery was accessed via DigitalGlobe, a US geospatial imagery vendor.