Duncan and Davies have made two generations of electronic bottles together. Their first generation of devices, the ones that sailed the Ganges, had a lot of cell phone towers to ping along the way, so a SIM card would do. But the researchers also wanted to see how plastic bottles might behave once they reach the ocean. They therefore equipped a second generation of GPS. Here, they took inspiration from their previous work on tracking sea turtles: GPS works great on the high seas, away from cell service. (Their designs are open source, so any plastic researcher can create theirs and even improve the system.)
For both versions of the device, they had to figure out how to make a tube full of electronics behave like a real plastic trash can. “It all depends on the center of gravity, actually,” Davies says. They couldn’t, for example, charge all the batteries on one side of the bottle. They also left an open cavity in the bottle, so trapped air would give it buoyancy, keeping about half of the device above the waterline and the other half below. Critically, the bottle had to orient itself so that its antenna was pointed skyward and not towards the bottom of the river.
“We played a lot in buckets in our back gardens, floating the bottles, testing the setups, doing whatever it took,” Davies says. “The right wall thickness – the right all– until we have something that mimics a bottle. So we threw another bottle next to it and they were floating in the same orientation.
Confident of the seaworthiness of the GPS versions of the bottles, the team deployed them in Bangladesh, near the mouth of the Ganges, and also in the Bay of Bengal. They then observed how the aircraft moved around the Bay of Bengal along similar trajectories. One bottle traveled nearly 1,800 miles in 94 days. They tended to head west, towards the east coast of India, eventually getting caught up in powerful vortex systems. “On the map, we see a kind of spiral starting to happen,” Duncan says. “This is the indication of where we might find plastic accumulations.”
And that turns out to be the goal of carefully designing plastic bottles to survive grueling journeys down the Ganges and across the Bay of Bengal: it shows where garbage tends to collect in these waters. Previously, scientists had developed models—Depending on variables such as ocean currents, winds and the shape of coastlines — to show how pieces of plastic can move around in the environment. These patterns indicate that the trash tends to stay along the coast, wash off a bit outside, then come in, over and over again. The findings from this new work provide strong real-world evidence to support this dynamic: Electronic bottles tended to hug the coastline, traveling hundreds of miles parallel instead of immediately moving offshore.