Duncan and Davies actually cobbled together two generations of electronic bottles. Their first generation of devices, the ones that sailed along the Ganges, had plenty of cell 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 get to the ocean. So they outfitted a second generation with GPS. Here they took inspiration from their prior work tracking sea turtles: GPS works great on the open ocean, far away from any cell service. (Their designs are open source, so any plastic researcher can build their own, and even improve upon the system.)
For both versions of the device, they had to figure out how to make an electronics-stuffed tube behave like a real piece of plastic trash. “It’s all about the center of gravity, really,” says Davies. They couldn’t, for instance, load all the batteries onto one side of the bottle. They also left an open cavity within the bottle, so the trapped air would give it buoyancy, keeping about half the device above the waterline and half below. Critically, the bottle had to orient itself such that its antenna pointed skyward, not toward the river bottom.
“We played a lot in buckets in our back gardens, floating the bottles, testing configurations, getting it just right,” Davies says. “The right thickness of wall—the right everything—until we got something that would mimic a bottle. So we threw another bottle in beside it, and they would float 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 watched how the devices scooted around the Bay of Bengal along similar trajectories. One bottle traveled nearly 1,800 miles in 94 days. They tended to head westward, toward the east coast of India, eventually getting caught up in strong eddy systems. “On the map, we see kind of spiraling starting to happen,” Duncan says. “That’s the indication of where we might be finding accumulations of plastic.”
And that turns out to be the point of carefully engineering plastic bottles to survive grueling journeys down the Ganges and across the Bay of Bengal: It shows where trash tends to gather in these waters. Previously, scientists had developed models—based on variables like ocean currents, winds, and the shapes of coastlines—to show how pieces of plastic might travel around the environment. These models indicate that trash tends to stick around the coast, washing a little ways out, then washing back in, over and over. This new work’s findings lend strong real-world evidence to back up that dynamic: The electronic bottles tended to hug the coastline, traveling hundreds of miles parallel to it instead of immediately washing far out to sea.