Emerging chemical threat in waterways can be deadly to salmon, trout

Lapeer, Mich. — Most people depend on their cars to transport them to their outdoor destinations. And despite widely publicized, spirited debates regarding the merits of electric powertrains and autonomous driving, for the foreseeable future, those cars will still have tires.

Perhaps lesser know is that the rubber in those tires contains a chemical called N-(1,3- dimethylbutyl)-N’-phenyl-pphenylenediamine, or 6PPD for short. The chemical 6PPD is used by tire manufacturers to extend a tire’s life but as a tire wears, it leaves behind small rubber particles containing 6PPD which, after a heavy rain, can wash into rivers and streams and transform into a compound called 6PPD-quinone – a compound that studies have shown to be deadly to coho salmon in the northwestern United States.

“6PPD-quinone is an emerging concern because it affects the reproductive potential of trout and salmon,” said Michigan Department of Natural Resources Fisheries Division Chief Randy Claramunt.

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The connection between cars used to transport people to their favorite fishing destinations and the potential damage to the fisheries themselves (literally where the rubber meets the road) is the subject of a recent study led by the Water Resources Division of the Michigan Department of Environment, Great Lakes, and Energy (EGLE).

Prior to the publication of the EGLE study, most research focused only on tire wear as the primary source of 6PPD-quinone contamination. Little investigation into other potential sources – specifically, the use of recycled tires for road paving – was done, and none in Michigan.

While recycling is generally viewed as an environmentally friendly way of disposing of old tires that would otherwise end up in a landfill, some used tire recycling solutions can lead to the creation of 6PPD-quinone.

One way that used tires are recycled is by grinding them into small bits – called crumb rubber – and using them as turf in indoor sports arenas (they are the dark crumbs often seen during slow-motion replays when football players drag their feet across the turf). Because of the low likelihood of being washed into a river or creek, crumb rubber in sports arenas poses little risk of 6PPD-quinone contamination. That is not the case when these same ground-up tires are used in the paving of roads.

Crumb Rubber Mix Asphalt (CRMA) is a pavement material made from a mix of regular asphalt and crumb rubber, and in addition to its perceived environmental benefits, is a quieter, more durable road surface. It’s no surprise then that CRMA use is very popular, consuming 220 million pounds of recycled tires across the country each year – the equivalent of about 12 million tires according to a 2016 report by the U.S. Environmental Protection Agency.

Other pavement materials that combine rubber from recycled tires and asphalt are Asphalt- Rubber Chip Sealer and Asphalt- Rubber Cape Sealer. Unlike crumb rubber, these solutions blend melted rubber from recycled tires with asphalt for a gooey, honey-like coating that restores a smooth surface to roads for a fraction of the cost of a new asphalt surface.

In 2021, Asphalt-Rubber Chip Sealer was used to repair 100 miles of Michigan roads, some of which were adjacent to or intersected known salmon spawning rivers, which provided an opportunity to study the relative risks of all potential sources of 6PPD-quinone contamination.

The EGLE study collected samples from three such waterways, as well as samples from waterways near historic and recently paved CRMA roadways.

Another sample was collected from a waterway upstream of a recently paved CRMA road to distinguish between tire wear and CRMA sources of 6PPD-quinone. Five additional samples were collected directly from puddles on CRMA and Asphalt-Rubber Chip Sealer road surfaces to assess 6PPD-quinone creation before it enters a watershed. Samples were also taken downstream from two industrial crumb rubber storage facilities.

Seventeen water samples from Michigan waterways were collected in all, and 6PPD-quinone was detected in two of them: Plaster Creek, an industrial site downstream of a crumb rubber stockpile, and Millpond Drain, a highway water retention drain near Bay City. All puddle samples contained 6PPD-quinone, the highest concentration sample being at the entrance to an industrial crumb rubber storage location where trucks hauling rubber crumb material enter and leave the facility.

Detection of 6PPD-quinone in every puddle sample and two of the 17 waterway samples confirmed that at least some of the deadly compound is finding its way into Michigan’s watersheds, but additional research is needed to determine the extent to which it is affecting reproductive success in Michigan’s trout and salmon streams.

“It (6PPD-quinone) may not be the only thing driving a decrease in trout and salmon reproduction, but it’s another variable that’s going in the wrong direction,” Claramunt said.

Given the cultural and economic importance of Michigan’s trout and salmon fisheries, this preliminary study highlights the importance of continued investigation.