Hurricane Helene dumped a huge share of yearly microplastics on remote mountain streams

When Hurricane Helene slammed into the southern Appalachian Mountains on September 25, 2024, it did more than flatten forests and flood valleys. It also delivered an extraordinary pulse of microscopic plastic pollution to some of the most remote waterways in the eastern United States. According to a new study led by researchers at Virginia Tech in collaboration with Western Carolina University, Highlands Biological Station, and the University of North Carolina at Chapel Hill, rainfall from the storm drove microplastic deposition at three long-term monitoring sites in Western North Carolina to levels three to 22 times higher than what had been measured before the hurricane arrived. At one site, the single storm accounted for nearly one-fifth of all the microplastics deposited over the entire year, a finding that is forcing scientists to rethink which landscapes should be considered affected by plastic pollution.

The research, published in the journal Integrated Environmental Assessment and Management, is the first study in the United States to measure microplastic deposition before, during, and after a hurricane. That timing was largely a matter of circumstance. The research team had already deployed atmospheric deposition samplers at three sites to track how many plastic particles settle out of the air in the headwaters of the Richland Creek Watershed and the Cullasaja River Basin. When Helene struck with 20 to 30 inches of rain and catastrophic flooding, the instruments were running continuously, capturing one of the most complete records ever assembled of how a major storm reshapes the movement of airborne plastics through a mountain landscape.

The three monitoring sites told a consistent but geographically variable story. At Highlands Biological Station, operated with Western Carolina University in the high-elevation town of Highlands, the hurricane accounted for 18.8 percent of the total annual microplastic deposition, meaning that a few days of storm weather delivered nearly a fifth of a full year’s plastic fallout. Farther north, the storm contributed 4.2 percent of annual deposition at Iron Duff in Haywood County and 4.6 percent at Coweeta Hydrologic Laboratory in Macon County. The differences likely reflect local rainfall totals, topography, and the trajectories of moisture-laden winds, but the direction of the effect was unmistakable at every site: the storm dramatically amplified the rain of plastic particles falling onto even the most protected forested catchments.

What makes these numbers striking is where they were recorded. Microplastics, the fragments and fibers smaller than five millimeters that shed from textiles, packaging, tires, and countless other sources, are usually studied in urban watersheds near roads, industry, and dense development. The headwaters sampled in this study are rural, forested, and generally regarded as pristine or minimally impacted. Austin Gray, a biologist at Virginia Tech who leads the Aquatic Toxicology and Ecology Lab, said the microplastic footprint Helene left behind was substantial and that the particles deposited after a storm could have originated from areas far away. In other words, the plastic found in a mountain stream may have been lofted from cities and industrial regions hundreds of miles distant, carried aloft, and then washed out of the sky by torrential rain.

The scale of the atmospheric reservoir that makes such long-distance transport possible is staggering. A recent study published in Nature estimated that more than 600 quadrillion microplastic particles are suspended in the atmosphere at any given time, a number so large it requires a six followed by fifteen zeros. Working with Hosein Foroutan, a civil engineer at Virginia Tech who models atmospheric transport, Gray and his collaborators estimated that Hurricane Helene released an additional 385 quadrillion microplastic particles into the air above normal background levels. Storms of this magnitude, the researchers suggest, act like enormous pumps, scouring particles from surfaces, churning them into the boundary layer, and redistributing them across entire regions as the rain comes down.

The study also revealed a subtler aftermath effect that may prove important for understanding how plastics cycle through ecosystems. In the weeks following Helene, the researchers recorded an uptick in microplastics deposited on dry days, when no rain was falling to scrub particles from the air. Nathaniel Barrett, a Virginia Tech graduate student in biological sciences working in Gray’s lab, offered a possible explanation: microplastics deposited by the storm may have stuck to the surfaces of leaves, and subsequent winds could have blown those particles back off the vegetation, causing elevated deposition on later dry days. The team is now investigating this resuspension pathway in follow-up work, because if storm-deposited plastics can be re-entrained by wind, a single hurricane could continue seeding the landscape with plastic long after the floodwaters recede.

The implications extend well beyond the sampling sites. Small headwater streams, the kind monitored in this study, account for more than 70 percent of the total stream length in the continental United States, yet very little research has examined microplastic contamination in these low-order networks. Gray emphasized that whatever settles into the headwaters does not stay there; it flows downstream, eventually reaching larger rivers, reservoirs, and coastal waters. A storm-driven pulse of plastic delivered to a remote forest catchment therefore becomes a distributed source that can propagate through entire drainage networks, potentially exposing aquatic insects, fish, and the food webs that depend on them to particles whose origins may lie in distant metropolitan areas.

Several major questions remain open. The researchers acknowledge that the full spatial extent of Helene’s microplastic footprint has not been mapped, and the study captures only three sites in one mountainous region during one storm. The risks that storm-delivered microplastics pose to human health and to biodiversity are still poorly characterized, particularly in the Southeast, a region Gray described as home to a great deal of vulnerable biodiversity and one that faces storms of growing intensity and frequency. Better tools for tracking the movement of microplastics through the environment, including coordinated sampling networks and sustained funding support, will be needed to understand whether the patterns documented after Helene are typical of major hurricanes or exceptional to it.

Methodologically, the study’s design offers a template for future work. By distinguishing between particles that settled during dry periods and those that settled during rainy periods, the researchers could separate the contributions of dry deposition, in which particles simply fall or are blown onto surfaces, from wet deposition, in which raindrops and cloud droplets carry plastics to the ground. The fact that samplers were already in place and running when the hurricane hit, an accident of timing that is unlikely to be repeated often, underscores the value of continuous long-term monitoring at fixed sites. Without the pre-storm baseline, the extraordinary spike recorded during Helene would have been impossible to quantify, and the post-storm dry-day uptick would have gone unnoticed.

For now, the study stands as a vivid demonstration that no landscape is too remote to escape plastic pollution, and that extreme weather is a powerful and underappreciated vector for moving it. As climate change drives more intense rainfall and stronger tropical systems, the atmospheric conveyor belt that carries microplastics from cities to mountains is likely to run harder and more often. Gray noted that this is just one study, and the only one of its kind conducted in the United States, but the picture it reveals is sobering: a single hurricane can rewrite the annual pollution budget of a protected forest watershed in a matter of days, depositing plastics in places that maps and assumptions had long classified as untouched.

Subject of Research: Hurricane-driven atmospheric microplastic deposition in remote Appalachian headwater streams

Article Title: Hurricane Helene deposited nearly one-fifth of annual microplastics at one North Carolina site

Article References: Hurricane Helene deposited nearly one-fifth of annual microplastics at one North Carolina site. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: microplastics, Hurricane Helene, atmospheric deposition, headwater streams, Appalachian Mountains, North Carolina, plastic pollution, extreme weather, Virginia Tech, water quality, storm events, environmental monitoring