Hurricane Helene rained 18.8% of a year's microplastics onto one North Carolina station

The study's strength is its timing. Collectors were already running in the headwaters of the Richland Creek Watershed and the Cullasaja River Basin before Helene arrived ^[6], and the team was sorting particles that settled in dry spells from those that fell with rain ^[7]. Each site's pre-storm record is its own control, so the spike is not an artifact of switching sites or methods. According to phys.org, this is the first U.S. study to measure deposition before, during and after a hurricane ^[5]. The authors, from Virginia Tech, Western Carolina University, Highlands Biological Station and the University of North Carolina, published it in Integrated Environmental Assessment and Management ^[5].

The headline share needs its denominator. The 18.8% belongs to Highlands Biological Station alone; at Iron Duff and Coweeta the storm supplied 4.2% and 4.6% of the year ^[2]. Helene was roughly four times as large a slice of the year at Highlands as at the other two sites ^[1]. The published summary does not say how long the storm sampling window lasted, or which site saw the threefold rise and which the 22-fold one ^[1]. A claim that any site took in a fifth of its year in a few days goes beyond that record.

In my view the result still supports sampling that can resolve single storms. That support rests on three collectors and one hurricane. An annual total at Highlands would include Helene's 18.8% without showing that it arrived with one event. Austin Gray, the Virginia Tech biologist on the team, put the consequence in terms of place. "Microplastics are depositing all over terrestrial and aquatic areas, especially after a storm event, and that changes what we consider an impacted zone since the particles deposited could have originated from areas far away," he said ^[4].

The storm's contribution may also run past its rainfall. Collection continued for weeks afterwards ^[9], and deposition on dry days rose ^[14]. "We speculate that it was because microplastics deposited by the storm stuck to the surface of the leaves. Later, wind may have blown those particles back off the leaves, causing more microplastics to be deposited on subsequent dry days," said Nathaniel Barrett, a graduate student in Gray's lab ^[15]. If that holds, some storm-delivered plastic was logged as later dry deposition, and the shares above would understate Helene's contribution. "That's something that we're trying to investigate in future research," Barrett said ^[16].

The largest number in the report is a model estimate. Working with Hosein Foroutan in civil engineering at Virginia Tech, Gray's group estimated that Helene released 385 quadrillion additional particles into the atmosphere above normal levels ^[13]. A recent study in Nature put the atmosphere's load at more than 600 quadrillion ^[12]. Both figures are estimates. Taken at face value, the storm's release is at most about 64% of the Nature total ^[2].

"There's been very little investigation into microplastics within small headwaters, particularly low-order stream networks, but these small streams account for more than 70% of the total stream length in the continental U.S.," Gray said. "And if it's happening in the headwaters, then it's going to flow downstream." ^[11] He was plain about scope. "This is just one study," Gray said. "And it's the only one of its kind conducted in the United States." ^[17]