Researchers report largely clear watershed but warn of microplastic spread after Helene; prescribed burn planned

Dr. Peter Bates and watershed colleague Jerry Miller told the Town of Waynesville council on March 11 that long-term monitoring shows very clear water within the town's watershed but that extreme storms and regional sources are moving sediment and microplastics downstream.

The presentation summarized more than a decade of monitoring at multiple stations in the Richland Creek watershed, ongoing invasive-plant control and forest stewardship work, and regional sampling that found microplastics present in streams, flood deposits and biological samples.

The speakers said routine samples at the "Old Ball 1" monitoring station and most sites show very low suspended-sediment concentrations: more than 90% of samples are under about 10 milligrams per liter, a level the presenters characterized as clear water. But Hurricane Helene produced an extreme flood that briefly pushed turbidity and suspended-sediment values well above typical ranges and delivered large slugs of sediment to the lower Allen Creek channel and the reservoir, they said. The flood also moved substantial quantities of plastic debris downstream into Lake Junaluska and the reservoir area.

Why it matters: Town officials and stewardship partners said the watershed's overall water quality remains good under normal flow conditions, but episodic storm flows can rapidly change conditions and move sediment and contaminants into storage areas downstream. The presentation identified both local work (invasive plant removal, targeted thinning and planned prescribed fire) and broader regional issues (atmospheric deposition of microplastics) as relevant to future stewardship and planning.

Most of the stewardship work described is aimed at maintaining forest diversity and limiting non-native invasive plants near the reservoir. Bates said the watershed's conservation easement, held jointly by the Conservation Trust for North Carolina and the Southern Appalachian Highlands Conservancy, restricts treatments near water and requires easement-holder approval for plans. He described an effort to avoid using herbicides near the reservoir and to pilot non-herbicide methods for controlling multiflora rose in the 300-foot buffer where herbicides are discouraged.

Jerry Miller summarized monitoring methods and results. He described automated pressure transducers and ISCO samplers that record flow and collect samples during rising stages, and multi-parameter sondes that log turbidity, temperature and conductivity every five minutes. At Old Ball 1, which has a long record dating to 2008, more than 90% of samples are at or below about 10 mg/L suspended sediment. Miller said the flood associated with Hurricane Helene produced the highest discharges recorded at that site; an outlier suspended-sediment sample reached about 3,400 mg/L during short pulses likely tied to debris flows. He said conditions returned to baseline relatively quickly after the event.

Miller also presented a regional study of microplastics in the Richland Creek and Upper Pigeon River basins. The team sampled eight locations from headwaters through Waynesville and analyzed water, sediment and benthic organisms. Average concentrations in water were about 9.7 particles per liter (roughly 20 particles per two-liter bottle), with most particles fibers consistent with textile-derived materials (nylon/polyester). During high-flow events such as Helene, the number of particles passing downstream rose dramatically because of increased discharge: at one Waynesville cross section the team estimated particle loads climbed from roughly 1.2 billion particles per day to about 61 billion particles per day during the storm.

The presenters also reported microplastic concentrations in flood deposits and streambed sediments in ranges that they described as high compared with many global records, and they showed small aquatic organisms (caddisfly larvae) that contained microplastics, which could move plastics into the food web. Miller said atmospheric deposition (plastic-bearing dust and fibers falling during rain) appears to be a source in upland areas; atmospheric samplers at Highlands Biological Station and other high-elevation sites recorded high deposition rates, particularly during heavy rains associated with Helene.

On forest management, Bates described ongoing thinning, understory work and the goal of increasing native forest structural diversity to improve resistance and resilience to disturbance. He said the town has a 135-acre prescribed burn plan developed with the North Carolina Forest Service; easement holders are reviewing the plan because species of concern (a northern water shrew) occur near the proposed burn unit. Bates said the team hopes to complete the burn in the current prescribed-burn season if the easement holders grant approval.

During a brief question-and-answer period, council members asked whether the reservoir clarity matches the headwater sites and whether long-term capacity of the reservoir is threatened by the sediment influx. Miller said the town's water-treatment plant data, not part of this monitoring program, would better answer reservoir clarity; he added that most of the flood-deposited material was sand-sized and likely to be deposited near shorelines, and he did not expect a major reduction to reservoir capacity from the Helene deposits.

The presenters invited council members and staff to visit the watershed and review monitoring stations and stewardship work in the field.

Ending: Council members thanked the presenters and said they appreciated the long-term monitoring and the preparations for prescribed fire and invasive-plant control. No council action was required of the town based on the presentation at the meeting.