Microplastics in the Delaware River, Northeastern United States

Figure 1. U.S. Geological Survey scientists collecting a microplastics sample in the Delaware River at Callicoon, New York. Photograph by Donald Hamilton, National Park Service.

What microplastics are

Microplastics — plastic particles smaller than 5 millimetres across — are a contaminant of growing concern in water. Knowledge of them in fresh water has grown enormously over the past decade, but little is known about how common they are in National Park Service (NPS) waters, or how much living things there take up.

They come from many sources and are usually grouped by shape:

They reach water by many routes: litter, stormwater runoff, industrial and household wastewater, biosolids spread on land, fallout from the air, and the wearing down of buoys, boats and other gear.

Why ingestion matters

Once in the environment, microplastics keep breaking into ever smaller pieces through mechanical and chemical weathering, down to the nanoscale. The resulting range of sizes lets animals right across the food web — mammals, birds, fish, zooplankton — swallow them. The effects are poorly understood, but can include reproductive harm, oxidative stress, liver toxicity and cell damage.

The study

From 2015 to 2019 the USGS and the NPS studied microplastics and their uptake in park waters in three phases. Phases 1 and 2 covered the St. Croix National Scenic Riverway, Mississippi National River and Recreation Area, and Lake Mead National Recreation Area. Phase 3, summarised here, sampled nine sites with different land uses on the Upper, Middle and Lower Delaware Scenic and Recreational River and its tributaries.

Figure 2. Sampling sites on the Delaware River and selected tributaries. Urban land cover in 2016 from the National Land Cover Database.

Each site gave one water and one sediment sample, taken at base flow. To check uptake, fish and mussels were collected at some sites: open-water-feeding smallmouth bass (Micropterus dolomieu), bottom-feeding white sucker (Catostomus commersonii) and eastern elliptio mussels (Elliptio complanata). Particles were sorted by size — 0.35–1.0 mm and 1.0–5.6 mm for water and sediment, 0.125–5.6 mm for fish and mussels — counted, and sorted into the six types. Methods and all results are in a USGS data release (https://doi.org/10.5066/P9QVIVX3).

What they found

Microplastics were in 100 percent of water and sediment samples, 94 percent of fish and 45 percent of mussels.

Fibres dominated every kind of sample, as earlier studies at other park sites also found.

Figure 3. Microplastic concentrations in water and sediment at each site, upstream to downstream, by particle type.

• Water: highest in the Musconetcong River, a watershed with a relatively high share of urban land (19 percent); lowest in the Delaware at Callicoon, the least urban watershed.
• Sediment: highest in Bushkill Creek, the most urban watershed (33 percent); lowest in the Delaware at Burlington.
• Tire particles stood out in the Bushkill Creek sediment: 598 particles per kg dry weight, 33 percent of all its microplastics. Tire leachate has been shown to be toxic to some fish, and similar concentrations have been found in streambed sediment in other urban watersheds. Sources may include tire wear, crumb rubber on sports fields and playgrounds, and rubberised asphalt.
• Size: small particles (0.35–1.0 mm) outnumbered large ones (1.0–5.6 mm) 4.6 times in water and 2.4 times in sediment. Fish and mussel particles were not sorted by size.

Figure 4. Average microplastics per animal in mussels (eastern elliptio) and fish (smallmouth bass and white sucker) at three Delaware River sites; fish were not sampled at Callicoon.

Figure 5. The share of each particle type in each kind of sample.

What it means

Microplastics are not only present in the park-managed stretches of the Delaware — aquatic animals are eating them. This study did not look at the effects, but earlier studies have found lethal and sublethal ones. Future work could clarify:

1. where the river's microplastics come from and how they get there;
2. how common particles smaller than 0.35 mm and nanoplastics are — left out of this study, but possibly as important or more so for living things;
3. the effects on individual species and the ecosystem.

Sources

• Baldwin, A.K., Spanjer, A.R., Hayhurst, B. (U.S. Geological Survey), and Hamilton, D. (National Park Service), 2021, Microplastics in the Delaware River, northeastern United States: U.S. Geological Survey Fact Sheet 2020-3071. https://pubs.usgs.gov/publication/fs20203071
• The infographic, site table and figures come from the fact sheet's PDF, which the web version lacks.
• Rewritten in hubnx's own words.