ELEVATED MICROPLASTIC CONCENTRATION IN A SMALL TRIBUTARY STREAM UNDER HIGH-FLOW CONDITIONS: IMPLICATIONS FOR THE MOHAWK RIVER, NEW YORK

We sampled water in 21 tributaries of the Mohawk River in upstate NY for microplastic particles in June-July 2018. The tributaries included the three major and 18 smaller tributaries to the Mohawk River. All but one of the streams (Hans Groot Kill, HGK) were under low-flow conditions during sampling; we re-sampled HGK during low-flow conditions. We used a 335-µm net with a 1m x 0.5m opening to trap microplastic particles in flowing water; sampling time ranged from 8 to 27 minutes.

Microplastic particles were present in all of the sampled tributaries. In the low-flow samples, abundance ranged from 10 to 859 particles and concentration ranged from 0.08 to 11.8 particles/m³ of water sampled. With ~35% counted thus far, the HGK high-flow sample stands out: abundance (7,128 particles) is >2x total abundance in all other samples combined (3,078) and concentration (125 particles/m³) is ~10x higher than the highest low-flow sample. In contrast, the HGK low-flow sample had 75 particles and 6.62 particles/m³.

High abundance and/or concentration of particles appears to correlate with storm-drain inflow, sewage leakage, and runoff that transports plastic waste to the stream channel. HGK is exposed at the surface for only ~1.2 km; upstream and downstream reaches are carried in culverts. Samples were collected at the downstream end of the exposed section. Storm-drain and other pipe outfalls punctuate the banks of the open-air HGK. High Enterococci and E. coli levels in HGK suggest that sewage is reaching the stream. Similarly, the low-flow stream with the highest particle abundance and concentration has a documented history of sewage contamination by leaking pipes.

Our research supports the conclusion that microplastic pollution is pervasive in the aquatic environment and that one small stream at high flow (HGK, ~1.5 m wide at low flow, ~4 m at high flow) can deliver more microplastic particles to the Mohawk River than 20 larger streams at low flow during longer time intervals. The dramatic difference between abundance and concentration of particles in the HGK low-flow and high-flow samples hints at the role of high-flow events as major mobilizers of microplastics in the watershed. Truly extreme events such as hurricanes likely result in microplastic transport far exceeding anything that occurs under average flow conditions.