Vertical Distribution of Microplastics in the Water Column and Surficial Sediment from the Milwaukee River Basin to Lake Michigan

Abstract

Microplastic contamination was studied along a freshwater continuum from inland streams to the Milwaukee River estuary to Lake Michigan and vertically from the water surface, water subsurface, and sediment. Microplastics were detected in all 96 water samples and 9 sediment samples collected. Results indicated a gradient of polymer presence with depth: low-density particles decreased from the water surface to the subsurface to sediment, and high-density particles had the opposite result. Polymer identification results indicated that water surface and subsurface samples were dominated by low-density polypropylene particles, and sediment samples were dominated by more dense polyethylene terephthalate particles. Of the five particle-type categories (fragments, films, foams, pellets/beads, and fibers/lines), fibers/lines were the most common particle-type and were present in every water and sediment sample collected. Fibers represented 45% of all particles in water samples and were distributed vertically throughout the water column regardless of density. Sediment samples were dominated by black foams (66%, identified as styrene–butadiene rubber) and to a lesser extent fibers/lines (29%) with approximately 89% of all of the sediment particles coming from polymers with densities greater than 1.1 g cm–3. Results demonstrated that polymer density influenced partitioning between the water surface and subsurface and the underlying surficial sediment and the common practice of sampling only the water surface can result in substantial bias, especially in estuarine, harbor, and lake locations where water surface concentrations tend to overestimate mean water column concentrations.

Microplastics in Lake Mead National Recreation Area, USA: Occurrence and biological uptake

“…Concentrations in surficial sediment samples in the current study (87.5-1,010 particles/kg dw) are similar to those reported in the Milwaukee Harbor and nearshore Lake Michigan (Wisconsin, USA; 39.5-319 particles/kg dw) [60]; nearshore Lake Ontario (Canada; 40-4,270 particles/kg dw) [23]; and an urban lake in London (England; 539 particles/kg dw) [61]. The overlap in sediment microplastic concentrations between Lake Mead NRA and these more urbanized locations may point to a dispersed pathway such as atmospheric deposition of microplastic particles on the landscape [8,11,53].…”

“…Concentrations of microplastics in water samples in the current study (0.44-9.70 particles/ m 3 ) are somewhat lower than a reported value for Brownlee Reservoir, another large reservoir in the western U.S. (13.7 particles/m 3 ; Snake River, Idaho/Oregon) [52]. Concentrations in water samples from the St. Croix and Mississippi Rivers (0.80-4.84 particles/m 3 ) [58], and from the much more urban Milwaukee Harbor and nearshore Lake Michigan (Wisconsin, USA; 0.21-5.23 particles/m 3 ) [60], were similar to those in the current study. However, the St. Croix/Mississippi and Milwaukee Harbor/Lake Michigan studies used a larger mesh size (333 μm) than the current study, potentially biasing their concentrations low.…”

A Practical Overview of Methodologies for Sampling and Analysis of Microplastics in Riverine Environments

“…Indeed, recent studies have pointed out a vertical distribution of MPs. High concentrations of them can also be found through the water column and at the sediment level in marine [44,[68][69][70] and riverine environments [42,50,58,71], reinforcing that the common strategy of sampling only the surface layer of water is insufficient and can result in considerable bias, particularly in estuarine, harbour, and lake environments [50].…”

“…Since the majority of synthetic polymers have a lower density than the seawater, in the past it was considered that microplastic particles could foremost float at surface level and only a small fraction could accumulate in the water column [11,58]. Nevertheless, several factors like flow turbulence, current conditions, the adsorption of a biofilm that increases MPs density and speeds up the sedimentation process, the different composition, density, and shape of polymers, move and homogenise the MPs' distribution throughout the water column in rivers, especially since freshwater density is lower than that of marine water [42,50,58]. Indeed, recent studies have pointed out a vertical distribution of MPs.…”

Effects of hydrodynamics on the cross‐sectional distribution and transport of plastic in an urban coastal river

“…Transport of most riverine contaminants has been well studied (Lick, 2008; Rijn, 1993); however, the mechanisms underlying transport and fate of plastics are not well understood (Alimi, Farner Budarz, Hernandez, & Tufenkji, 2018; Horton & Dixon, 2018). Previous case studies have typically collected data either at the water surface or sediments/near the bottom (Dris et al., 2015; Horton & Dixon, 2018); yet, it is now understood that the focus on surface samples may poorly estimate the true magnitude of plastic loads (Kooi et al., 2016; Lenaker et al., 2019).…”