Seite - 7 - in Freshwater Microplastics - Emerging Environmental Contaminants?

5.1 EnvironmentalTransportation Many plastic materials that enter the environment will not remain stationary. Instead theywill be transported between environmental compartments (e.g. from land to freshwater and from freshwater to marine environments), with varying residence times in each. For example, the movement from land to river systems willdependuponprevailingweatherconditions,distancetoaspecificriversite,and land cover type. The collection of plastic litter at roadside habitats is easily observed, and the regular grass cutting practices of road verges in some countries means that littered items are quickly disintegrated bymowing equipment [8]. The movement ofMPs from land towatermay thenoccur throughoverland run-off or dispersion (via cutting action) to roadside ditches. Themovement of bulk plastics andMPswithin the riverine systemwill be governed by its hydrology (e.g. flow conditions, daily discharge) and the morphology (e.g. vegetation pattern) at a specific river site thatwillhavea largeeffectuponthepropagationof litterbecause of stranding and other watercourse obstructions such as groynes and barrages [2]. Compared to larger plastics, MPs may also be subject to different rates of degradation as they will be transported and distributed to various environment compartments at quicker rates than macroplastics. The formation of MP-associated biofilms has been investigated for LDPE in marine setting [62]. Transport to sediments and the formation of biofilms over the surface of MPs may also limit rates of degradation as this removes exposure to light. The modelling of MP fate and transportation in freshwaters is discussed further in Kooi et al. [63],whileMP-associatedbiofilmare discussed inHarrison et al. [64]. 5.2 EnvironmentalPersistence andDegradation Themajority of our current understanding regardingplastic degradationprocesses isderivedfromlaboratorystudies thatoften investigateasinglemechanismsuchas photo-, thermal, or bio-degradation [65]. There is limited information on the degradationofplastics under environmentally relevant conditionswhere anumber ofdegradationmechanismsoccurat together.Where information isavailable these studieshave tended to focusonweight loss, changes in tensile strength,breakdown of molecular structure, and identification of specific microbial strains to utilise specific polymer types. The degradation processes are defined in accordancewith the degradationmechanismunder investigation (e.g. thermal degradation) and the experimental result generated. In contrast, particle formation rates are often not investigated.This is importantbecausepolymers suchasPEdonot readilydepoly- merise and generally decompose into smaller fragments. These fragments then further disintegrate into increasingly smaller fragments eventually forming nano- plastics [66–68]. MicroplasticsAreContaminants ofEmergingConcern inFreshwater. . . 7