Metals and Microplastics in the First Industrial City: Fluvial sediment contamination in the upper Mersey and Irwell catchments, UK

UoM administered thesis: Phd

  • Authors:
  • Rachel Hurley

Abstract

Rivers have been the recipients of waste for millennia. However, since the onset of industrial and urban development, major degradation of fluvial systems has been observed globally. This has encompassed a wide range of contaminants from numerous potential sources. Despite efforts to reduce inputs, contamination continues to persist in many catchments. Metal contamination of fluvial sediments is a well-established problem. Conversely, microplastics are an emerging contaminant, for which there is a paucity of data regarding their sources, behaviour, and fate. Based on the onset of industrialisation, Manchester is often heralded the ‘first industrial city’. During the industrial period, the fluvial network became heavily contaminated. By the 1970s, it was amongst the most polluted river systems in Europe. Despite this background, no study has thus far undertaken a systematic, catchment-wide survey of sediment-associated contamination. This study assesses patterns of metal and microplastic contamination in fluvial sediments of the Irwell and upper Mersey catchments (1527 km2), which comprise the Manchester river network, from the onset of the industrial period to the present day. Five metal(loid)s have been studied and microplastics are assessed by type, size, and density for the first time. Metal concentrations in fine-grained bed sediments are heavily enriched across the entire fluvial network, even in headwater reaches. By examining spatial patterns, it is possible to attribute a portion of this to the reworking of historically-contaminated material; although, modern, urban sources are also important. Sources of metals to channel beds are numerous and spatially complex. This is also the case for microplastic contamination; although, microplastic particles are not bound to natural sediments and exert more transient behaviour in fluvial systems. Following an extreme flood event on Boxing Day 2015 and a sustained period of high flows (winter 2015/16), metal contamination was shown to present markedly conservative behaviour despite significant reworking of bed sediments. Metal mobility was generally low and was not affected in the long-term by hydrological processes. Despite this, the results indicate that bed sediment-associated metal contamination is likely to persist into the future at levels that exceed sediment quality guidelines. In contrast, flooding is very effective in flushing high concentrations of microplastics from channel beds. This suggests that microplastic contamination can be effectively reduced through source control. The environmental significance of microplastic contamination was directly observed through the ingestion of microplastics by freshwater Tubifex worms at the distal end of the Irwell system (Salford Quays). Microplastic concentrations within worm tissue were high, indicating an increased risk for trophic transfer. This also presents a potential link to the human food chain. Furthermore, both metals and microplastics accumulate in floodplain deposits. Floodplains are effective in preserving microplastics, recording the temporal evolution of microplastic contamination over the last 75 years. Maximum values are observed in the late 1960s/early 1970s. Conversely, elevated metal concentrations occur much earlier and reflect catchment-wide patterns of industrialisation and urban growth. Reworking of channel banks forms a secondary source of metals and microplastics to the active river channel and downstream environments that will persist long into the future. Thus far, microplastics have passed under the monitoring radar and sediment contamination is rarely given due consideration in assessments of river quality, such as in the Water Framework Directive. However, this study shows that both metals and microplastics within fluvial sediments are important contaminants and have significant implications for the health of the entire aquatic system.

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Original languageEnglish
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Award date1 Aug 2018