Reconstructing the first record of historical microplastic accumulation from lake sediments in Aotearoa New Zealand: A case study at Lake Wiritoa.

Microplastics are widespread anthropogenic contaminants found in the modern environment. By defini- tion, these are particles smaller than five millimetres and are predominantly derived from the mechanical breakdown of larger plastic debris. Sediments in rivers, lakes, estuaries, coastal and marine settings are important sinks for environmental microplastics. The burial of microplastics in these sediments can produce historical records of plastic production, usage, and release into the environment. Profiling the abundance and characteristics of microplastics in sediment can inform the extent of contamination and reflect the preferential accumulation of specific polymer types. These archives provide ideal opportunities to study biotic interaction and toxic impacts to aquatic life at all levels of the food chain, and potential consequences to human health. This thesis aims to contribute a better understanding of the history and extent of microplastic pollution in New Zealand’s environment. Firstly, a method for reliably recovering microplastics from sediment is developed, motivated by the lack of harmonized microplastic extraction procedures among existing literature. Density separation is the most commonly applied technique, where sediment containing microplastics is mixed with a solution of intermediate density, allowing lighter microplastics to float out. The primary objective was to adapt or develop a separation method to robustly extract and quantify microplastics in fine-grained lake sediments, while minimizing cost and hazard. I compared recoverable proportions of eight types of microplastics varied by use of three brine solutions that spanned a density gradient. The results show that the densest solution (composed of 25% sodium polytungstate and 75% saturated sodium chloride) yields significantly higher recovery rates for denser microplastics, such as polyethylene terephthalate (PET), plasticised polyvinyl chloride (PVC-P), and polylactic acid (PLA) (p<0.0001), but offers negligible advantage for lighter microplastics, including polystyrene (PS), polypropylene (PP), and high-density polyethylene (HDPE) (p>0.05). The relative density between the separation medium and the microplastics is a significant predictor of recovery rate, but the precursor material, size, and shape (fragment or fibres) of the particle also play a role. A solution with a density of approximately 1.46 g/cm3, ideally a mixture of denser sodium polytungstate diluted with non-toxic sodium bromide or sodium chloride, is recommended for recovering microplastics from fine-grained lacustrine sediments. At this density and with these materials, the chemical costs and hazards are minimized while the expected recovery rates for most common polymers remain high. The chosen method for density separation was then applied to a sediment core from Lake Wiritoa, a small lowland lake approximately six kilometers southeast of Whanganui in New Zealand’s Manawatu region. Lake Wiritoa has both agricultural and recreational land use in its 171-hectare catchment and is situated downwind of the Whanganui urban centre. The use of plastics for farming (e.g., mulch films) in the terrestrial catchment, proximal recreational plastic littering, and downwind deposition of small plastic particles are three likely sources of microplastics for Lake Wiritoa. Pyrolysis methods are used to identify and quantify recovered trace-markers of microplastics, allowing for the novel analysis of small masses of lake sediment and replacing the need for large bulk sieving. The microplastic concentration profiles from Lake Wiritoa’s core show a typical trend of increasing concentration towards present day, reflecting temporal changes in the transport and accumulation of plastic debris. However, the profiles did not match the exponential increase in plastic consumption well-described from the early 1950s in Europe and North America. Instead, microplastics first occur in c. 1947 CE (polyethylene, polystyrene) but do not significantly increase in concentration until c. 2000 CE (p<0.05). Polyethylene is the most abundant polymer with a peak concentration of 1158.97 μg per g sediment in 2020, whereas polypropylene was not detected until the 2000s but reaches a peak of 567.9 μg per g sediment also in surface sediments. The profile from Lake Wiritoa represents the first record of temporal microplastic accumulation in New Zealand, extending otherwise limited direct environmental monitoring back to a first occurrence.