Quantifying Microplastic Contamination in Coastal Sediments of the Durham Heritage Coast

In this study the abundance and distribution of microplastics (1 µm – 5 mm, MPs) were investigated in sediments along the intertidal zones of thirteen beaches of the Durham Heritage Coast (DHC). This study is the first study performed in the UK to examine a stretch of coastline in order to analyse the distribution of MPs in an area, as well as their overall abundance and potential sources. The durability and persistence of MPs within the marine environment, means that their environmental impacts such as their harm to organisms and humans can continue for many decades. This study contributes to the understanding and extent of MP pollution in the marine environment in the North East of England and provides a reliable and reproduceable methodology for extracting and quantifying MPs. The beaches were chosen due to their accessibility, sediment supply and varying anthropogenic impacts (tourism and storm water drains). Density separations via salt solutions were used in order to separate the MPs from the sediment. Sodium Chloride, with a density of 1.2 g cm-3 was used in order to extract low density MPs (e.g. polypropylene and polyethylene) and Sodium Iodide, with a density of 1.8 g cm-3 was used to separate high density MPs (e.g. polyester and acrylics). These salts were chosen due to their low environmental impact, relatively low cost, and low risk of any further degradation/denaturation of the MPs. Microscopic analysis of the filtered supernatants allowed for the quantification and analysis of the MPs within the samples using standard characteristics of shape, colour and size. The investigation results showed that MPs were detected at all thirteen beaches. The concentrations found within the DHC sediment (442 ± 168 MPs/kg of dry weight (dw) sediment) are comparable with other studies conducted in coastal sediments, with microfibres (MFs) found to be the most abundant (39.1%) MP type, followed by fragments (26.9%). 83.9% of the MPs were ≤ 1000 µm in size and the three modal colours were black (18.8%), orange (18.0%) and blue (17.5%). The Pearson’s Correlation Coefficient and the Kruskal-Wallis Statistical analysis tests were used to identify abundance and distribution variability, respectively. The Pearson’s Correlation Coefficient (Rp) showed a negative correlation (Rp = - 0.43), between medium grain size of sediment at each beach with mean number of MPs found within the corresponding sample, excluding all outliers in MP abundances. A p-value of < 0.05 shows that the results are statistically significant and there is no correlation between grain size and MP abundance. Instead, the abundance of MPs has been found to be associated with a multitude of sources such as poor waste management at wastewater treatment plants, fishing, agriculture, roads etc., which all directly affect the DHC. Additionally, the Kruskal-Wallis statistical test showed that the distribution of MPs along the DHC is statistically different (H = 33.4 > Chi-square value of 21.03, when p = 0.05). The MP distribution has proposed to have been affected by the prevailing southward coastal drift movement and obstructions along the coast such as headlands and harbours.