Investigating the Environmental Fate and Pathways of Microplastics from Agricultural Catchments to Freshwater

The term microplastics (MPs) encompasses a suite of contaminants of varying size (0.001 to 5 mm), shape, and plastic polymer type. Their small size and resistance to degradation renders them a significant contaminant of risk in our environment. Research into the occurrence, behaviour, and impacts of MPs in the environment remains largely in its infancy, with most studies on the topic spanning just the last decade. Although much of this research has been focused on the marine environment, attention more recently has turned to terrestrial systems, and particularly to agricultural soils, where MPs in these systems may present risks to food security. MP sources to agricultural soils are many and varied and include potential inputs from soil amendments (including biosolids and compost), degradation of larger plastic (from, for example, plastic mulching), the fragmentation of tyres from vehicles and machinery, and atmospheric fallout. Once incorporated into the soil, MPs may have high retention times, accumulating and consequently impacting the physical and chemical properties of the soil, and impacting plant biota. Conversely, they may also be remobilised from soil systems and transported into freshwater environments. Our knowledge, however, on the processes governing MP movement remains limited, and working towards quantifying the export of MPs from soils is therefore important for both river basin managers and policymakers. This thesis presents work that addresses relevant knowledge gaps concerning the pathways of MPs from agricultural catchments into freshwater systems. Through a series of field and laboratory experiments, two potential MP transport pathways from agricultural settings were investigated, namely, vertical migration through soil, and overland movement across field surfaces through surface runoff. Vertical MP migration from soil to surface waters and groundwater was assessed in a field-based study in two agricultural settings (Field A and B) in the south-east of Ireland. Both settings were characterised by extensive, but similar, histories of annual biosolid land treatment over a period of two decades. A third agricultural setting (Field C) near Fields A and B, and with similar geological, physiographic, and topographic characteristics, but which was never exposed to the application of biosolids, served as a reference. The capacity for vertical MP mobility through the soil matrix was assessed in Field A through the extraction of six (2 m deep) cores in two cross-slope transects (three replicate cores from each transect at lateral intervals of 2 m) in the upper and lower sections of the field. MP abundances were determined with core depth, as were MP concentrations in groundwater samples extracted from the core boreholes following core extraction. The potential for MP export through overland and interflow (lateral movement of water in the unsaturated, or vadose zone) pathways to surface waters was determined in Fields A and B from MP abundances determined from 15 no. shallow surface cores (0.1 m) along a down-slope transect in each field, and through MP concentrations in effluent samples collected from a sub-surface land drain in Field A. The analysis showed that MPs were recovered in soils at depths up to 35 cm, with no recovery of MP below the plough zone of the field. The presence of the plough zone at this depth was confirmed through Itrax scanning of the extracted cores. Furthermore, the average MP concentration in the top 0.1 m of soil across all samples was 365 ± 302 MP kg-1, with 0.3 MP L-1 and 1.6 MP L-1 recovered from the groundwater and drainage water samples, respectively. Results confirmed a relatively even distribution of MPs across the studied fields with no significant differences being observed between MP abundances and spatial position. The hypothesis that MPs may accumulate in lower lying areas of fields, or at locations where field gradients flatten out, was therefore, not proven. The research points towards ploughing as a significant driver of MP mobility under the tested conditions. Furthermore, given the decreasing abundances of MP with increasing core depth, results would suggest that risks of contamination to groundwater resources from vertical MP migration is low. To assess MP export in surface runoff overland flows, a series of laboratory-based rainfall simulation experiments were undertaken, and these examined the processes which influence MP mobility. Parameters investigated included rainfall pattern and intensity, catchment slope, the presence of surface vegetation, direction of cultivation, and the characteristics of the MP particles. Across all experiments, rainfall intensity was observed to be the most influential parameter in mobilising MPs, with increased rainfall intensity promoting higher levels of MP transport across land surfaces. Resistance characteristics of both the soil surface and the tested MP (surface morphology), together with catchment or field slope and the presence of surface vegetation (agricultural ryegrass) were shown to be important influences on MP mobility. Other MP characteristics were also influential, with smaller particles being significantly more mobile across all experiments. Overall, these findings suggest that extreme rainfall and field tillage (soil preparation by mechanical agitation) may represent crucial events for mobilising MP from agricultural catchments, while mitigation measures such as riparian buffer zones may be effective in reducing MP inputs into freshwater. Through implementing results from the rainfall simulation experiments, a proof-of-concept model was developed based on a source-pathway-receptor methodology. The work represents one of the first attempts at quantifying the export of MPs from agricultural sources to freshwater receptors. The model, which examines the overland mobility of MPs through surface runoff, is limited in its capability due to a lack of empirical data, however, due to its simple and flexible structure, the model can be easily modified as and when new data becomes available. The final aspect of the thesis uses insights from this modelling to map areas where the spreading of biosolids in agricultural systems should be restricted. This land suitability map was based on current legislative limits relating to antecedent concentrations of heavy metal in soils, areas underpinned by ‘extreme’ groundwater vulnerability, national guidelines that underpin good practice in land management (e.g., the need for 20 m riparian buffers) and critically, areas where high levels of surface runoff are likely to present conditions conducive to MP transport and export. In this regard, the inclusion in the mapping of surface runoff characteristics (through the EPA Diffuse Tools project) was shown to be a significant contributor in reducing the areal extent of suitable land for the spreading of biosolids nationally.