Microplastic Mobility and Retention in Geosynthetic Clay Liners

Abstract

Microplastics (MPs) are plastic particles smaller than 5 mm which have recently emerged as environmental contaminants of concern. Landfills serve as major reservoirs of plastic and potential sources of MP pollution due to the accumulation and degradation of plastic waste. MPs have been detected in landfill leachate, with concentrations exceeding 30,000 pieces per litre, well above levels typically observed in natural water bodies. Engineered barrier systems in landfills typically rely on geosynthetic clay liners (GCLs) and an underlying attenuation layer to restrict leachate migration. The swelling behaviour of bentonite in GCLs allows it to achieve low hydraulic conductivity and self-sealing capacity. While GCLs’ ability to prevent the migration of dissolved contaminants has been extensively investigated, their capacity to retain suspended particulate contaminants such as MPs remains poorly understood. This knowledge gap is especially concerning under field-representative conditions, where exposure to wet-dry cycles and calcium-rich leachates is known to reduce bentonite swelling and self-sealing behaviour, potentially compromising long-term barrier performance. An important methodological obstacle in addressing this gap is that conventional laboratory rigid soil columns, typically used to assess contaminant transport, are not suitable for low-permeability soils. This thesis is the first to provide a systematic experimental and numerical investigation of MP transport through GCLs. In all experiments with GCLs hydrated with deionized water, effluent MP concentration remained below the analytical detection limit, and no breakthrough was observed within the test durations, regardless of whether the bentonite was in powder or granulated form. By contrast, in GCLs containing granular bentonite that were exposed to 100 mM CaCl2 and subjected to two wet-dry cycles, early breakthrough was observed for all studied MPs.