Water retention and hydraulic properties of a natural soil subjected to microplastic contaminations and leachate exposures.

The influences of microplastics (MPs) contamination on soils have been extensively studied recently. Most of previous studies focus on saturated hydraulic conductivities and water retention of loose soils under laboratory conditions. The effects of MPs on the hydraulic properties of compacted soils for engineering purposes have not been well understood. This paper presents the laboratory investigation of water retention capacity, saturated (ksat) and unsaturated (kθ) hydraulic conductivities of a compacted natural soil contaminated by MPs and exposed to fresh, medium-aged, and stabilized leachates. The saturated (kg) and unsaturated air conductivities (kgθ) are calculated. The MPs with maximum particle sizes of 500, 150 and 50 μm were added to soils to obtain samples with mass ratios of 0.5, 1.0, 2.0, and 5.0 %, respectively. Under similar ranges of dry densities, permeation of fresh leachates decreases ksat of the compacted soils by 30 % while exposure to stabilized leachates increases ksat by 10 %, due to the viscosities of liquids. The flow channel properties of the compacted soils contaminated with different sizes and concentrations of MPs vary. The most complex flow channel can be found in samples with 5 % 50 μm MPs. The inclusions of MPs decrease residual moisture contents of the compacted soils regardless of MP sizes and percentages. The effects of MPs on air-entry pressures and parameter n depend on the sizes of MPs. The kθ (kgθ) of compacted soils with MPs depend on the combined effects of ksat (kg) and tortuosity parameter (l). Though l ranges from -0.85 to 2.12 with different levels of MP exposures, it does not have a significant influence on the relative hydraulic (kθ/ksat) and air conductivities (kgθ/kg) of the compacted soils. Future studies can focus on the long-term hydraulic properties of soils under MP contamination.