Microplastic Polymer Types Alter Soil Water Flow Across Differing Soil Textures

A laboratory investigation examining water movement through soils containing microplastic particles has demonstrated that saturated hydraulic conductivity changes significantly even under trace contamination levels. Conducted across a narrow concentration window spanning 0.001 percent to 0.015 percent by dry weight, the experiments revealed that plastic polymers alter internal void networks, producing contrasting hydraulic responses between loam and sandy loam textures. Saturated hydraulic conductivity values followed an overall hierarchy of polyethylene terephthalate exceeding polyvinyl chloride, which in turn exceeded polyethylene. Conductivity reached its maximum measured rate of 2.1 centimeters per hour in sandy loam soil containing larger polyethylene terephthalate particles at 0.015 percent concentration. Conversely, conductivity dropped to its lowest value of 1.02 centimeters per hour in loam soil subjected to identical polyethylene terephthalate particles at the highest concentration. While polymer type and soil texture strongly governed hydraulic conductivity, differences between microplastic size fractions of 0.5 to less than 1 millimeter and less than 0.5 millimeter yielded no statistically significant variations.

The study highlights a growing concern over the global accumulation of microplastics across terrestrial environments, where contamination is estimated to be four to twenty-three times greater than in oceanic waters. Microplastics, classified as synthetic particles measuring below 5 millimeters, increasingly contaminate agricultural fields through direct disposal and the application of plastic films. Mechanical disruption during tillage, alongside ultraviolet exposure, thermal degradation, and biochemical weathering, fragments large agricultural plastics into secondary particles exhibiting varied densities, charges, morphologies, and size distributions. Finer secondary particles integrate into soil aggregates, where their presence reduces macroaggregate stability from 84 percent down to between 65 and 71 percent, while decreasing aggregate diameters from 1.4 millimeters to 1.0 to 1.1 millimeters. While previous studies have examined biological and chemical shifts caused by microplastics, their influence on fundamental physical soil properties remains less understood. Saturated hydraulic conductivity controls infiltration, drainage, and chemical leaching, yet research addressing hydraulic behavior within pristine soils containing trace microplastic concentrations has remained scarce.

To investigate hydraulic responses under undisturbed baseline conditions, soil samples were gathered from two distinct campus locations at Chittagong University displaying notable textural contrasts. Loam soil was acquired from an uncultivated field near Ansar camp at coordinates 22 degrees 27 minutes 56 seconds North and 91 degrees 46 minutes 46 seconds East. Sandy loam soil was collected from the Department of Soil Science experimental field at coordinates 22 degrees 27 minutes 58 seconds North and 91 degrees 46 minutes 49 seconds East. Sampling targeted the upper soil layer between 0 and 15 centimeters depth. The retrieved soils were air-dried, fragmented with a wooden hammer, and passed through a 2-millimeter stainless steel sieve to ensure uniform preparation. Granulometric analysis via the Bouyoucos hydrometer method confirmed that the sandy loam contained 62 percent sand. Baseline physicochemical characterization evaluated soil pH in a 1-to-5 suspension, alongside cation exchange capacity, calcium, magnesium, and organic carbon determined by the Walkley-Black method using the 1.724 Van Bemmelon factor.

The research evaluated three major recalcitrant plastic polymers frequently identified in natural environments: polyethylene terephthalate, polyvinyl chloride, and polyethylene. Sourced from local markets to reflect common consumer plastics, the materials were mechanically processed into micro-sized fragments. Sharp scissors were first utilized to slice the plastics into thin flakes, which were subsequently reduced into fine particles. Standardized sieving separated the resulting microplastics into two distinct size classes: 0.5 to less than 1 millimeter and less than 0.5 millimeter. These prepared polymers were systematically blended into sieved soil samples at four precise dosing rates of 1 milligram, 5 milligrams, 10 milligrams, and 15 milligrams per 100 grams of dry soil. These dosing quantities corresponded to dry weight concentrations of 0.001 percent, 0.005 percent, 0.010 percent, and 0.015 percent. This contamination window was deliberately selected to simulate initial, low-level pollution in previously pristine environments rather than heavily degraded agricultural settings.

To measure water conductivity under uniform physical conditions, the prepared soil-polymer mixtures were packed into aluminum cores, maintaining an identical bulk density of approximately 1.20 grams per cubic centimeter. The experimental setup comprised 144 treatment cores arranged in a full factorial scheme of two soil types, three plastic polymers, two particle size fractions, four application concentrations, and three replications. Additionally, six untreated control cores, three for each soil type, were included, bringing the total experimental assembly to 150 cores. Each individual plastic polymer accounted for 48 treatment cores across the evaluation. Hydraulic conductivity testing was performed using the constant-head method governed by Darcy’s law, measuring water discharge through saturated cores under fixed hydraulic head conditions. By establishing controlled bulk density and uniform packing across all cylinders, the experimental procedure ensured that any changes in fluid transmission rates directly reflected the physical modifications caused by the incorporated plastic polymers rather than compaction artifacts.

The resulting measurements illuminated two distinct physical mechanisms controlling saturated hydraulic conductivity: pore dimension enlargement and pore clogging. At low polymer concentrations, microplastic particles can expand void dimensions by altering internal particle arrangements, which increases macropore spaces and facilitates water transmission. Conversely, at higher concentrations, accumulating particles lodge within interstitial void channels, creating physical obstructions that diminish pore connectivity and reduce conductivity. Furthermore, the inherent hydrophobicity of plastic surfaces influences fluid movement through porous matrices. Hydrophobic particles repel water molecules, which can reduce wall friction and promote fluid passage through open pores, or conversely disrupt capillary continuity. Hydrophobicity is governed by the chemical composition, surface polarity, crystallinity, polymerization degree, and plasticizer additives of the polymer, alongside the density of surface functional groups. Whether hydrophobic repulsion accelerates water movement or pore clogging restricts fluid transmission depends fundamentally on the baseline pore size distribution and textural properties of the surrounding soil.

The interactions observed with polyethylene terephthalate clearly demonstrated how soil texture modulates these competing hydraulic processes. In loam soil, adding the lowest concentration of 0.001 percent polyethylene terephthalate increased hydraulic conductivity by 1.12 times relative to control cores. Because trace particle quantities did not clog the balanced pore spaces of the loam, the hydrophobic nature of the polymer promoted water movement. As polyethylene terephthalate concentrations increased, however, conductivity steadily declined to a study low of 1.02 centimeters per hour at 0.015 percent due to extensive pore clogging in the finer pore network. Conversely, in sandy loam soil containing 62 percent sand, larger polyethylene terephthalate particles measuring 0.5 to less than 1 millimeter produced a continuous rise in conductivity with increasing concentration, reaching 2.1 centimeters per hour. The low structural stability and macroporous network of sandy loam accommodated larger particles without clogging, allowing strong hydrophobic surface repulsion to enhance water flow.

Polyvinyl chloride generated distinct hydraulic responses that reflected its moderate hydrophobicity and lower contact angle compared to polyethylene terephthalate. In loam soil, adding polyvinyl chloride produced a steady decrease in hydraulic conductivity across both particle sizes as concentrations rose, directly driven by pore-clogging mechanisms. In sandy loam soil, larger polyvinyl chloride particles of 0.5 to less than 1 millimeter caused an initial increase in conductivity at the lowest concentration of 0.001 percent due to surface water repellency. However, at higher concentrations, pore blockage predominated, returning conductivity to baseline control values. For smaller polyvinyl chloride particles below 0.5 millimeter, conductivity in sandy loam dropped below control levels at the 0.015 percent concentration. Because polyvinyl chloride has a smaller contact angle and weaker hydrophobicity than polyethylene terephthalate, its particles interacted more strongly with water molecules, resulting in lower overall conductivity values across both evaluated soil textures.

Polyethylene consistently reduced saturated hydraulic conductivity across both loam and sandy loam soils as concentrations increased, establishing widespread pore clogging across all tested levels. Despite this uniform downward trend, a notable size effect appeared within sandy loam soil: smaller polyethylene particles measuring less than 0.5 millimeter maintained higher conductivity rates than larger particles measuring 0.5 to less than 1 millimeter. This difference was attributed to the greater specific surface area of smaller particles, which generated stronger cumulative hydrophobic water repellency that partially counteracted pore restriction. Although polyethylene possesses higher hydrophobicity than polyvinyl chloride, its particles effectively constricted pore necks, resulting in the lowest average conductivity among the three tested polymers. When analyzed across the complete experimental matrix, however, particle size variations did not produce statistically significant differences in conductivity. Polymer type and soil texture remained the primary factors determining water transport through the soil columns.

The findings contrast with earlier studies that assessed elevated microplastic additions, such as 0.3 percent or 0.4 percent polyester fibers, which reported altered porosity and water retention without identifying significant changes in saturated hydraulic conductivity. In contrast, this study demonstrates that minimal concentrations between 0.001 percent and 0.015 percent substantially impact hydraulic conductivity in previously unpolluted environments. Because hydraulic conductivity governs irrigation, drainage, erosion, and the transport of dissolved nutrients and agrochemical pollutants, these subtle alterations carry broad implications for soil water management. The researchers emphasize that environmental regulations and plastic disposal standards must account for polymer composition and soil texture when evaluating agricultural pollution risks. Furthermore, because this study focused on cut flakes without examining particle morphology, the authors conclude that future investigations must evaluate diverse microplastic shapes and production methods to fully understand their hydrological impacts in natural soil systems.

Subject of Research: Impact of low-concentration microplastics on soil saturated hydraulic conductivity across different soil textures and polymer types

Article Title: Saturated hydraulic conductivity in microplastics incorporated soils: Effects of soil texture, polymer type, particle size, and concentration

Article References: Hossain, R., Hakim, A., Roy, S., Tanu, F. Z., & Akhtaruzzaman, M. (2025). Saturated hydraulic conductivity in microplastics incorporated soils: Effects of soil texture, polymer type, particle size, and concentration. Discover Polymers, 2(1), Article 11. https://doi.org/10.1007/s44347-025-00022-8

Image Credits: AI Generated

DOI: 10.1007/s44347-025-00022-8

Keywords: saturated hydraulic conductivity, microplastics, soil texture, polyethylene terephthalate, polyvinyl chloride, polyethylene, sandy loam soil, loam soil, pore clogging, pore dimension, constant head method, soil water flow, soil physical properties, hydrophobic water repellency, agricultural soil contamination, Darcy's law, soil macropores