Drip Irrigation Promoted Migration of Microplastic Particles Across Vertical Soil Columns.

The widespread use of plastics has undeniably brought numerous advantages to society, facilitating countless advancements in technology, industry, and daily life. However, the proliferation of plastic debris in various environmental systems has become an escalating concern. Recognizing this pressing issue, our research team at the Civil Engineering Department of the Indian Institute of Technology (IIT) Roorkee has undertaken an experimental investigation to study the transport behavior of microplastics within soil matrices.

Specifically, we focus on coumarin 6 dyed microplastic particles, sized between 35 to 40 microns, as tracers to understand their migration patterns. This study employs vertical soil columns as experimental setups, designed to mimic natural subsurface conditions. Each soil column has a depth of 30.5 cm and is packed with carefully prepared soil types to replicate varying real-world scenarios. By using artificial drip irrigation systems to simulate rainfall or water infiltration, we aim to elucidate the mechanisms by which microplastics are transported through soil systems, potentially leaching into underlying groundwater reservoirs.

The primary objective of the study is to systematically investigate the factors influencing the downward movement of microplastics in different soil types. For this purpose, two types of soil have been used: fluvial sand and gravel soil. The influence of several variables on microplastic transport was examined, including variations in soil pH, organic matter content, drip irrigation intensity. These parameters were chosen because of their potential to alter the physicochemical properties of the soil environment, thereby affecting the mobility of microplastics.

To begin the experiments, vertical soil columns were packed with either fluvial sand or gravel soil. The soil was pre-conditioned to achieve specific pH levels and organic matter contents, ensuring controlled and reproducible conditions across trials. Microplastic particles stained with coumarin 6 dye were introduced at the top of the soil column along with water droplets, mimicking natural infiltration processes under varying drip irrigation intensities.

The effluent from the outlet at the bottom of the soil column was collected at regular intervals to quantify the number of microplastic particles that had traversed the column. These collected samples were then subjected to analysis under a fluorescent microscope, which enabled accurate detection and quantification of microplastic particles.

The study also highlighted the impact of drip irrigation intensity on microplastic migration. Higher flow rates were found to promote greater transport of microplastics, as the increased water velocity reduced the residence time of particles within the soil and minimized opportunities for retention or adsorption. Conversely, lower flow rates allowed for more pronounced interactions between the microplastics and the soil matrix, leading to increased retention.

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