Higher concentrations of microplastics in runoff from biosolid-amended croplands than manure-amended croplands

Microplastics (MPs), polymer particles less than 5 mm, are a growing environmental concern due to their persistence and potential to sorb contaminants and support biofilm growth. Land application of municipal biosolids, a byproduct of wastewater treatment, has emerged as a potential major source of MPs in agricultural ecosystems. While previous research has explored MP accumulation in biosolid-amended soils, the quantification of MP transport via runoff from these fields remains largely unaddressed. This study aims to fill this knowledge gap by comparing MP concentrations in runoff from fields amended with biosolids, manure (a less likely source of MPs), and control plots. The study also investigates the influence of MP morphology on transport and estimates the potential MP release from corn and soybean production in the US, providing crucial data for understanding and mitigating this emerging pollution pathway. The land application of municipal biosolids as a fertilizer and soil conditioner is a common practice, with application rates depending on the crop's nitrogen needs. Although valuable nutrients are present in biosolids, they also contain contaminants like trace organic compounds, heavy metals, and pathogens, impacting water quality. Recent studies have demonstrated that biosolids contain significant concentrations of MPs. The potential for MPs to transport via stormwater runoff in urban and suburban areas, and subsequently to wastewater treatment plants, is well established. However, the role of land-applied biosolids in MP transport to surface waters is less understood. A few studies have examined MP fate and transport in agricultural soils, showing potential for vertical transport via crop roots and horizontal transport via rain-induced runoff. However, more research is needed to quantify MP concentrations in runoff from biosolid-amended fields and to investigate how different MP morphologies influence transport.

Existing literature highlights the presence of microplastics in various environmental matrices, including air, soil, water, and even human and animal tissues. Concerns regarding microplastic pollution stem from their ability to adsorb harmful substances such as heavy metals, their susceptibility to biofilm formation, and their potential ingestion by organisms. Studies have shown that MPs can be transported through stormwater runoff in urban areas, and that wastewater treatment plants efficiently remove and accumulate MPs in biosolids. Although a few studies have looked at MP accumulation in soils amended with biosolids, information on MP transport via runoff from these fields is lacking. Previous research has shown that factors like MP size, density, and morphology influence their mobility in soil and water. Some studies have addressed the transport of MPs in agricultural settings, but mostly focusing on general agricultural practices or using single field plots, leaving a large gap in knowledge about the scale of the problem.

Six 3.6 m × 10 m plots were constructed at the University of Nebraska's Rogers Memorial Farm. Two plots received biosolids, two received manure, and two served as controls. Sorghum was planted in all plots. Runoff was collected following rainfall events exceeding 18 mm. MPs were extracted from runoff, biosolids, manure, and soil samples using a modified method involving oxidation with Fe(II) and H2O2, density separation with NaCl and ZnBr2 solutions, and filtration. MP morphology and polymer type were identified using microscopy and ATR-FTIR spectroscopy. Statistical analysis (PROC GLIMMIX in SAS) was performed using repeated measures analysis, accounting for the temporal correlation of data. SEM and confocal laser scanning microscopy were used to examine MP surface characteristics and biofilm growth. The potential for MP transport from corn and soybean production areas in the US was estimated using reported biosolids application rates and the proportion of MPs transported in runoff from the experiment. Recovery rates for the MP extraction methods were evaluated using spiked samples. Details about the chemical and physical properties of the biosolids and manure used are presented in supplementary materials, along with more detailed information about soil characteristics in each plot, rainfall data, and the calculations used to estimate the potential for MP release from corn and soybean production areas.

The average MP concentration in biosolids was 9.1 ± 1.7 particles/g (dry weight), with fibers and fragments dominating. Manure showed a much lower average concentration of 1.5 ± 0.2 particles/g (dry weight). Soil MP concentrations were higher in biosolid-amended plots (2.6 ± 0.6 particles/g) compared to manure-amended (1.1 ± 0.3 particles/g) and control plots (0.9 ± 0.1 particles/g). Runoff MP concentrations were significantly higher in biosolid-amended plots (16–31 particles/L) than in manure-amended (8–20 particles/L) and control plots (10–14 particles/L), although not significantly different from manure and control plots at every time point. Fibers and fragments were the most abundant MP morphologies in runoff. SEM imaging revealed that beads had rougher surfaces and more biofilm growth compared to fibers and fragments, which likely explains their lower transport in runoff. Polyethylene (PE) was the most common polymer type identified in the biosolids, followed by polyethylene terephthalate (PET) and polypropylene (PP). Based on the study's findings, an estimated 64 billion MPs are potentially transported to surface water annually via runoff from biosolid-amended corn and soybean fields in the US. Iowa, Illinois, Minnesota, and Nebraska were identified as states with the highest potential for MP transport.

The significantly higher MP concentrations in runoff from biosolid-amended plots compared to manure-amended and control plots strongly suggest that biosolids are a substantial source of MPs in agricultural runoff. The observed preferential transport of fibers and fragments likely stems from their lower surface roughness, which reduces biofilm attachment and improves mobility. The dominant polymer types identified in biosolids (PE, PET, PP) are commonly found in various consumer products and align with previous findings. The estimated 64 billion MPs annually transported from corn and soybean fields in the US highlights the scale of this nonpoint source pollution problem. This study's findings extend beyond previous research by quantifying MP transport from biosolid-amended fields at a larger scale and considering various MP morphologies. These findings underscore the need for better management practices to reduce MP pollution from biosolids.

This study demonstrates that land application of biosolids significantly contributes to microplastic contamination of surface waters via agricultural runoff. The higher concentrations of microplastics in runoff from biosolid-amended plots compared to manure-amended and control plots highlight the importance of considering biosolids as a key source of microplastic pollution. The study's findings suggest that best management practices designed to reduce nutrient and sediment runoff may also be effective in mitigating microplastic transport. Further research should focus on refining MP transport estimations by incorporating environmental variables and investigating the effectiveness of existing agricultural best management practices in removing MPs from runoff.

The study was conducted at a single field location, limiting the generalizability of the results to other regions with varying soil types, climate conditions, and agricultural practices. The study's rainfall events might not fully represent the range of precipitation patterns observed in agricultural areas throughout the year. The methods used to extract and analyze microplastics may not capture all MPs present in the samples, potentially leading to underestimation of concentrations. The estimation of MP transport from corn and soybean fields is a model using average application rates; actual rates may vary regionally.