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

Municipal biosolids are widely land-applied as fertilizers and soil conditioners based on crop nitrogen requirements. Although nutrient-rich, biosolids can contain contaminants including organic chemicals, heavy metals, pathogens, and microplastics (MPs). MPs (<5 mm) are pervasive in air, soil, water, and biota, can sorb contaminants and harbor biofilms, and pose ecological and potential human health risks. Wastewater treatment plants remove most MPs which accumulate in biosolids, creating a possible pathway to terrestrial environments via land application. While MPs have been shown to be transported by urban stormwater runoff, their transport from biosolid-amended croplands remains poorly quantified. This study asks: Do biosolid applications increase MP concentrations in agricultural runoff relative to manure and control plots? Are certain MP morphologies preferentially transported? How do biosolids compare to manure as MP sources? What is the potential scale of MP transport from U.S. corn and soybean lands receiving biosolids?

Prior work shows MPs occur in biosolids at up to 286 particles/g dry sludge and are widely distributed in the environment. Studies have examined vertical/horizontal transport in soils and stormwater contributions to WWTPs, with >95% of influent MPs removed to biosolids. Few studies assessed MP accumulation in soils from biosolid applications, with reports of 8.7–14 particles/g soil and evidence of export to soils, and one field plot suggesting MPs are transported in runoff from biosolid-amended fields. Gaps remain in quantifying expected MP concentrations in runoff from agricultural fields, morphology-specific transport behavior, comparing biosolids with manures as MP sources, and scaling potential MP release from U.S. cropland.

Field experiment at University of Nebraska’s Rogers Memorial Farm (Aksarben silty clay loam, 4% slope). Six 3.6 m × 10 m plots, bordered by sheet metal to 10 cm depth; runoff collected in downslope troughs into tanks. Treatments: two plots received municipal biosolids (177 kg/plot ≈ 4 dmt/acre), two received an equal mass of cattle manure, and two were controls. Materials were manually distributed and sorghum planted. Runoff grab samples collected after five natural precipitation events (>18 mm) between July–September 2020; tanks mixed prior to sampling; total runoff volume and pH recorded. Soil cores (0–5 cm; 5–15 cm) collected pre-application and post-experiment; biosolids and manure sampled at application time. Microplastics extraction and counting: Runoff concentrated by drying to 10 mL; wet peroxide oxidation with Fe(II)/H2O2 at 70°C; sonication; density separations with saturated NaCl and ZnBr2; filtration (12 µm glass fiber); stereomicroscopy counting. Recovery for water matrix: 88 ± 1.6% (1 mm) and 88 ± 3.3% (500 µm). Solid matrices (biosolids/manure/soil): oxidation step, sequential density separations (NaCl, then NaCl:ZnBr2), filtration and microscopic counting. Recovery for solids: ~90% (10 particles spiked) and 93 ± 4.7% (≤500 µm). Characterization: SEM to assess surface roughness and biofilm presence; confocal laser scanning microscopy with propidium iodide staining to visualize biofilm on MPs, followed by SEM confirmation on same particles. ATR-FTIR on 27 MPs (>300 µm) to identify polymer types using spectral libraries. Quality assurance: Field wet deposition sampled via rain gauge; lab dry deposition monitored with covered petri dishes; glassware rinsed thrice; samples covered with foil; extractions minimized transfers. Statistics: Repeated measures analysis of runoff MP concentrations across five events using SAS PROC GLIMMIX with AR(1) covariance and Kenward–Rogers df; comparison of treatment means at each time point. Soil first-layer MP concentrations analyzed via linear mixed model ANOVA with Tukey-adjusted post hoc comparisons. U.S. scaling: Estimated MPs at risk of transport from biosolids applied to corn and soybean fields using EPA typical application rates (assumed 7.5 and 12.5 dmt/acre, respectively), 0.1% land application coverage, measured biosolids MP concentration (9.1 particles/g), and observed runoff transport fraction (0.4%). State-level planted acreage from USDA FSA used; Nebraska county-level analysis performed.

• Biosolids and manure MP content: Biosolids averaged 9.1 ± 1.7 particles/g (dry wt); manure averaged 1.5 ± 0.2 particles/g. Estimated MPs applied per plot: biosolids 1.8×10^6; manure 2.9×10^5.
• Soil MPs (0–5 cm): Control 0.9 ± 0.1 particles/g; manure 1.1 ± 0.3; biosolids 2.6 ± 0.6. Pre-application control similar to post-experiment control.
• Runoff MPs (five events): Control 10–14 particles/L; manure 8–20 particles/L; biosolids 16–31 particles/L. No MPs detected in rainfall samples (July 21, 27). Runoff pH 6.5–7.5.
• Export fractions: Biosolids plots transported an estimated 7000 MPs (0.4% of applied); manure plots 0.3% of applied MPs transported.
• Rainfall vs concentration: Events ranged 1.7–5.1 cm; no clear relationship between rainfall intensity and MP concentration.
• Statistics: Time-by-treatment p=0.0562 overall; significant pairwise differences at specific times. On July 30: biosolids > manure (significant). On Sept 10: biosolids > manure and biosolids > control (significant). No significant manure vs control differences at any time; soil first-layer ANOVA p=0.0966 with no significant post hoc pairwise differences.
• Morphologies in runoff (all plots average): fragments 63%, fibers 33%, films 3%, foams 2%; no beads detected. Preferential transport of fibers/fragments attributed to lower surface roughness vs beads/films/foams.
• Biofilm and surface roughness: SEM and confocal showed greater biofilm on rough-surfaced beads and rougher films/foams; fibers/fragments smoother, consistent with higher mobility in runoff.
• Polymer types in biosolids (ATR-FTIR; plastics only): PE 34%; PET 17%; PP 13%; PS 8%; PVC 8%; PC (acrylic) 8%; NY 4%; PES 4%; PR 4%.
• U.S. scaling: Estimated ~64 billion MP particles/year potentially transported to surface waters via runoff from biosolids applied to corn and soybean fields. Top states by estimated transport: Iowa 8.0 billion, Illinois 7.7 billion, Minnesota 5.7 billion, Nebraska 5.2 billion. Nebraska county-level mapping provided.

The study demonstrates that land application of municipal biosolids increases MP concentrations in agricultural runoff relative to manure-amended and control plots, addressing a key knowledge gap on nonpoint source MP transport from croplands. The preferential occurrence of fibers and fragments in runoff, combined with microscopy evidence of lower surface roughness and reduced biofilm attachment compared to beads/films/foams, supports a mechanistic basis for differential transport by overland flow. Runoff MP concentrations from biosolid-amended plots are comparable to or exceed values reported for urban/suburban stormwater in other studies, suggesting biosolid-amended croplands can be substantial terrestrial sources of MPs to surface waters. Polymer profiling indicates common consumer and packaging plastics (PE, PET, PP) dominate MPs in biosolids, consistent with WWTP influent sources and partitioning into sludge. Scaling analysis indicates that even modest land application coverage (0.1% of cropland) could lead to tens of billions of MP particles mobilized annually to surface waters, particularly in the U.S. Corn Belt. These findings highlight agricultural runoff as an important pathway linking terrestrial MP reservoirs (biosolid-amended soils) to aquatic systems and underscore the need to evaluate agricultural best management practices (e.g., grass buffers, constructed wetlands) for MP removal.

This work provides field-based quantification showing biosolid application elevates microplastic concentrations in agricultural runoff compared to manure and control plots, with fibers and fragments preferentially transported. It characterizes MP morphologies and polymer types in biosolids, links surface roughness and biofilm growth to transport behavior, and offers one of the first national-scale estimates of MP flux from biosolid-applied croplands to surface waters (~64 billion particles/year). Future research should: (1) evaluate performance of agricultural best management practices for MP retention/removal; (2) incorporate environmental variability (soil texture, slope, vegetation, rainfall regimes) into transport estimates; (3) expand multi-site, multi-season field studies to improve generalizability; (4) investigate vertical transport to groundwater and long-term soil accumulation/aging; and (5) assess additional agricultural plastic sources (e.g., mulch films, irrigation).

Runoff concentrations and transport fractions were derived from a single field location, soil type, slope, and crop, limiting generalizability. Statistical differences among treatments were event-specific, with no consistent significance across all dates and marginal evidence in soils. The scaling analysis assumes uniform biosolids MP concentrations (9.1 particles/g) and a fixed runoff transport fraction (0.4%) across regions, not accounting for variable environmental factors (soil texture, vegetation cover, rainfall intensity/frequency, slope) that influence transport. Only larger MPs (>~300 µm subset characterized by ATR-FTIR) were polymer-typed, and beads were not observed in runoff, which may reflect detection limits or entrapment in matrices. The study period did not capture long-term aging or seasonal variability.