by Art Hirsch, AHirsch@Terralogicss.com
Plastic and microplastic contamination is prevalent in large and small agricultural operations, posing a threat to current and future food production systems, human health, and the environment. Microplastics are small plastic materials, less than five mm in diameter, and are known to cause environmental risks to terrestrial and aquatic ecosystems and public health. The University of New Castle-Australia and the University of Vienna estimated that globally, each person ingests 0.1- 5 grams of microplastics per week from air, water and food ingestion.
There are two types of microplastics. Primary microplastics are less than 5 mm and manufactured at a specific size for a given application or function such as cosmetics, industrial cleaning operations, and residential cleaning materials. Secondary microplastics are also less than 5 mm in diameter and are generated from the degradation of plastic materials into the environment from single use plastics (plastic bags, water bottles), synthetic textiles, and construction materials.
Microplastics enter the environment through numerous sources and pathways. Synthetic microfibers can be generated from the degradation of clothing textiles by washing and drying. Microfibers can be released from automobile tires that contain 20% plastic material. Cosmetics and cleaning materials contain microbeads that enter surface water from waste treatment systems. Landfills contain plastic materials such as synthetic textiles from fast fashion disposal. Single use plastics are the #1 source of microplastics that enter the Great Lakes.
Agricultural operations use plastic extensively for numerous applications that promote the use or generation of microplastics, contaminating terrestrial and aquatic environments. Plastic mulch improves agricultural efficiency in water usage and moisture loss. This increase in agricultural productivity by plastic comes with a high environmental cost by generating microplastics. Plastics are highly integrated into agricultural operations such as greenhouse films, plastic mulches, sheeting for hoop houses, product, and plastic irrigation tubes which generate microplastics and soil impacts. Microplastics are used to encapsulate pesticides, herbicides and seeds. Major agricultural sources and pathways for microplastics include biosolids, stormwater, encapsulation, and plastic sheeting mulch.
Pathways for Microplastics
Biosolids are the treated materials produced during the processing of wastewater at municipal wastewater treatment plants. Biosolids contain nutrients and organic matter and are used as soil amendments and conditioners that are beneficial to agriculture and municipalities. Biosolid quality and proper use are regulated by the Michigan Department of Environment, Great Lakes and Energy (EGLE). Biosolids are a major source of microplastics and Per- and Polyfluorinated Substances (PFAS), known as forever chemicals.
For years, biosolids have been a stable and valuable source of plant nutrients and soil structural enhancements in Michigan. Michigan State University estimated that over 83,100 dry tons of biosolids were applied to agricultural fields in 2000. Statewide, about 175 different wastewater treatment plants applied biosolids to about 18,000 acres of agricultural land in 2016, according to the Michigan Department of Agriculture and Rural Development (MDARD). EPA estimates that nationwide, about 50% of all biosolids are being recycled onto land.
In addition to PFAS, biosolids contain diverse types of pollutants such as heavy metals, viruses and microplastics from secondary municipal waste operations in which pollutants adsorb onto microplastic solids. Biosolids contain over 10,400 microplastic particles per kilogram of biosolid, (Rayford, Frontiers in Soil Science) with runoff water containing 31 particles per liter (Beni, Communication Earth & Environment). Microplastics in biosolids can release and adsorb toxic pollutants such as nickel and chromium (VI) that can be transported into the vegetation and the overall environment (High Concentrations of Microplastics in runoff from biosolids amended croplands that mature amended croplands, Nature, 2023). The accumulation of microplastics can impact plants by pollutant uptake and incorporation and can change soil chemical and biological characteristics. Microplastics can influence the function and health of soils by changing pH, carbon storage, electrical conductivity and porosity (Verma, Nano-Microplastics and Agro-Ecoystems, Frontiers in Soil Science, 2023). The presence of microplastics in soils can potentially impact the quality of fruits and vegetables (Conti, Environmental Research), 2020).
The Michigan agricultural community and many agricultural extension service representatives are not fully aware of the microplastic pollution issues from agricultural operations. This is based upon the author’s conversations with Michigan Agricultural Extension Service agents and Michigan State University (MSU) professionals. Michigan Agricultural Extension Services, MSU and other universities need more awareness and research to understand microplastic loading, fate and life cycle transport of microplastics to develop microplastics’ best management practices.
Biosolids applied to agricultural fields are susceptible to both soil erosion and transport via stormwater discharges and aerial dispersion. Stormwater discharges are very much like non-point source discharges of nutrients entering the Great Lakes which EGLE and the U.S. Environmental Protection Agency (EPA) struggle to control. Agricultural stormwater contains high concentrations of microplastics leaving the fields and entering surface water tributaries to the Great Lakes. Agricultural operations using biosolids need to implement best management practices to control sediment stormwater discharges. One best practice is the development and use of vegetative buffer zones adjacent to ditches, streams, rivers and lakes. The use of regenerative farming can reduce and/or eliminate the need for biosolid applications. There needs to be an increase in microplastic outreach and education, and onsite evaluations given to the Michigan agricultural community by state agricultural services to manage stormwater discharges. Water quality legislation and regulation should be considered for large agricultural operations to control microplastics in the agricultural environment, if education and voluntary control is not successful.
One of the least known and most concerning sources of microplastic pollution is the deliberate addition of encapsulated pesticides, herbicides and seeding applications used in industrial agriculture (Center for International Environmental Law, 2022). The encapsulation is like a time released aspirin where the microplastic cover starts to break down over time thus releasing the encapsulated materials and microplastics. The microplastics do not dissolve away and are broken down into smaller and smaller pieces thus accumulating in the soil structure of the agricultural soils. Millions of microplastics are added and eventually dispersed annually over agricultural fields in Michigan and nationwide. In addition, over time these small microplastics can be transported through the soil column and into the groundwater. Groundwater can transport microplastics to residential wells used for drinking, agricultural wells for irrigation, or into receiving stream systems. The use of microplastic encapsulation should be banned, requiring a non-plastic material substitution such as encapsulated silica or a nature-based coating.
Plastic microplastic pollution can enter agricultural systems from damaged, degraded and discarded agricultural products and leakage from polluted water sources, air and waste. Many agricultural plastics that generate microplastics are single-use plastic sheet products with short life spans, degrading within one year. Ron Goldy from the MSU Agricultural Extension Service estimates that on average, over 230 pounds of plastic mulch is added per acre in Michigan, where over 10,000 acres use plastic mulch material. Plastic mulching materials degrade from sun ultraviolet radiation, rain, wind and abrasion, and impact agricultural soils.
Improper plastic management and disposal of plastic wrapping, containers and plastic sheeting are additional microplastic sources. Degraded PVC plastic irrigation piping is another agricultural source that can transport microplastics via air dispersion or direct soil introduction.
Needed Agricultural Actions
Stopping the accumulation of microplastic pollution in the environment is critical for protecting human health, biodiversity, and even the climate. The quality and integrity of our food and agricultural environment is at stake. The Center for International Law believes that agriculture is one of the most controllable sources of microplastics by using best practices and material substitution. The following are needed agricultural actions that can be done in Michigan to control microplastic generation and transport:
- Develop legislation that will prohibit the use of microplastic encapsulation for agricultural operations for pesticide, herbicides and seeding applications.
- Precautionary microplastic management by the Michigan agriculture agencies and industrial agricultural operations is needed to create proactive best management practices designed to eliminate significant microplastic sources.
- Increase research on the source, fate and transport of agricultural microplastics.
- Reduce the dependency of industrial agriculture on chemical fertilizers and pesticides.
- Increase the public awareness about microplastics in the agricultural environment and food ingestion.
- Initiate and maintain agricultural awareness and education regarding the implementation of best management practices for plastic management and soil erosion/transport.
- Use organic mulching materials instead of plastic mulches such as crop residues, tree leaves, rice straws, husks, wood dust and other natural materials.
- Eliminate the use of microplastic contaminated wastewater for agricultural irrigation.
- Use regenerative agricultural practices using natural mulches, cover crops and no till practices.
- Use agricultural manure that is low in microplastics to replace the biosolids.