Microplastics: State of the Science and Future Perspective

glass beakers
By Cayla Cook

Plastic pollution is one of the world’s most pressing and observable modern issues, possessing the potential to significantly impact human health, aquatic species health, and, undoubtedly, environmental health. This pollution is categorized by the size in which plastics occur due, in part, to varied fate and transport in soil, water, and even atmospheric compartments. Beyond the very visible nuisance of macro-and mesoplastics, researchers and regulators are now turning their attention to less visible microplastics, which, according to the California Water Resources Control Board (WRCB), includes nanoplastics.

Despite being only 5 mm to 1 nm in size, these sometimes nonvisible polymeric particles are present everywhere. They amass in the environment, potentially accumulating in deep ocean sediment, and are even found in rainfall in the most remote regions of earth. Despite connections to many EPA priority pollutants and antibiotic resistant bacteria, bacterial communities and adsorbed contaminants are not believed to be the primary vector for toxicological impact—this key predictor relies solely on particle size and possibly particle count according to recent modeling efforts. Microplastics are also known to bioaccumulate in aquatic species and, similarly, a portion of ingested microplastics may translocate and deposit within human tissue: in fact, greater than 95 percent of humans studied in multiple countries have been found to excrete microplastics. Furthermore, the particle itself isn’t the only concern as microplastics constitute an entire suite of polymer types and include harmful additives, including the now-infamous bisphenol-A (BPA).

These findings are now compelling utilities, industries, and scientists to focus on monitoring methods that help us better understand our exposure to microplastics and begin assessing mitigation strategies to reduce contamination from sources. Current efforts have identified unlikely culprits for human exposure, including bottled beverages, and similarly unexpected culprits for aquatic exposure, including tire wear particles and washing machine effluent that is high in microfibers.

Nonetheless, our drinking water and wastewater utilities remain a central focus in the current debate.

Regulatory action, research and confirmed material identification of microplastics by size.

Where We Stand Today

Although the toxicity of microplastics is still being studied, sufficient concern encouraged California, in 2018, to issue Senate Bill (SB) 1422, which seeks to develop preliminary health thresholds for microplastics in drinking water, if needed, and establish a four-year water quality monitoring program beginning in July 2021. Led by the WRCB, these efforts aim to proactively address the potential adverse effects posed by microplastics and prepare standardized methods of testing that inform California as well as an international cohort of researchers invested in the study of this topic.

In June 2020, the WRCB, with input from the public and the EPA, fulfilled SB 1422’s first regulatory deadline by establishing the world’s first regulatory definition for microplastics in drinking water:

Solid polymeric materials to which chemical additives or other substances may have been added, which are particles which have at least three dimensions that are greater than 1 nm and less than 5,000 micrometers (μm).

Passed concurrently with SB 1422, SB 1263 next requires the WRCB and Ocean Protection Council (OPC) to create a Statewide Microplastics Strategy by 2022 and recommend source-reduction strategies by 2026. Most recently, the OPC published a report highlighting a precautionary framework for potential microplastic toxicity, which mentioned wastewater more than 20 times and highlighted ongoing research associated with ambient water thresholds. The report also frequently mentioned biosolids, given that research increasingly shows microplastic contamination may result in accumulation in agricultural fields, runoff into nearby water bodies, and, perhaps most heavily debated, uptake by agricultural crops.

While many unknowns remain regarding the impacts of microplastics and how to confidently detect nanoplastics, we do know that as much as 96 percent of microplastics are not included in current mass balances. The most recent studies estimate that microplastics ranging from 1 to 100 μm account for up to 95 percent of particles present in finished drinking water and treated wastewater, respectively, and that removal rates are as low as 35 percent in rural treatment facilities. Furthermore, quantities of microplastics ranging from 1 to 5 μm have been shown to actually increase up to 16 percent in advanced treatment facilities following oxidation, suggesting that current processes can fracture particles to an infinitesimally smaller size which may increase toxicity.

One of the fundamental challenges facing the industry is that, although we are unable to detect these small sizes, they are still included in the regulatory definition and may pose greater risks, both potential and demonstrated. That is to say, current estimates of microplastics in treatment facilities may be dramatically underrepresented.

A recent timeline of regulatory action on microplastics.

Where We’re Headed

One key limitation has been the lack of consensus regarding how to efficiently test for microplastics in water. Multiple American Standards for Testing Materials (ASTMs) have been recently developed to begin to address this loophole with the hope of identifying ASTM methods that begin to detect in the nanoplastic range. The water board is currently working to identify the most effective method to test drinking water and the current opinion is that a tiered monitoring method will provide cost-effective, swift, and increasingly reliable results to support the increasing need for advanced material testing.

As California establishes these aggressive goals, there is still some uncertainty surrounding future regulatory movement at other state or the federal levels. In addition to SB 1422 and SB 1263, there is one law that may facilitate change in microplastics, and more specifically, microfibers – the Break Free from Plastic Pollution Act (BFFPPA) of 2021. Introduced by representatives from California and Oregon, this federal legislation is intended to define sustainable plastic-reduction strategies, such as establishing minimum recycled content standards, banning single-use plastic bags nationwide, and, most crucially, shifting the task and financial burden of designing, collecting, reusing, recycling, and disposing plastic products and packaging from utilities to producers and manufacturers. As of May 2021, the BFFPPA has over 98 co-sponsors in the House of Representatives and 12 in the Senate.

In addition to monitoring these developments, the EPA continues to prepare their own actions and research, investigating microplastics and the persistent organic pollutants that cling to them through the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), also known as Superfund, and awarding grants for the microplastic clean-ups of water bodies such as Lake Tahoe. In addition, there are other on-going efforts related to industrial discharge permits in states like Texas, who has considered the large microplastic pellet discharge a violation resulting in a $50 million fine.

The ultimate goal is to involve stakeholders to increase our understanding of microplastic contamination and begin efforts to reduce, reuse, and remove plastic from water resources.


Cayla Cook

Cayla Cook is the microplastics lead and deputy regional Carollo Research Group (CRG) lead with more than seven years of experience studying the nexus of polymer science and water resources. She spearheads all micro- and nanoplastic-related efforts at Carollo for water, wastewater and biosolids as principal investigator, project manager and technical advisor, among other roles. 

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