Increasing reliance on treated wastewater as an alternative irrigation source is prompting US researchers to investigate whether microplastics could transport antibiotics into agricultural soils and potentially increase their uptake by food crops.

The work by the US Department of Agriculture’s Agricultural Research Service (ARS) could have implications for irrigated crops, including potatoes, as drought and pressure on freshwater resources increase interest in reclaimed municipal wastewater as an agricultural water source.

Researchers are examining how microplastics entering wastewater from sources including laundry, personal care products and dishwashing interact with pharmaceuticals that can remain in treated water.

“Microplastics can be introduced to wastewater from multiple sources, including personal care products, laundry, and dish washing,” said Clinton Williams, soil scientist and research leader at the ARS Water Management and Conservation Research unit in Maricopa, Arizona. “Once in the wastewater these plastics can adsorb (hold onto) organic molecules, including pharmaceuticals.”

According to ARS, microplastics carried into agricultural soils through irrigation can accumulate around the interface between roots and soil. Pharmaceuticals attached to the particles may subsequently be released, creating the possibility that they could be taken up by crops.

Researchers are also examining whether antibiotics transported in this way could create conditions favouring the development of antimicrobial resistance among soil microorganisms.

Williams said synthetic materials including nylon and acrylics showed the greatest potential to transport antibiotics. Further research will seek to quantify whether antibiotics associated with microplastics can be taken up by edible crops, while changes in antimicrobial resistance within soil microbial communities are also being measured.

Conventional wastewater treatment facilities are primarily designed to remove organic carbon and nitrogen rather than trace concentrations of pharmaceuticals. ARS said antibiotics can occur at concentrations below 10 parts per billion and are not directly removed during treatment.

Williams and researchers at Clemson University and the University of North Carolina at Greensboro are studying the combined effects of microplastics and antimicrobials in agricultural ecosystems by applying the materials together to soils and measuring subsequent antimicrobial resistance.

The findings could become increasingly relevant as water-stressed agricultural regions seek alternatives to conventional freshwater irrigation.

“[Irrigation with treated wastewater] will increase,” Williams said. “It’s a resource that can be used to increase water supplies without significant costs, [but] if microplastics are shown to increase the uptake of antibiotics and other micro-organics into food crops, there may be a need to remove microplastics from treated wastewater.”

The research has not established that microplastics are increasing antibiotic uptake in food crops; ARS is currently investigating the scale of the potential risk and whether additional treatment of reclaimed water could ultimately be required.