Research is intensifying on the impact of spreading sewage sludge on farmland in spreading microplastics into watercourses.
Urban wastewater treatment plants were identified as one of the largest point sources of microplastics which are plastic particles less than 5mm in diameter.
They are formed either through the breakdown of large plastic particles or through intentional production for products such as cosmetics and cleaning agents.
The production of sewage sludge has increased over the years, and in 2015 more than 58,000 tonnes were produced in the Republic of Ireland.
Although there are many disposal and reuse pathways, in Ireland up to 80pc is currently reused on agricultural land.
Dr. Anne Marie Mahon of Galway Mayo Institute of Technology told an Oireachtas committee examining a proposed law prohibiting certain products containing such microbeads in Ireland, sewage sludge is used as a fertiliser for tillage, not for crops for human consumption.
"Microplastics, which are incorporated into the sewage sludge, may find their way back onto the land due to spreading of sewage sludge practices," she said.
She also noted that there are regulations or guidelines regarding the application of sewage sludge.
"For example, there are regulations regarding the distance from a watercourse and what kind of whether one should or should not spread in. "Some sewage sludge is applied untreated. Most are applied treated.
"These treatments, which we looked at in one study, have an impact on the microplastics.
"They can exacerbate the problem by breaking the particles down further. "One of the main threats or reasons we are concerned about microplastics is the size.
"The smaller they get, the more environmental compartments they can penetrate. They can go into the lower bottom of the food chain and can be transported up readily.
"Once they go into the nanometer range, they can cross a cell membrane. The smaller they get, the more concerned we are about it," she said.
Dr Mahon said her research team are currently intensifying their investigations on land and by taking some sediment cores on sites that have been land-spread with sewage sludge for 20 years to see what the vertical profile of the microplastics would be.
"We hope that we will get an idea from that as to whether the microplastics will be mobilised after a rain event, therefore increasing the risk to local, nearby watercourses or whether they would be transported vertically and perhaps enter the groundwater.
"Either way, it is not ideal," she said.