Diverting streams and rivers to irrigate crops or provide drinking water may “significantly extend” the time microplastics spend in river catchments, new research states.
Diverted water from rivers disperse tiny plastic particles across farmland from where they may be flushed back into other watercourses or neighbouring catchments, the research by an international group of scientists led by the University of Birmingham has found.
The findings have been published in scientific journal Water Research.
The research team studied two paired rivers in Colorado, North America – the Boulder Creek and its less urbanised tributary, South Boulder Creek.
The researchers found that microplastic concentration patterns in both rivers were related to the degree of catchment urbanisation.
Data from both streams suggests a link between microplastic concentration and urbanisation, they state.
They found that microplastic concentrations in Boulder Creek with a more urbanised catchment were higher in both surface water and sediment than in South Boulder Creek.
Microplastic concentration increased in the downstream direction when passing more urbanised areas, they noted.
“We discovered strong links between the degree of urbanisation in the river catchment and observed river microplastic concentrations, highlighting how human activities resulted in immediate increase in microplastics in this mountainous catchment,” lead author Anna Kukkola from the University of Birmingham said.
“A key novelty of this study is the application of the loading approach which is used here for the first time for the quantification of microplastics fluxes,”she explained.
This enabled the team to “not only identify microplastic sources, but also determine the downstream evolution of microplastic transport patterns, and in this case also the diversion of microplastics out of the river catchment”, she said.
“These results for microplastics are consistent with our results for other urban-derived elements such as chloride, where we are seeing three to nine times more loading in the more urbanized Boulder Creek watershed,” Co-author Rob Runkel, from the United States Geological Survey,said.
The international team furthermore discovered that the magnitude of flow diversions from both streams resulted in large quantities of microplastic being removed from each stream and being transported out of their actual catchment.
They measured microplastic removal through flow diversions of over 500 microplastic particles per second (or 1,800,00 per hour) from the two rivers studied.
To put this into perspective,the researchers state that in 2012, 241 km3 of water were diverted for agricultural purposes in North America alone, with 2,670 km3 having been diverted globally.
By using conservative estimates based on the >63 µm particle threshold of their study, the researchers say that they estimated that this could result in around 41 trillion microplastic particles being redistributed out of river networks into the terrestrial environment in North America every year.
They calculated as many as 459 trillion particles could be redistributed globally.
“How we manage our streams and rivers can have a substantial impact on the transport of microplastics, yet these effects have not been incorporated into global models that assume downstream convergence of microplastic fluxes along river networks,” co-author and principal investigator Prof Stefan Krause, from the University of Birmingham, said.
“Our current models may, therefore, underestimate the quantities and residence times of plastics held in river catchments and overestimate the speed with which microplastics are transported into our oceans,”Krause said.
While toxicity assessment was not a focus of the current study, co-author Professor Iseult Lynch from the University of Birmingham said the “results of this study are highly relevant for estimating ecotoxicological impacts on aquatic and terrestrial environments and ecosystems”.
Participating institutions with the University of Birmingham included the United States Geological Survey, USA; Centre for Environment, Fisheries and Aquaculture Science, Lowestoft, UK; and University of Lyon, France.
The research was part of the Leverhulme Trust-funded 100 Plastic Rivers project
The paper is available here
