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Waterborne pathogens becoming more common due to climate change

New research reveals the increased frequency of intense storms and floods due to climate change is contributing to increased levels of waterborne pathogens and diseases in lakes, rivers and coastal waters.

A new study published in Scientific Reports outlines how a rise in the amount of organic matter washed into bodies of water after storms and floods can prevent pathogen-killing UV rays from penetrating the water’s surface.

This ‘browning’ of coastal and inland bodies of water is part of a trend that’s expected to continue as climate change leads to more extreme rainfall and thawing of permafrost.

To conduct the study, researchers from Miami University in the US, in collaboration with the National Center for Atmospheric Research (NCAR), analysed water samples from bodies of water around the world, including the US, Chile and New Zealand. This is the first time researchers have quantified the impact of dissolved organic matter on the ability of UV radiation to kill waterborne pathogens.

"Much of the research emphasis up to this point has been on the browning itself, not the ecological consequences," said lead author Craig Williamson, an ecologist at Miami University.

"We were able to determine that in some cases, browning is decreasing the ability of sunlight to disinfect water by a factor of 10. This could have serious implications for drinking water supplies and coastal fisheries across the globe."

Scientists at the NCAR combined this information with modelling from the Tropospheric Ultraviolet-Visible model to calculate each one’s solar inactivation potential (SIP), which is an index of the expected disinfecting power of UV light in a body of water.

Because scientists already have some understanding of which wavelengths of UV light do the most damage to which waterborne pathogens, the scientists were able to calculate the SIP for each lake.

The results allowed them to quantify the impacts of dissolved organic matter. For example, the summertime SIP for one lake in northeastern Pennsylvania in the US — which, along with other regional lakes has undergone significant browning in recent decades — dropped by about half between 1994 and 2015.

The scientists also showed how SIP can dramatically decrease after a heavy rainfall event, using water samples collected from the region where the Manitowoc River flows into Lake Michigan, which supplies drinking water to more than 10 million people in the US.

Modeling based on samples taken before and after a strong storm moved through in mid-2011, showed that its SIP might have dropped by as much as 22% due to the extra dissolved organic matter that washed into the area.

The study highlights possible challenges for water supply managers and public health workers as the climate continues to warm and extreme precipitation events become more common. Not only does an increase in dissolved organic matter make it more difficult for sunlight to disinfect bodies of water, it also makes it more difficult for water treatment plants to work effectively, Williamson said.

The research also underscores the importance of working across scientific disciplines to fully understand the impacts of climate change across the Earth system, said Sasha Madronich, an atmospheric chemist with the NCAR).

"What happens in the atmosphere affects what happens in lakes," he said.

"These are not separate compartments of the world. These things are all connected."

The study was an outgrowth of collaboration among multiple scientists from different disciplines who serve on the United Nations Environment Programme Environmental Effects Assessment Panel (UNEP EEAP).