Climate Change May Stunt Ability Of Ultraviolet Light To Disinfect Lakes & Coastal Waters, Exacerbating Drinking Water Scarcity

October 31st, 2017 by James Ayre

Climate change, through the impacts of increasingly extreme rainfall events and melting permafrost on the organic matter content of various bodies of water, may lead to a stunting of the ability of the sun’s ultraviolet light to disinfect the world’s lakes and coastal waters, according to a new study published in Scientific Reports.

To explain, as the organic matter content of water rises, so does the inability of the sun’s light to penetrate very deeply into the body of water in question — which means that more and more of the pathogenic microbes washed into the world’s lakes and coastal waters from cities and agricultural operations will persist for longer periods of time, presuming the study findings are accurate.

“The pristine waters of Lake Tahoe’s Sand Harbor contrast with the brown water in the lake’s Star Harbor, where people and boats are active. Dissolved organic matter from human activity and from heavy rains can cloud the water and reduce solar disinfection.” Photo by Andrew Tucker

To date, a “browning” of the world’s waters has already been observed owing to rising levels of soil erosion and increasingly extreme rainfall events in some areas.

“Much of the research emphasis up to this point has been on the browning itself, not the ecological consequences,” stated 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.”

The press release provides more: “For the study, Williamson and his colleagues relied on water samples collected from lakes around the world, from Pennsylvania and Wisconsin to Chile and New Zealand. The water samples were tested to determine how much dissolved organic matter each contained, as well as the wavelengths of light — including ultraviolet wavelengths — absorbed by that organic matter.

“Then NCAR scientist Sasha Madronich used this information as well as modeling results from the Tropospheric Ultraviolet-Visible model to calculate the solar inactivation potential (SIP) for each lake. SIP is an index of the expected disinfecting power of UV light in a particular body of water, based on its dissolved organic matter and other characteristics.

“Madronich also analyzed reflection and refraction off each lake’s surface to calculate how much light penetrates the lakes and then, finally, how deeply it reaches. 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 use the model output to calculate the SIP for each lake. In some cases, they also calculated this measure of expected disinfecting power across different parts of, or for different time periods in, the same lake.”

So, what were the the findings? To use one random lake in Pennsylvania as an example, the summertime SIP at that site dropped by around half during the 1994–2015 time period.

That lake has experienced a similar level of browning in recent decades as most other lakes in the region, according to the researchers.

So, why does this matter? Because as things continue to worsen in this regard, water treatment plants will become increasingly expensive to operate, and surface waters will become more and more dangerous. This will be compounded directly by the reality that as temperatures continue climbing, pathogenic bacteria will become more and more common in surface waters anyways.

As it stands, around 12 million to 19 million people already become sick from waterborne pathogens every year.











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