Summary: Climate-driven increases in precipitation dissolve more organic matter into surface waters, reducing sunlight’s ability to destroy waterborne pathogens through UV radiation. This effect threatens drinking water safety for millions who rely on surface-water supplies, including Lake Michigan’s 10+ million consumers.

Why UV Inactivation of Pathogens Matters for Drinking Water

Solar ultraviolet radiation (UV-A and UV-B) is nature’s free disinfectant. In clear, shallow surface water, UV photons damage the DNA and RNA of bacteria, viruses, and protozoa—rendering pathogens like Cryptosporidium, Giardia, and norovirus unable to replicate. Water utilities and regulators have long considered this natural die-off as a supplemental barrier between source water and treatment plants.

The mechanism of concern identified by Williamson et al. (2017) is straightforward: dissolved organic matter (DOM)—the brown, tannin-rich material leached from soil and decaying vegetation—absorbs UV wavelengths before they reach pathogens suspended deeper in the water column. As precipitation intensifies and permafrost thaws, DOM loading in rivers and lakes is accelerating worldwide, a process called “browning.” A 10–30% reduction in UV transparency has already been measured in many northern lakes over the past two decades.

Water treatment engineers must account for this trend by increasing UV dose in treatment systems, adopting supplemental disinfection barriers, and monitoring turbidity and DOM concentration in real time. For municipal systems drawing from surface water, this research reinforces the necessity of multi-barrier treatment—coagulation, filtration, UV disinfection, and residual chlorination—rather than relying on any single step.

Reverse osmosis (RO) systems remove dissolved organic matter at the membrane stage, reducing the chlorine demand and potential for disinfection by-product (DBP) formation downstream. Pairing RO pre-treatment with UV disinfection produces water that is both pathogen-free and low in DOM, addressing both the direct contamination risk and the secondary browning problem described in this study.

Author Full Names: Williamson, Craig E.; Madronich, Sasha; Lal, Aparna; Zepp, Richard G.; Lucas, Robyn M.; Overholt, Erin P.; Rose, Kevin C.; Schladow, S. Geoffrey; Lee-Taylor, Julia
Source: SCIENTIFIC REPORTS, 7 10.1038/s41598-017-13392-2OCT 12 2017
Language: English

Abstract: Climate change is accelerating the release of dissolved organic matter (DOM) to inland and coastal waters through increases in precipitation, thawing of permafrost, and changes in vegetation. Our modeling approach suggests that the selective absorption of ultraviolet radiation (UV) by DOM decreases the valuable ecosystem service wherein sunlight inactivates waterborne pathogens. Here we highlight the sensitivity of waterborne pathogens of humans and wildlife to solar UV, and use the DNA action spectrum to model how differences in water transparency and incident sunlight alter the ability of UV to inactivate waterborne pathogens. A case study demonstrates how heavy precipitation events can reduce the solar inactivation potential in Lake Michigan, which provides drinking water to over 10 million people. These data suggest that widespread increases in DOM and consequent browning of surface waters reduce the potential for solar UV inactivation of pathogens, and increase exposure to infectious diseases in humans and wildlife.

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Source: Water Feed