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July 11th, 2002

Wildfires will hurt Colorado water quality and fish, CU-Boulder expert says

Colorado's record-setting wildfire season will leave behind potentially harmful conditions in water supplies, according to a University of Colorado at Boulder researcher and former U.S. Forest Service firefighter.

Mark Williams, a fellow at CU's Institute of Arctic and Alpine Research and co-associate director of the institute's undergraduate academy, said that a potentially cancerous compound called trihalomethane (THM) could show up in drinking water drawn from water bodies that drain burned areas.

"THM has been found to be cancer-causing in some laboratory tests on rats," said Williams, associate professor of geography and environmental studies and former firefighter at Bridger-Teton National Forest near Big Piney, Wyoming.

THM forms when dissolved organic carbon--which is present in higher levels in water near burned areas--reacts with chlorine disinfectant at treatment plants. Municipalities that get drinking water from surface waters affected by wildfire will most likely develop elevated levels of THM unless the dissolved organic carbon can be removed before the water is disinfected with chlorine, Williams said.

"There are a lot of communities that will be affected," he said. THM is more likely to occur in areas that use surface water, because it's more likely that organic matter will be found in surface water supplies as opposed to deep ground wells, he explained.

"We have a major problem here in Boulder of high THM levels caused by naturally occurring dissolved organic carbon in snowmelt runoff," Williams said, noting that Boulder officials use a sand filtration technique and divert streams away from water supplies when carbon levels are high.

Since 1979 community water systems that serve 10,000 or more have been required to check for THM when a disinfectant such as chlorine is used. But treatment budgets could take a hit this year because of the wildfires.

"If they don't already check for THM, any treatment strategy will incur extra expenses that aren't currently budgeted," Williams said.

Fish could suffer from depleted oxygen levels in water long after wildfires are put out.

Williams said that for several years, water from burned areas will have five to 100 times higher concentrations of nutrients like nitrate, ammonium, nitrogen and phosphorus. Water bodies near steep slopes will carry more nutrients, he said, because slopes make rainfall and snowfall more efficient in transporting nutrients from soils at higher elevations.

Downstream, this causes eutrophication, a process that stimulates growth in aquatic plant life and usually results in the depletion of oxygen in the water.

"If we have the worst-case combination of higher nutrients in the water and another drought year, you'll see fish kills because of reduced oxygen in water bodies caused by decomposition of the increased organic matter," Williams said.

There isn't a simple way to block the nutrients and prevent the consequences downstream, he said, but hydrological controls that divert runoff into groundwater channels might be the most cost-effective mitigation tool.

"A more speculative problem that should be investigated is that fish may also be threatened by greater transport of mercury to large water bodies," Williams said. Wildfire frees mercury that is naturally stored in vegetation. Mercury transport is enhanced by soil and stream water that is high in dissolved organic carbon, which is often the case in the wake of a fire, he explained.

"Mercury levels are already high in several reservoirs in Colorado. These large fires may result in an increase in the mercury content of game fish," he said.

Williams noted that in 1998, mercury content of fish already exceeded EPA advisory levels in five state reservoirs: McPhee and Nauraguinnep in southwestern Colorado, Sanchez and Navajo in south central Colorado, and Teller Reservoir near Colorado Springs.