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#           Wildfire-driven changes in hydrology mobilize arsenic and metals from legacy mine waste      

      July 2, 2020    

Wildfires burning in watersheds that have been mined and since revegetated pose unique risks to downstream water supplies. A wildfire near Boulder, Colorado that burned a forested watershed recovering from mining disturbance that occurred 80-160 years ago allowed us to 1) assess arsenic and metal contamination in streams draining the burned area for a five-year period after the wildfire and 2) determine the fire-affected hydrologic drivers that convey arsenic and metals to surface water. Most metal concentrations were low in the circumneutral waters draining the burned area. Water and sediment collected from streams downstream of the burned area had elevated arsenic concentrations during and after post-fire storms. Mining-related deposits were the main source of arsenic to streams. An increased proportion of overland flow relative to infiltration after the fire mobilized arsenic- and metal- rich surface deposits and wildfire ash into streams within and downstream of the burned area. The deposition of this sediment into stream channels resulted in the remobilization of arsenic for the five-year post-fire study period. It is also possible that enhanced subsurface flow after the fire increased contact of water with arsenic-bearing minerals exposed in underground mine workings. Other studies have reported that wildfire ash can be an important source of arsenic and metals to surface waters, but wildfire ash was not an important source of arsenic in this study. Predicted increases in frequency, size, and intensity of wildfires in the western U.S., a region with widely dispersed historical mines, suggest that the intersection of legacy mining and post-wildfire hydrologic response poses an increasing risk for water supplies.

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                  -              Data           
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June 30, 2020

####                 Chemistry of water, stream sediment, wildfire ash, soil, dust, and mine waste for Fourmile Creek Watershed, Colorado, 2010-2019              

Water Resources Mission Area

#### Sheila Murphy

##### Research Hydrologist

Water Resources Mission Area

sfmurphy@usgs.gov

303-541-3023

#### R. Blaine McCleskey

##### Research Chemist

Water Resources Mission Area

rbmccles@usgs.gov

303-541-3071

#### JoAnn Holloway

##### Research Physical Scientist

Geology, Geophysics, and Geochemistry Science Center

jholloway@usgs.gov

303-236-2449

## Related Content

June 30, 2020

####                 Chemistry of water, stream sediment, wildfire ash, soil, dust, and mine waste for Fourmile Creek Watershed, Colorado, 2010-2019              

Water Resources Mission Area

#### Sheila Murphy

##### Research Hydrologist

Water Resources Mission Area

sfmurphy@usgs.gov

303-541-3023

#### R. Blaine McCleskey

##### Research Chemist

Water Resources Mission Area

rbmccles@usgs.gov

303-541-3071

#### JoAnn Holloway

##### Research Physical Scientist

Geology, Geophysics, and Geochemistry Science Center

jholloway@usgs.gov

303-236-2449

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####             Water Resources Mission Area - Headquarters        

USGS Headquarters12201 Sunrise Valley DriveReston, VA20192United States

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