Wildfires in Old Mining Areas Pose Risk to Water Quality

Wildfires can impair surface-water quality, reduce reservoir storage, and increase costs to water providers and customers, particularly in the western United States. The most frequently reported water-quality impairments after fires include increased sediment and nutrient concentrations; these are caused by increased surface runoff, erosion, and transportation of ash and soil into surface waters. However, changes in metal export from past mining activities in burned watersheds to surface waters have been largely unknown.

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The 2010 Fourmile Canyon Fire near Boulder, Colorado burned a watershed that had been mined from the 1860s to the 1940s and subsequently reforested. To test the impacts of wildfire on downstream water quality, USGS scientists measured arsenic and metal contamination in streams affected by both fire and legacy mining during the five years following the wildfire event. They found that although most metal concentrations were low, water and sediment collected downstream of the burned area had elevated arsenic concentrations during and after post-fire rain events. The scientists determined that increased surface runoff after the fire, and potentially increased flow through mine workings, mobilized arsenic- and metal-rich legacy mining deposits, in addition to wildfire ash and soil, into streams. While other studies have reported that wildfire ash can be an important source of arsenic and metals to surface waters, wildfire ash was not a major source of arsenic in this study.

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This USGS study determined that wildfires burning in watersheds that have been mined and then revegetated pose unique risks to downstream water supplies. Potential ways that metals can be transported to surface water after wildfire in historically mined areas include:

• Precipitation that falls directly onto exposed mine waste (caused by loss of canopy vegetation) can dissolve and transport the metals from the waste to streams.
• Increased surface runoff and stream flow during storms can erode and transport metal-rich sediment from hillslopes and streambanks to streams.
• Minor increases in stream flow can remobilize metal-rich sediments deposited on stream banks.
• Metals can dissolve from storm-deposited sediment in streams as a result of a change in chemical state.
• Increased underground flow of water through mine workings can cause increased flushing of metals into streams.
• Greater water-level fluctuations within mine workings could promote the oxidation of sulfide minerals and increase arsenic mobility.

Frequency, size, and intensity of wildfires in the western U.S., a region with widely dispersed historical mines, are predicted to increase in the future. Therefore, the intersection of legacy mining and post-wildfire hydrologic response poses an increasing risk to many water supplies in this region due to the risk of increased metal concentrations. High levels of metals in drinking water can impair human health and leads to greater costs for water treatment. USGS research is advancing our ability to measure, model and predict potential impacts of wildfires on water supplies.

The paper, “Wildfire-driven changes in hydrology mobilize arsenic and metals from legacy mine waste”, was published in Science of the Total Environment.

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