Intersection of Wildfire and Legacy Mining Poses Risks to Water Quality

Abstract

Mining and wildfires are both landscape disturbances that pose elevated and substantial hazards to water supplies and ecosystems due to increased erosion and transport of sediment, metals, and debris to downstream waters. The risk to water supplies may be amplified when these disturbances occur in the same watershed. This work describes mechanisms by which the intersection of mining and wildfire may lead to elevated metal concentrations in downstream waters: (1) conveyance of metal-rich ash and soil to surface waters, (2) increased dissolution and transport of dissolved metals due to direct contact of precipitation with mine waste, (3) increased erosion and transport of metal-rich sediment from mining waste, (4) remobilization of previously deposited metal-contaminated floodplain sediment by higher postfire flood flows, and (5) increased metal transport from underground mine workings. Predicted increases in wildfire size, frequency, and burn severity, together with the ongoing need for metal resources, indicate that improved mapping, monitoring, modeling, and mitigation techniques are needed to manage the geochemical hazard of the intersection of wildfire and mining and implications for water availability.

Keywords: compound events; disturbances; geochemical hazards; metals; water supplies; western United States; wildland fire.

Figure 1

(A) Map of the western U.S. showing mines and prospects (excluding gravel, sand, and borrow pits and quarries), wildfires (1984–2022), and an index of relative importance to surface drinking water (based on average annual water yield multiplied by a drinking water protection model that includes population served and intake locations) and (B) graph showing cumulative area burned and cumulative number of wildfire-affected mining sites in the western U.S. (intersection of wildfire perimeters and point locations of mine sites; western U.S. here refers to the states of Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, Wyoming).

Figure 2

Conceptual diagram of water-quality hazards from the intersection of wildfire and mining. Number labels identify metal contamination sources and long-term hazards in diagrams and photographs. Yellow arrows denote pathways of dissolved and particulate metals. (A) Potential hillslope and headwater metal sources. (B) Mine waste within the 2010 Fourmile Canyon Fire burned area, Colorado, eroded by postwildfire floods. (C) Mine tailings impoundment in Grand County, Colorado. (D) Mine adit within the 2010 Fourmile Canyon Fire burned area, Colorado. (E) Conceptual model of downstream metal contamination sources and long-term hazards showing inset A in the basin headwaters. Photographs by Sheila Murphy.