Water scarcity is an increasing concern globally. (1,2) Impairment of water for its intended usage has been identified as a type of water scarcity, (3−5) and, thus, water-quality deterioration can exacerbate water-supply shortages (6) and harm ecosystem health. (7) Hydroclimatic extremes and climate shifts can be primary drivers of surface water quality decline. (7,8) Climate drivers in combination with landscape disturbances and other hazards, termed “compound events,” can be particularly deleterious to water quality. (9,10) Despite synergistic process interactions that can cause severe effects on water quality in response to compound events, traditional risk assessments consider single drivers in isolation, potentially underestimating hazards to water quality. (10) As compound events become increasingly common (11,12) and the subsequent water-quality effects are recognized, (13−15) guidance for land, water, and ecosystem management will need to include the effects of overlapping climate extremes and landscape disturbances.

Mining and wildfires are two major disturbances that can have substantial impacts on downstream water quality. Water discharged from mines can have extreme pH and (or) be rich in metals (and metalloids, grouped here with metals), (16) and metal-rich mine waste can be transported 10s to 100s km downstream and stored for extended periods (1,000–100,000 years) in floodplains and lake sediments. (17−22) Mining-affected floodplains are now the primary source of metals to rivers in the United States (U.S.) and western Europe. (22) These metals are being remobilized by floods, (19,22,23) and due to predicted increases in rainfall intensity, flooding-driven redistribution of mining-affected floodplain sediment will likely worsen in the future. (18,24,25) Wildfires can lead to enhanced erosion and sediment transport and subsequent increases in sediment, nutrients, and metals in downstream waters, (26,27) resulting in stream habitat degradation and inflated water treatment costs. (5,28,29) Downstream effects may extend for 100s of km (30−32) and last more than a decade. (29,33−35) Modeled projections suggest that a third of western U.S. watersheds will have >100% more sedimentation by 2050 because of wildfire. (36)

Wildfires are now burning in mining-affected watersheds in many areas of the world, including North America, (37−41) South America, (42) Australia, (43−46) Europe, (47) Asia, (48) and Africa. (49,50) In the western U.S., the intersection of these hazards often coincides with important surface water supply watersheds (39,40,46) (Figure 1), with substantial water-quality and land management implications. For example, wildfire in a legacy mining area in Colorado led to elevated stream concentrations of arsenic for at least five years after the fire (39) and required removal of a ∼70-year-old arsenic- and lead-rich tailings deposit to protect downstream water supplies. (51) A postfire debris flow in Montana mobilized mine waste into a stream, leading to a multimillion-dollar cleanup effort. (38,52) Observed and anticipated increases in extreme wildfire behavior and severity, (53) in concert with amplification of storm intensity in many parts of the world, (54,55) suggest that the intersection of wildfire and mining and subsequent risk to water supplies may worsen. Previous work described potential pathways of metals to surface water in areas affected by wildfire and legacy mining. (39) The objective of this work is to connect current concepts about remobilization of legacy mine waste (18,22,56−58) with recent advances in understanding how wildfire affects landscapes and thus the erosion, mobilization, and transport of sediment. (59−62) We also illustrate the hazards to water supplies posed by the wildfire-accelerated mobilization of mining-derived metals in the western U.S. and identify challenges and opportunities for targeted research...

Figure 1. (A) Map of the western U.S. showing mines and prospects (excluding gravel, sand, and borrow pits and quarries), (131) wildfires (1984–2022), (132) 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) (133) and (B) graph showing cumulative area burned (134) and cumulative number of wildfire-affected mining sites in the western U.S. (intersection of wildfire perimeters (134) and point locations of mine sites; (131) western U.S. here refers to the states of Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, Wyoming).