The role of geology and legacy mines in post-wildfire water quality

Abstract

Wildfires can lead to increased surface runoff, erosion, and conveyance of sediment, ash, pollutants, and debris to surface water during storm events. This can result in decreased water quality, loss of reservoir storage capacity, stream habitat degradation, and increased treatment costs for drinking-water providers. Studies have shown that the range and magnitude of post-wildfire water-quality effects vary widely and that important factors include burn severity, wildfire extent and post-fire precipitation. Less is known about the role of underlying geology and mineralogy. Wildfires have become a common occurrence in the western U.S., a region with a diverse array of underlying geology and mineralogy as well as a large number of legacy mining sites. 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 delivery of metals to water bodies. There are several potential post-wildfire pathways for metal transport to surface water after wildfire: precipitation falling directly onto mine waste surfaces after vegetation on the waste has burned, leading to the dissolution and transport of metals to streams; increased surface runoff and stream flow that erode and transport metal-rich sediment deposits from hillslopes and streambanks to streams; remobilization of the metals from streambeds; and increased flow, and greater fluctuation, of water through underground mine workings and delivery of this water to the stream network. The role of these processes in post-wildfire water quality must be accounted for in different regions across the western U.S. The USGS is working to advance our ability to measure, model and predict potential impacts of wildfires on water supplies.