POST-WILDFIRE SURFACE WATER QUALITY IN MINED WATERSHEDS

Surface waters draining mined areas can have an elevated risk of metals pollution due to increased mineral surface areas and altered flow paths created during the mining process. Storm events, which can be exacerbated by climate change, can increase flooding and remobilize metal-rich mine waste from hillslopes or floodplain deposits. Vegetation in mining areas can be elevated in metals, which are made available for export when wildfire converts the trees to ash. Wildfires reduce vegetation canopy, alter soil properties, and leave ash and burned debris vulnerable to erosion. A combination of these multi-stressors - mining, wildfires, and flooding - creates risk to water quality through increased fluxes of sediment, nutrients, and metals to surface waters. Identifying pathways of metal transport and physical and chemical processes that may increase mobility or bioavailability of metals are important to understanding the effects of these multi-stressors on water quality. This presentation compares the potential pathways and chemistry of stressors by comparing the geochemistry of soils, stream sediments, and water from two burned watersheds in the southern Rocky Mountains: Fourmile Creek near Boulder, CO and Gallinas Creek near Las Vegas, NM. Post-fire soil geochemistry revealed similar manganese (Mn) and arsenic (As) concentrations in the Gallinas Creek and Fourmile Creek watersheds. Mn concentrations in stream sediments from both watersheds were also generally similar, 491 to 4,051 mg/kg, with some higher values in Gallinas Creek. Manganese concentrations in unfiltered water ranged from 2 to 611 µg/L (median 37 µg/L) in Fourmile Creek and from 0.88 to 967 µg/L (median 265 µg/L) in Gallinas Creek. Average As concentrations in stream sediments from Fourmile Creek were 84.6 mg/kg, approximately 20 times higher than the average concentration in Gallinas Creek. As in unfiltered water is generally 30 times higher in Fourmile Creek than Gallinas Creek. These results reflect differences and similarities in the geochemistry of stream sediment, soils, and water within these two watersheds, suggesting that geology and mineralization play a critical role in post-fire geochemistry.