Dominance of Particulate\nMercury in Stream Transport\nand Rapid Watershed Recovery from Wildfires in Northern California,\nUSA

Frequency and intensity of wildfires are expected to\nincrease due\nto climate change, especially in areas with a long summer drought.\nForests are a major sink for the global pollutant mercury (Hg), and\nfluvial transport of Hg from recently burned watersheds has not been\nwidely investigated. Here, we examined two years of fluvial transport\nof Hg and its speciation (total Hg, methyl-Hg, particulate, and dissolved\nforms) under storm events and baseflow in two recently burned watersheds\nwith different burned proportions and one nonburned reference watershed\nin the Coastal Ranges of northern California. We examined postfire\nstorm-event transport of Hg and its methylated form (methyl-Hg), addressed\nthe importance of the “initial runoff pulse” to postfire\nHg fluvial transport and its predominant association with suspended\nsolids, and elucidated potential sources of Hg exports from the burned\nlandscapes using geochemical indicators, which suggested that ash\nmaterials were likely the significant sources of particulates in the\nfirst high-flow season postfire but not subsequently. The maximum\ntotal suspended solid and total Hg levels in the “first pulse”\nat the severely burned watershed were 442 and 46 times higher, respectively,\nthan those at the reference watershed. Stream suspended solid and\nHg levels declined substantially in the burned watersheds after just\na few months of rainfall likely due to the rapid regrowth of vegetation\ncommonly observed in postfire landscapes, implying that the wildfire\neffects on immediate Hg inputs from the burned landscape are at most\ntransient in nature.