The Effect of Wildfire on Soil Mercury Concentrations in Southern California Watersheds

Abstract

Mercury (Hg) stored in vegetation and soils is known to be released to the atmosphere during wildfires, increasing atmospheric stores and altering terrestrial budgets. Increased erosion and transport of sediments is well-documented in burned watersheds, both immediately post-fire and as the watershed recovers; however, understanding post-fire mobilization of soil Hg within burned watersheds remains elusive. The goal of the current study is to better understand the impact of wildfire on soil-bound Hg during the immediate post-fire period as well as during recovery, in order to assess the potential for sediment-driven transport to and within surface waters in burned watersheds. Soils were collected from three southern California watersheds of similar vegetation and soil characteristics that experienced wildfire. Sampling in one of these watersheds was extended for several seasons (1.5 years) in order to investigate temporal changes in soil Hg concentrations. Laboratory analysis included bulk soil total Hg concentrations and total organic carbon of burned and unburned samples. Soils were also fractionated into a subset of grain sizes with analysis of Hg on each fraction. Low Hg concentrations were observed in surface soils immediately post-fire. Accumulation of Hg coincident with moderate vegetative recovery was observed in the burned surface soils 1 year following the fire, and mobilization was also noted during the second winter (rainy) season. Hg concentrations were highest in the fine-grained fraction of unburned soils; however, in the burned soils, the distribution of soil-bound Hg was less influenced by grain size. The accelerated accumulation of Hg observed in the burned soils, along with the elevated risk of erosion, could result in increased delivery of organic- or particulate-bound Hg to surface waters in post-fire systems.

Singular and Combined Effects of Blowdown, Salvage Logging, and Wildfire on Forest Floor and Soil Mercury Pools

“…Otherwise, no significant relationship existed between C and Hg within the forest floor, suggesting as have others, that localized heterogeneity resulting from variations in the relative stage of decomposition, degree of burn, and Hg sorption capability of remaining charred surface materials varies substantially within the forest floor. 62,63 Implications. We see important implications from this research, especially considering that blowdown is among the major disturbance agents in US Lake States forests, 34−36 possibly becoming even more prevalent under a changing 42,64 Further, postdisturbance salvage logging is currently among the more contentious forest-management issues in the U.S. and worldwide.…”

Fields and Forests in Flames: Vegetation Smoke and Human Health

“…In a study following fires in 2005 and 2006 in three watersheds in Southern California, researchers found organic or particulate-bound mercury in surface soils can be more readily deposited in waterways after a fire 31 . Awareness of that tendency could lead to actions such as better testing of fish in affected waterways or improved sampling for water quality if the waterways are a drinking water source.…”

Characterization and evaluation of controls on post-fire streamflow response across western US watersheds

“…In terms of water quality, contaminant levels can be dramatically increased for many years after a wildfire in both soil (Burke et al, 2010) and stream systems (Emelko et al, 2011;Stein et al, 2012;Burke et al, 2013), increasing the workload on source water protection organizations in communities reliant upon burned watersheds for drinking and agricultural water. Furthermore, wildfires are readily attributed as the cause of substantial increases in debris flows (Benavides-Solorio and MacDonald, 2001;Cannon et al, 2001;Meyer et al, 2001).…”