Trace Elements in Stormflow, Ash, and Burned Soil following the 2009 Station Fire in Southern California

Abstract

Most research on the effects of wildfires on stream water quality has focused on suspended sediment and nutrients in streams and water bodies, and relatively little research has examined the effects of wildfires on trace elements. The purpose of this study was two-fold: 1) to determine the effect of the 2009 Station Fire in the Angeles National Forest northeast of Los Angeles, CA on trace element concentrations in streams, and 2) compare trace elements in post-fire stormflow water quality to criteria for aquatic life to determine if trace elements reached concentrations that can harm aquatic life. Pre-storm and stormflow water-quality samples were collected in streams located inside and outside of the burn area of the Station Fire. Ash and burned soil samples were collected from several locations within the perimeter of the Station Fire. Filtered concentrations of Fe, Mn, and Hg and total concentrations of most trace elements in storm samples were elevated as a result of the Station Fire. In contrast, filtered concentrations of Cu, Pb, Ni, and Se and total concentrations of Cu were elevated primarily due to storms and not the Station Fire. Total concentrations of Se and Zn were elevated as a result of both storms and the Station Fire. Suspended sediment in stormflows following the Station Fire was an important transport mechanism for trace elements. Cu, Pb, and Zn primarily originate from ash in the suspended sediment. Fe primarily originates from burned soil in the suspended sediment. As, Mn, and Ni originate from both ash and burned soil. Filtered concentrations of trace elements in stormwater samples affected by the Station Fire did not reach levels that were greater than criteria established for aquatic life. Total concentrations for Fe, Pb, Ni, and Zn were detected at concentrations above criteria established for aquatic life.

Fire Promotes Arsenic Mobilization and Rapid Arsenic(III) Formation in Soil via Thermal Alteration of Arsenic-Bearing Iron Oxides

“…A United States Geological Survey study explored As speciation in soils and ash subjected to wildfire and found a predominance of As(V), as well as some As(III), readily mobilized in water leachate (Wolf et al, 2010). Although a number of studies have reported elevated As in surface waters after wildfires (Leak et al, 2003;Burke et al, 2013;Burton et al, 2016;Abraham et al, 2017), none of these studies present geochemical mechanistic explanations for why this might be the case -aside from invoking enhanced sediment erosion and likely desorption from higher pH ash/burned soil. The findings of this study provide a mechanism which may help explain past-observations of elevated As in surface waters after wildfires -yet simultaneously, the findings also highlight the need for future field-focused research.…”

“…Importantly, fires can also enhance trace metal/metalloid mobilization from burnt soils, altering soil properties and potentially degrading surface water quality in burnt catchments (Bladon et al, 2014;Abraham et al, 2017;Burton et al, 2019). An increasing body of field-based research provides examples of elevated trace metal/metalloid concentrations in soils and surface waters after wild-fires (e.g., Stein et al, 2012;Burke et al, 2013;Odigie and Flegal, 2014;Campos et al, 2015Campos et al, , 2016Burton et al, 2016;Abraham et al, 2018). However, in general there remains a distinct lack of mechanistic studies exploring fire-induced geochemical changes to metal/metalloid speciation or to the mineralogy of their host-phase(s) and how this may be contributing to observations of enhanced trace metal/metalloid mobility (Cerrato et al, 2016).…”

Metal Reactivity in Laboratory Burned Wood from a Watershed Affected by Wildfires

“…Several post fire investigations have reported increased concentration of metals such as Fe, Pb, Ni, Zn, Al, As, Cd, Mn and Cu in the watershed affected by increased loading of ash and sediments. [7][8][9][10] The Valles Caldera National Preserve (VALL) in north central New Mexico is an example of a site with frequent wildfire activity in recent years. 11 Two major wildfires have affected the VCNP watershed since 2011: (1) The Thompson Ridge wildfire burned 23,965 acres in VCNP in 2013 12 and (2) The catastrophic Las Conchas, one of the largest in New Mexico history, which burned over 156000 acres of area in the Jemez Mountains in 2011.…”

“…After the 2009 station fire in California, total concentrations of As, Pb, Zn and Ni were reported higher in the burned watersheds. 10 Trace elements (e.g., Fe, Mn, Hg) from burned soil and ash were also found in elevated concentrations in nearby streams after the fire. 10 Elevated concentrations of As, Al, Cd, Cr, Fe, Pb, Hg, Ca, Mg, Mn, Ba and K have also been observed in sediments and streamflows in fire-affected watersheds, several months after the fire events.…”

“…10 Trace elements (e.g., Fe, Mn, Hg) from burned soil and ash were also found in elevated concentrations in nearby streams after the fire. 10 Elevated concentrations of As, Al, Cd, Cr, Fe, Pb, Hg, Ca, Mg, Mn, Ba and K have also been observed in sediments and streamflows in fire-affected watersheds, several months after the fire events. 7,[24][25][26][27] A previous study by Ignatavicius et al 28 of trace metals were found in some samples.…”