The impact of over 100 years of wildfires on mercury levels and accumulation rates in two lakes in southern California, USA
Sarah E. Rothenberg
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Matthew E. Kirby
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Broxton W. Bird
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Margie B. DeRose
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Chu-Ching Lin
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Xinbin Feng
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Richard F. Ambrose
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Jennifer A. Jay
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M. E. Kirby B. W. Bird M. B. DeRose Department of Geological Sciences, California State University
, Fullerton, Fullerton,
CA 92834, USA
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S. E. Rothenberg (&) R. F. Ambrose Environmental Science and Engineering Program, Department of Environmental Health Sciences, University of California
,
Los Angeles
, Box 951772,
CA 90095-1772, USA
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Present Address: S. E. Rothenberg X. Feng (&) State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences
, Guiyang 550002,
China
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C.-C. Lin J. A. Jay Department of Civil and Environmental Engineering, University of California
, Box 951593,
Los Angeles, CA 90095-1593, USA
In southern California, USA, wildfires may be an important source of mercury (Hg) to local watersheds. Hg levels and Hg accumulation rates were investigated in dated sediment cores from two southern California lakes, Big Bear Lake and Crystal Lake, located approximately 40-km apart. Between 1895 and 2006, fires were routinely minimized or suppressed around Big Bear Lake, while fires regularly subsumed the forest surrounding Crystal Lake. Mean Hg concentrations and mean Hg accumulation rates were significantly higher in Crystal Lake sediments compared to Big Bear Lake sediments (Hg levels: Crystal Lake 220 93 ng g-1, Big Bear Lake 92 26 ng g-1; Hg accumulation: Crystal Lake 790 1,200 lg m-2 year-1, Big Bear 240 54 lg m-2 year-1). In Crystal Lake, the ratio between post-1965 and pre-1865 Hg concentrations was 1.1, and several spikes in Hg levels occurred between 1910 and 1985. Given the remote location of the lake, the proximity of fires, and the lack of point sources within the region, these results suggested wildfires (rather than industrial sources) were a continuous source of Hg to Crystal Lake over the last 150 years.
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Mercury (Hg) is a naturally occurring element in the
Earths crust, which may be liberated through natural or
anthropogenic means. Once released Hg may be
transformed into methylmercury (MeHg), which is efficiently
biomagnified in the aquatic food web. In the troposphere,
Hg exists as gaseous elemental Hg (Hgo), reactive gaseous
mercury (RGM, consisting of oxidized gaseous Hg(II)
compounds), or Hg may be bound to particulates (Hgp)
(Lindberg et al. 2007). RGM and Hgp have relatively short
atmospheric residence times and may be deposited within
hours to weeks, while the tropospheric residence time for
Hgo is between 6 months and 2 years (Schroeder and
Munthe 1998).
Wildfires and biomass burning are significant sources of
Hg emissions to the atmosphere (Brunke et al. 2001; Engle
et al. 2006; Friedli et al. 2001, 2003a, b; Harden et al.
2004; Veiga et al. 1994; Wiedinmyer and Friedli 2007).
During a wildfire, Hg is primarily emitted from vegetation
and soils in the form of Hgo. In a smoke plume, most Hg
([87%) is in the form of Hgo, while the remaining is bound
to particulates (Hgp); however, the ratio between the two is
fuel-dependent (Friedli et al. 2003a, b). Although Hg is
released primarily as Hgo, increased MeHg levels have
been observed in the vicinity following a wildfire. Caldwell
et al. (2000) reported sediment MeHg levels peaked within
3 months after a wildfire (64-km downwind from the fire
origin) due to increased storm runoff and transport of Hg
and MeHg, while Amirbahman et al. (2004) observed
elevated MeHg levels in soils burned 45 years prior to Hg
analysis, indicating soil changes following a fire (e.g.,
higher soil pH) persisted for years. Fires also enhance
productivity in lakes through deposition of nutrients
(Spencer et al. 2003), which may increase MeHg levels in
fish tissue by lengthening the food chain (Kelly et al.
2006), or nutrients may decrease MeHg biomagnification
in fish tissue due to algal bloom dilution (Chen and Folt
2005; Pickhardt et al. 2005).
In southern California, March to October is considered
the fire season; however, the length of the season is
dependent on winter precipitation levels. A number of
water bodies in this region are listed as impaired for
Hg in water, sediments and/or fish tissue (USEPA 2008),
although there are no coal burning power plants and few
other atmospheric point sources of Hg within the air shed.
In this semi-arid region, fires may be a significant source of
Hg. Forest fires produce ash, which may contain heavy
metals that are easily solubilized and may impair water
quality (Plumlee et al. 2007). Soils coated with ash are
more water repellent than unburned sediments, causing
increased erosion and higher runoff (Onda et al. 2008;
Shakesby and Doerr 2006). While the effect of fires on lake
sedimentation and water chemistry has been well
documented (Gresswell 1999; Malmon et al. 2007; Minshall
et al. 1989; Spencer et al. 2003), the impact of wildfires on
historical sediment (...truncated)