Effects of Wildfire on Mercury, Organic Matter, and Sulfur in Soils and Sediments

Webster, Jackson Paul

Graduate Thesis Or Dissertation

Effects of Wildfire on Mercury, Organic Matter, and Sulfur in Soils and Sediments Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/n870zq98g

Abstract

The transport and deposition of mercury from natural and anthropogenic sources has led to its enrichment in surficial soils on a global scale. In arid climates, such as the western United States, the common occurrence of wildfire mobilizes accumulated mercury. In addition to atmospheric re-emission, wildfire enhances the transport of mercury from forest soils to the sediments of lakes and reservoirs through increased watershed runoff and soil destabilization. Little information is reported on the fate mercury in waters affected by wildfire. To address the knowledge deficit, multiple studies were conducted throughout forested watersheds of Colorado, United States, subjected to historical and recent wildfire to characterize mercury, organic matter, and sulfur oxidation state in soils and sediment. Simulated wildfire heating of soil was conducted using a muffle furnace to observe the influence of heat on sulfur oxidation. Batch experiments were conducted on soils to assess aqueous mercury release from burned, unburned, and laboratory heated soils. To examine the sulfur speciation and mercury behavior following ash-laden sediment deposition in a reservoir, reservoir cores containing these sediments were obtained and analyzed. Additionally, redox transitions occurring during sediment deposition were simulated using microcosms. To measure how oxidizing (heating) and reducing (reservoir deposition) conditions influence sulfur speciation, X-ray absorbance near-edge structure (XANES) spectroscopy was employed. Changes in mercury binding capacity were quantified using a competitive ligand exchange method. In Mesa Verde National Park, it was found that soils affected by historical wildfires contained about 50 % less organic matter and mercury than unburned soils collected adjacent to the burned samples, which corresponded to mercury release of 19 ± 8.2 g ha-1. In aqueous release experiments from historically burned soils, there was half the amount of carbon and mercury as unburned soils. In contrast, laboratory heating increased the release of organic matter and major ions from soils. Mercury release was linked to organic matter release, though losses from volatilization during heating reduced the total release into solution. Soil subjected to heating contained more oxidized sulfur species and concomitant losses of intermediate oxidation state species, while sulfur oxidation states in ash-laden sediments shifted toward reduced forms following deposition in a reservoir. Laboratory experiments simulating each of these shifts resulted in increased mercury binding capacity. Following heating a 1.5 fold increase in strong mercury binding sites was measured. Following simulated reservoir deposition, there was a 10-fold increase in the number of strong binding sites in the ash-laden sediments. Overall, this study characterizes the underlying processes influencing mercury behavior in soils and sediments following wildfire.

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