USGS2002
Field and laboratory studies were conducted between April 30, 2001 and July 19, 2001 to provide the first direct measurements of the benthic flux of dissolved (0.2-micrometer filtered) mercury species (total and methylated forms) between the bottom sediment and water column at three sites: two in the southern lobe and one in the northern lobe of Lahontan Reservoir, Nevada (Background, Fig. 1). Dissolved-mercury species and predominant ligands (represented by dissolved organic carbon, and sulfides) were the solutes of primary interest. Benthic flux, sometimes referred to as internal recycling, represents the transport of dissolved chemical species between the water column and the underlying sediment. Water-quality managers often assess and prioritize remediation strategies for aquatic systems, in particular Super Fund sites that have been adversely affected by anthropogenic activities. In the case of the Lahontan Reservoir along the Carson River, mercury associated with historic gold and silver extraction has been fluvially transported and accumulated in the bottom sediments. Frequent demands have been made by Super Fund site managers and the general public to quantify the connections between fluxes of contaminants and the health, abundance, and distribution of biological resources (Kuwabara and others, 1999). As part of a comprehensive examination of transport processes affecting mercury dynamics in Lahontan Reservoir, this study focuses on a poorly understood, yet potentially predominant, source of mercury to the reservoir water column, which is internal recycling, or benthic flux of mercury species and associated ligands. Mobilization, flux, and biological availability of mercury into the water column of the reservoir are affected by physical (e.g., advection and diffusion), chemical (i.e., oxidationreduction reactions, complexation and repartitioning) and biological processes (Flegal and others, 1991; Kuwabara and others, 1996; Grenz and others, 2000, Topping and others, 2001). The results described herein followed from the integration of current project studies with information needs identified by the U.S. Environmental Protection Agency, Region 9 (USEPA) to provide initial determinations of dissolved total and methyl-mercury fluxes from the sediments into the water column of Lahontan Reservoir. Recent mercury distribution and transformation studies in the Carson River system by Marvin-Dipasquale and others (2001) indicated the potential importance of sediment-water interactions in describing mercury speciation, its sources and sinks. Quantifying and understanding the magnitude and variability of these interactions are critical to the accurate assessment of contaminant sources and loads as well as to the development of process-integrated water-quality models for this mining-affected system. With a variety of strategies under consideration to determine how to most efficiently improve the water quality in the Carson River system, the primary question posed in this study was, “What processes regulate the fate and transport of mercury species in Lahontan Reservoir? In particular, are sources and sinks of dissolved total and methyl mercury associated with the bottom sediment within Lahontan Reservoir significant relative to major surface-water inputs from the Carson River?” The question was motivated by a number of factors. First, extraction of precious metals up gradient of the reservoir provide a historic source of elemental mercury that continues to be fluvially transported in dissolved and particulate phases (Hoffman and Taylor, 1998; Carroll and others, 2000; Carroll and Warwick, 2001). Elevated total and methyl mercury concentrations into the reservoir have been well documented (Priessler and others, 1999; Jones and others, 1999; Allander and others, 2001, Marvin-Dipasquale and others, 2001). Therefore, determining whether some fraction of this sediment-associated mercury can remobilize for transport to the overlying water and subsequently to down-stream portions of the Carson River is necessary. Second, elevated concentrations of mercury species in reservoir water, sediment, and fish have prompted a compilation, and comparison of dominant contaminant sources so that appropriate remedial strategies can be designed and implemented. Third, changes in oxidizing or reducing (redox) conditions and nutrient availability near the sediment-water interface (e.g., during phytoplankton blooms) can dramatically alter the mobility of metals and ligands associated with the bottom sediment as episodic sources of carbon settle out and accumulate (Thompson and others, 1981). Finally, there is a growing body of evidence from other aquatic systems that benthic flux or internal recycling of contaminants and nutrients is an important process to consider in developing appropriate ecosystem water-quality models (Wood and others, 1995; Kuwabara and others, 2000). The need for more refined numerical and conceptual models for mercury dynamics within the Carson River system thus clearly exists. This report is formatted unconventionally in a pyramid-like structure to effectively serve the needs of diverse parties interested in reviewing or acquiring information at various levels of detail (Appendix 1). The report enables quick transitions between the initial summary information (figuratively at the top of the pyramid) and later details of methods or results (that is, figuratively towards the base) using hyperlinks to supporting figures and tables, and an electronically linked Table of Contents.