EFFECTS OF STREAM WATER CHEMISTRY
ON MERCURY CONCENTRATIONS
IN BROOK TROUT
IN SHENANDOAH NATIONAL PARK

C. D. Snyder¹, R. Webb², J. Atkinson³ , S. Spitzer3

¹US Geological Survey,
Leetown Science Center, Aquatic Ecology Branch,
11649 Leetown Road,
Kearneysville, WV 25443

²University of Virginia,
Department of Environmental Sciences,
Clark Hall,
291 McCormick Road,
Charlottesville, VA 22904


2006

³National Park Service,
Shenandoah National Park,
3655 U. S. Highway 211 East,
Luray, VA 22835

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BACKGROUND AND JUSTIFICATION

Mercury (Hg) is a toxic element that naturally occurs in aquatic systems in very low concentrations. Past human use of the metal for industrial and agricultural purposes has resulted in serious contamination of many surface waters. Even in remote, relatively pristine areas where direct anthropogenic inputs are lacking, long-range atmospheric transport of Hg from fossil fuel combustion and other sources has led to increased concentrations in freshwater systems and biota (Downs et al. 1998, Fitzgerald et al. 1998). Concentrations of Hg sufficient to prompt fish consumption advisories (i.e., > 0.5 ug/g) have been reported for predatory fish from relatively remote areas with no on-site anthropogenic or geologic sources of Hg (e.g., Abernathy and Cumbie 1977, Bodaly et al. 1984).

The chemistry of Hg is complex and consequently its behavior is difficult to predict in nature. Total mercury concentrations in the environment have not been found to be effective predictors of bioaccumulation in fish. Depending on physical, chemical, and biological conditions at a site, Hg can remain largely tied up in sediments, released from sediments to the water column, be lost to the atmosphere, be transported with sediment particulate matter to other locations, or be taken up by aquatic biota where it may concentrate and become a threat to humans and other fish-eating animals (reviewed in Ullrich et al. 2001). Although the precise factors controlling the accumulation of Hg in aquatic biota are not fully understood, it is clear that fish and other aquatic species are much more efficient in accumulating methylmercury (MMHg) than the inorganic forms that predominate in the abiotic component of the environment (Mason et al. 1995). Thus, factors that influence the rate in which in inorganic Hg is transformed to MMHg also influence bioaccumulation as well.

Although the process of Hg methylation is complex, the results of numerous studies on contaminated lakes indicate that enhanced Hg methylation rates and bioaccumulation have been consistently linked to low pH, low salinity, and the presence of organic matter in low oxygen environments (reviewed in Ullrich et al. 2001). The relationship between water chemistry and Hg methylation has not been fully investigated in stream ecosystems.

Streams in SNP vary considerably in terms of pH and other important water chemistry parameters due to variation in dominant bedrock class underlying individual catchments. All the streams in SNP are characterized by low salinity or ionic strength. Streams underlain by siliciclastic bedrock have low acid neutralizing capacities (ANC) and consequently very low pH. As a result of their low pH, these streams may be the most vulnerable to mercury contamination. Streams underlain by basaltic bedrock have higher ANC and pH. Streams underlain by granitic bedrock have intermediate ANC and pH. We thus expect that bedrock distribution in SNP may reflect a gradient in watershed response to atmospheric deposition of mercury.

In addition to variation in water chemistry, there is variation in landscape setting that may be important. In particular, elevation has been shown to affect Hg deposition in some areas though results have been contradictory. For example, total Hg deposition has been shown to be greater in watersheds at higher elevations due to cloud deposition (i.e., wet Hg deposition associated with mist and fog) (Shanley et al. 2005). However, methylmercury in sediments and biota have been shown to be higher in lower elevation sites, presumably because the greater watershed areas support a larger number of wetlands and other sites where Hg methylation occurs (Kamman et al. 2005).

The objectives of this study were to 1) evaluate the potential threat of mercury to humans that consume fish caught in the park, and 2) determine the extent to which variation in bedrock geology and water chemistry influence mercury accumulation in brook trout, the primary game fish in the park.

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