~" .{ ELSEVIER 0269-7491(95)00112-3 Environmental Pollution, Vol. 92, No. 3, pp. 281-287, 1996 Copyright © 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0269-7491/96 $15.00 + 0.00 LEVELS OF METHYLMERCURY AND CONTROLLING FACTORS IN SURFACE SEDIMENTS OF THE CARSON RIVER SYSTEM, NEVADA Y. Chen, J. C. Bonzongo & G. C. Miller* Center for Environmental Sciences and Engineering- MS. 199, University of Nevada, Reno, NV 89557-0013, USA (Received 2 June 1995; accepted 16 November 1995) Abstract Spatial and temporal distribution of methylmercury (MeHg) was determined in surficial sediments collected from a river-reservoir system impacted by Hg-con- taminated mine wastes. Despite the fact that total mer- cury concentrations (Hgr) in surface sediments of the Carson River system were in the #g.g-1 range, levels of MeHg varied from about 2 to 28ngHg.g -1 dry weight, representing less than 3% of Hgr. Concentrations of MeHg were well correlated with both the biotic (r = 0.95) and abiotic activity (r = 0.85) of the sediment, determined as the ability of each compartment to specifically reduce an alternative electron acceptor. However, the positive relationship between the two measured activities suggests that the abiotic activity may be due to reductant sub- stances produced by micro-organisms. When sediments collected from the Carson River were used in laboratory assays for the determination of potential rates of MeHg production, the addition of inorganic Hg (added as HgCI2) resulted in increased rates of methylation when the spike concentration was lower or equal to 15.3 lzg.g -1 dry weight. This trend was reversed for spike concentra- tion of inorganic Hg above 15.3 #g.g-t. The reduction of methylation rate was associated with an inhibition of microbial activity. These observations suggest that seaso- nal inputs into the river of significant amounts of inor- ganic Hg eroded from mill tailings during winter and spring flooding events could have an inhibiting effect on Hg-methylating micro-organisms. This observation could explain the low [MeHg]/[Hgr] ratios previously docu- mented in waters of the Carson River system. Copyright © 1996 Published by Elsevier Science Ltd Keywords." Mining activity, sediment, Hg, microbial activity, methyl-Hg production. INTRODUCTION Elevated levels of mercury (Hg) found in fish tissues have generated public concern over human health risks *To whom correspondence should be addressed. 281 associated with consumption of contaminated fish (Bjorkland et al., 1984; Fitzgerald & Clarkson, 1991; Lindqvist, 1991). The ecological and human health effects of Hg are generally related to the environ- mental transformation of inorganic Hg to the toxic and biomagnification-prone methyl mercury (MeHg) (Compeau & Bartha, 1985). More than 80% of Hg accumulated in fish tissues is present as MeHg (Grieb et al., 1990; Bloom, 1992). However, processes leading to MeHg production in natural aquatic systems are still poorly understood. Despite some disagreements over whether the methylation and demethylation of Hg 2+ are biologically or abiotically mediated, experimental evidence is available to support contributions from both mechanisms in natural environments (Weber, 1993). Previous investigations on the methylation of Hg have shown that MeHg production occurs in sediment (Compeau & Bartha, 1985; Xun et al., 1987; Gilmour & Henry, 1991; Gilmour et al., 1992), as well as in the water column (Xun et ak, 1987). Even though the degree to which sediments act as a source of MeHg to aquatic biota is unknown, both inorganic Hg and MeHg are mainly concentrated in sediments relative to the water column (Gilmour & Henry, 1991). The Carson River system in Nevada has been contaminated by mine wastes for over a century by the historic Comstock mines. Precious metals were ext- racted by an amalgamation process in which liquid Hg was used to free the gold and silver from milled ores. A significant portion of Hg-contaminated waste tailings generated by this activity have directly or indirectly entered the Carson River (Smith, 1943) and accumu- lated in bed and river bank sediments (Cooper et aL, 1985). Several investigations have been conducted recently to document the extent of Hg contamination in the Carson River drainage basin (Cooper et al., 1985; Gustin et al., 1994); however, no data exist on concentrations of MeHg in sediments. The purpose of our study was to determine concentrations of MeHg in surficial sediments along the Carson River, as well as factors controlling MeHg production in sediment.