Looking downstream at the sampling site for investigating daily variations in mercury concentrations in the Madison River, Yellowstone National Park, Montana. A mobile water-quality laboratory and the gage house for a USGS streamflow station (Site ID: 06037500) are on the far bank. Water samples from the river were pumped through tubing into the mobile laboratory.
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Like many streams across the nation, some streams in Montana have fish consumption advisories due to high mercury concentrations. USGS scientists and their colleagues have been studying two Montana streams to determine what controls the concentrations of highly toxic methyl mercury. They reported in the journal Science of the Total Environment that due to changes in the chemistry of the streams caused by daily fluctuations in temperature, sunlight intensity, and other factors, methyl mercury concentrations were significantly higher in the afternoon. This finding is consistent with earlier findings that showed concentrations of some metals in stream waters also had daily fluctuations. Although concentrations of methyl mercury in water were low (less than 1 nanogram per liter, or one part per trillion) in these streams, the mercury concentrations in fish were high enough to advise fisherman to limit consumption of the fish they catch. This study advances our understanding of the highly variable conditions that cause the production of methyl mercury in the environment. These findings are important for designing field studies of mercury contamination in fish and for interpreting the processes that affect methyl mercury cycling in the environment.
Methyl Mercury
Mercury is present in streams as several chemical species, or forms, with methyl mercury being the most important because it is the mercury species that bioaccumulates through the food chain. Methyl mercury can occur in fish at concentrations that could be harmful to people who consume fish. Methyl mercury also was the species that had the largest daily concentration cycles. In both streams, concentrations of dissolved methyl mercury were lowest in the early morning and increased 68 to 93 percent during the day, reaching maximum concentrations in the afternoon. Concentrations then decreased during the night. Similar fluctuations in methyl mercury concentrations in several lakes have previously been reported, but this is the first study to examine daily mercury cycles in streams. These findings indicate that the time of day water samples are collected during field studies of mercury contamination could impact the results and conclusions drawn from the study.
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