The results of a new study suggest that an improved understanding of the processes mobilizing mercury in soils will be necessary to predict water quality impacts.

The heavy metal mercury is highly toxic to the nervous systems of humans and other living organisms. Emissions of mercury come from natural sources, such as volcanic eruptions, as well as anthropogenic activities, including the burning of coal and the production of gold, and are widely dispersed in the atmosphere before drifting to the ground as dust or falling as rain or snow. Once mercury makes its way into a river system, it can be converted into a more toxic form and transported long distances by streams, posing a widespread environmental threat.

The concentration of mercury in streams varies widely, and studies attempting to account for this variability across different types of watersheds have been largely unsuccessful. Although concentrations of dissolved mercury are strongly associated with the amount of dissolved organic carbon in stream water, their exact ratio varies significantly. To better constrain this relationship and potentially harness it for predictions, Stoken et al. evaluated the ratio of these constituents relative to the organic carbon content of watershed soils and other variables in 19 largely undisturbed watersheds across the United States.

The results show that soil organic content accounted for 81% of the variation in the ratio of the dissolved species, indicating that watershed soils play a crucial role in determining dissolved mercury and organic carbon concentrations in streams. These results suggest that soil organic carbon data sets could be used to make indirect, regional-scale estimates of dissolved mercury concentrations in watersheds where such data are available, including many across North America and Europe, without the need for extensive measurements in the field.

On the basis of these results, the researchers argue that a better understanding of the processes that mobilize mercury from soils will be needed to accurately predict the effects of new emissions policies on water quality, including those under changing climate scenarios. (Water Resources Research, doi:10.1002/2015WR017849, 2016)

—Terri Cook, Freelance Writer

Citation: Cook, T. (2016), How much dissolved mercury is present in streams?, Eos, 97, doi:10.1029/2016EO051901. Published on 10 May 2016.