Atmospheric mercury deposition corresponds to volcanic and anthropogenic events over the past 270 years. Preindustrial deposition rates can be conservatively extrapolated to present time (4 ng/L; in green) to illustrate the increase during the past 100 years (in red) and significant decreases in the past 15-20 years.
http://toxics.usgs.gov/pubs/FS-051-02/pdf/fs-051-02.pdf
U.S. Geological Survey