Effects of Timber Harvest on Mercury Cycling in the Pacific Northwest, USA
Thursday, 18 December 2014
The Pacific Northwest receives some of the highest rates of atmospheric mercury deposition in the USA (>14 µg/m2/year) which is largely driven by the large volume of precipitation this region receives. Forested landscapes have been shown to act as a net sink for atmospheric Hg inputs which has resulted in a legacy of mercury accumulated in the soil. Timber production and harvesting (typically via clearcutting) is a common land-use feature throughout the Pacific Northwest, accounting for roughly 25% of the total land area. Timber harvesting and associated activities (road construction and slash removal) have fundamental effects on watershed hydrology and biogeochemicial processes, and can also alter mercury cycling dynamics. The objective of our study was to understand how different forest harvest practices impact key components of the mercury cycle including: mobilization in runoff, methylation, volatilization, and downstream bioaccumulation. The study utilized a paired harvested and un-harvest sub-watershed experimental design. Flumes were located at the outflow of each of the 8 sub-watershed studied and were continuously monitored for flow volume and other water quality parameters. Water samples for filtered and particulate total and methylmercury (as well as several ancillary parameters) were measured approximately monthly for 1.5 years following harvest and represented the full range of hydrological conditions. Surface-air flux measurements were conducted seasonally using the dynamic flux chamber (DFC) approach. The results showed increased fluxes of mercury in streams and to the atmosphere in response for forestry operations. The stream fluxes showed higher loads in the filtered phase than associated with particulates. Methylmercury concentrations remained low (<0.05 ng/L) in all of the headwater catchments studied. Overall, these results highlight the effects of forestry operations on several important aspects of mercury cycling in forested landscapes.