Stream Mercury Export in Response to Contemporary Timber Harvesting Methods (Pacific Coastal Mountains, Oregon, USA)

Abstract

Land-use activities can alter hydrological and biogeochemical processes that can affect the fate, transformation, and transport of mercury (Hg). Previous studies in boreal forests have shown that forestry operations can have profound but variable effects on Hg export and methylmercury (MeHg) formation. The Pacific Northwest is an important timber producing region that receives large atmospheric Hg loads, but the impact of forest harvesting on Hg mobilization has not been directly studied and was the focus of our investigation. Stream discharge was measured continuously, and Hg and MeHg concentrations were measured monthly for 1.5 years following logging in three paired harvested and unharvested (control) catchments. There was no significant difference in particulate-bound Hg concentrations or loads in the harvested and unharvested catchments which may have resulted from forestry practices aimed at minimizing erosion. However, the harvested catchments had significantly higher discharge (32%), filtered Hg concentrations (28%), filtered Hg loads (80%), and dissolved organic carbon (DOC) loads (40%) compared to forested catchments. MeHg concentrations were low (mostly <0.05 ng L) in harvested, unharvested, and downstream samples due to well-drained/unsaturated soil conditions and steep slopes with high energy eroding stream channels that were not conducive to the development of anoxic conditions that support methylation. These results have important implications for the role forestry operations have in affecting catchment retention and export of Hg pollution.

Increasing temperature and flow management alter mercury dynamics in East Fork Poplar Creek

“…The patterns in Hg T were similar to patterns in TSS leading to moderate to strong positive correlations between these two variables for the whole creek and within each creek reach. Numerous investigators have reported similar positive correlations between Hg concentration and TSS (Brigham et al, 2009; Eckley et al, 2018; Hurley et al, 1998; Whyte & Kirchner, 2000).…”

“…between Hg concentration and TSS (Brigham et al, 2009;Eckley et al, 2018;Hurley et al, 1998;Whyte & Kirchner, 2000). S6).…”

Effects of Clearcutting and Residual Biomass Harvesting on Hillslope Mercury Mobilization and Downgradient Mercury Accumulation

“…However, the THg yields observed from these unharvested hillslopes (6–26 ng m −2 day −1 ) were often 2–3x higher than observed in stream waters at catchment scales (0.8–10 ng m −2 day −1 ; Bishop et al., 2009; de Wit et al., 2014; Eckley et al., 2018; Eklöf et al., 2014; Porvari et al., 2003). Similar to the unharvested yields on these hillslopes, the post‐harvest THg yields (12.1–23.5 ng m −2 day −1 ) were consistently higher than those reported in the broader catchment literature (1.3–14.5 ng m −2 day −1 ; Bishop et al., 2009; de Wit et al., 2014; Eckley et al., 2018; Eklöf et al., 2014; Porvari et al., 2003). Post‐harvest, the highest catchment THg yields from the literature were from organic‐soil dominated boreal forests in Europe (de Wit et al., 2014; Eklöf et al., 2014; Porvari et al., 2003), while the lowest were from the more mineral‐soil dominated and steep forests of the western USA (Eckley et al., 2018).…”

“…Eckley et al. (2018) observed changes in both the intercepts and overall slopes of DOC‐filtered/particulate Hg relationships in western USA harvested catchments, while Skyllberg et al. (2009) observed an increase in Hg‐DOC ratio in streamwater draining clearcut Swedish boreal forests within 0–4 yr. Conversely, de Wit et al.…”

Paleolimnological characterization of metal(loid) sources and transport processes in Canadian subarctic lakes

“…Vegetation can alter catchment water flow and limit soil erosion. For example, logging operations and wildfires that deplete vegetation can increase catchment soil erosion and the transport of metals to streams and lakes by orders of magnitude (Garcia and Carignan, 1999;Ice et al, 2004;Nappi et al, 2004), and this impact is amplified in catchments that have a steep topography compared to flatter catchments (Davis et al, 2006;Eckley et al, 2018). The parent material (surficial and bedrock geology) is important because it defines the baseline metal concentration and distribution in soils (Utermann et al, 2019;Wuana and Okieimen, 2011).…”