Mercury and Organic Carbon Dynamics During Runoff Episodes from a Northeastern USA Watershed

Abstract

Mercury and organic carbon concentrations vary dynamically in streamwater at the Sleepers River Research Watershed in Vermont, USA. Total mercury (THg) concentrations ranged from 0.53 to 93.8 ng/L during a 3-year period of study. The highest mercury (Hg) concentrations occurred slightly before peak flows and were associated with the highest organic carbon (OC) concentrations. Dissolved Hg (DHg) was the dominant form in the upland catchments; particulate Hg (PHg) dominated in the lowland catchments. The concentration of hydrophobic acid (HPOA), the major component of dissolved organic carbon (DOC), explained 41-98% of the variability of DHg concentration while DOC flux explained 68-85% of the variability in DHg flux, indicating both quality and quantity of the DOC substantially influenced the transport and fate of DHg. Particulate organic carbon (POC) concentrations explained 50% of the PHg variability, indicating that POC is an important transport mechanism for PHg. Despite available sources of DHg and wetlands in the upland catchments, dissolved methylmercury (DmeHg) concentrations in streamwaters were below detection limit (0.04 ng/L). PHg and particulate methylmercury (PmeHg) had a strong positive correlation (r 2 =0.84, p<0.0001), suggesting a common source; likely instream or near-stream POC eroded or re-suspended during spring snowmelt and summer storms. Ratios of PmeHg to THg were low and fairly constant despite an apparent higher methylmercury (meHg) production potential in the summer. Methylmercury production in soils and stream sediments was below detection during snowmelt in April and highest in stream sediments (compared to forest and wetland soils) sampled in July. Using the watershed approach, the correlation of the percent of wetland cover to TmeHg concentrations in streamwater indicates that poorly drained wetland soils are a source of meHg and the relatively high concentrations found in stream surface sediments in July indicate these zones are a meHg sink.

Comparison of total mercury and methylmercury cycling at five sites using the small watershed approach

“…4), suggesting common sources for the two species. At Sleepers we have inferred that near-stream or in-stream particulate organic matter, mobilized during storms (Hall and St. Louis, 2004), is the common source for THg and MeHg (Schuster et al, 2008). Mobilization of particulate organic matter is also a plausible mechanism at Panola, where in-stream sediment is high in organic matter, and at Icacos, where bed sediment and suspended sediment are low in organic matter but there is a high correlation of THg and POC (see below).…”

“…The long-term data and biogeochemical understanding from nearly two decades of intensive research at these sites provide rich context for the interpretation of Hg behavior. This recent focus on Hg in the WEBB program builds on previous work at three of the sites: Vermont (Shanley et al, 2002;Schuster et al, 2008); Colorado (Mast et al, 2005); and Wisconsin (Krabbenhoft et al, 1995).…”

Specific ultra-violet absorbance as an indicator of mercury sources in an Adirondack River basin

“…Instances where the FTHg-SUVA 254 association is about as strong, or stronger than the FTHg-DOC association tend to be from synoptic-type studies in which surface waters with diverse land cover have been sampled (Hall et al 2008;Tsui and Finlay 2011;Burns et al 2012), with the exception of the current study. In contrast, the FTHg-SUVA 254 relation tends to be relatively weaker than that of FTHg-DOC or FTHg-UV 254 in watersheds with more uniform, largely forested upland land cover (Schuster et al 2008;Dittman et al 2010). Several studies in which Hg-DOC relations were explored may not have also considered whether FTHg concentrations and SUVA 254 values were related because another measure such as DOC concentration, UV 254 , or HPOA concentration was able to explain most of the variation in FTHg Dittman et al 2009;Riscassi and Scanlon 2011;Eklöf et al 2012).…”

The fate of mercury in Arctic terrestrial and aquatic ecosystems, a review

“…[76] In Amituk Lake in the Canadian High Arctic, snowmelt delivered the most THg during spring in June and early July [22,77] but 59 % of the THg delivered to Amituk Lake during the snowmelt period was directly discharged through lake outflow because of the limited mixing between surface and underlying water. [22] In Arctic regions, melt water concentrations of MeHg and THg were found to be elevated above full-column snowpack values at the onset of snow melt in some locations, [20,[77][78][79] but not all.…”