Mercury Distribution across 14 U.S. Forests. Part II: Patterns of Methyl Mercury Concentrations and Areal Mass of Total and Methyl Mercury

Abstract

This study characterized distribution patterns of monomethyl mercury (MeHg) and areal mass of total mercury (THg) and MeHg across U.S. upland forests. MeHg concentrations increased from surface litter (average: 0.14 μg kg(-1)) to intermediate (0.47 μg kg(-1)) and deeper, decomposed litter (1.43 μg kg(-1)). MeHg concentrations were lower in soils (0.10 μg kg(-1) at 0-20 cm depth; 0.06 μg kg(-1) at >20 cm depth). Ratios of MeHg to THg were higher in litter compared to soils. In soils, MeHg concentrations positively correlated with THg across all sites, and MeHg concentrations also increased with C content and latitude. THg areal mass ranged from 41.6 g ha(-1) to 268.8 g ha(-1). Largest THg mass at all sites was sequestered in soils (average of 91%), followed by litter (8%) and aboveground biomass (<1%). MeHg mass (litter plus soils only) ranged from 75 to 443 mg ha(-1), of which 88% was found in soils. Both THg and MeHg mass correlated with latitude, with average mass increases of 10.6 g ha(-1) (THg) and 20 μg ha(-1) (MeHg) per degree latitude, indicating that highest THg and MeHg accumulation in upland forests are expected in northern sites.

Mercury in the Soil of Two Contrasting Watersheds in the Eastern United States

“…Few studies have previously reported MeHg data in soil; however, some have reported MeHg/THg, with values as high as 5% [14], but most <1% [4], [6]. Here, we reported values of about 0.5% at FB and close to 1% at MC, consistent with most studies reviewed by Grigal [6] and Obrist [4].…”

“…Soil in areas with high geogenic Hg concentrations can show the reverse of the typical depth pattern, with greater Hg concentrations in the deep soil than near the surface [15]. Despite the greater Hg concentrations commonly measured in surface horizons, deeper mineral horizons generally store more Hg because of the much greater mass of mineral-dominated subsoil in most landscapes [4], [6], [16]. Total soil storage of Hg can vary from a few mg/m 2 to >50 mg/m 2 , and these values vary, similarly to those of Hg concentrations, as a function of source loads of atmospheric Hg deposition, the amount of SOM in soil, and geogenic concentrations in parent material [3], [6].…”

“…A recent study estimates the global soil pool size (year 2000) at 240,000 Mg Hg, and indicates that Hg storage has increased by about 20% since 1840 largely due to atmospheric Hg deposition primarily derived from anthropogenic emissions sources such as coal burning, cement manufacturing, and other industrial and mining activities [2]. Soil Hg concentrations vary widely from 10 ng/g to 1000 ng/g in rural and remote areas, and from 100 ng/g to >10,000 ng/g in urban, industrial, and mineralized/mined lands [3], [4]. This Hg is largely bound to soil organic matter (SOM), and Hg concentrations are typically strongly related to measures of SOM or soil organic carbon (SOC) [5]–[7].…”

“…The ratio of MeHg/THg varies widely in soil from <1% to >20% [4], [6], [8]. Wetland soil generally has greater MeHg concentrations and MeHg/THg than soil in steeper upland parts of landscapes, and more complex patterns are often superimposed on this general gradient.…”

Mercury in streams at Grand Portage National Monument (Minnesota, USA): Assessment of ecosystem sensitivity and ecological risk

“…Total Hg concentrations in these sediment were less than the median Hg level in surficial or modern lacustrine sediments from 20 lakes in forest landscapes of northeastern Minnesota (Engstrom et al, 2007), yet both total Hg and MeHg in GRPO sediment were greater than the median values for 257 stream sediments across the US (MeHg 0.51 ng/g dw, total Hg 30.3 ng/g dw; Scudder et al, 2009). Total Hg in GRPO surficial soils were similar to those measured at nearby Voyageurs National Park, MN (A-horizon range, 58 to 256 ng/g dw; Wiener et al, 2006), while both MeHg concentration and the fraction of total Hg present as MeHg were similar to or higher than those measured in soils of other forest ecosystems (Tabatchnick et al, 2011;Obrist, 2012). As with the streamwater samples, our analyses of GRPO sediments and soils are indicative of an ecosystem with active, substantial Hg methylation.…”