Vertical Profile Measurements of Soil Air Suggest Immobilization of Gaseous Elemental Mercury in Mineral Soil

Abstract

Evasion of gaseous elemental Hg (Hg(0)g) from soil surfaces is an important source of atmospheric Hg, but the volatility and solid-gas phase partitioning of Hg(0) within soils is poorly understood. We developed a novel system to continuously measure Hg(0)g concentrations in soil pores at multiple depths and locations, and present a total of 297 days of measurements spanning 14 months in two forests in the Sierra Nevada mountains, California, U.S. Temporal patterns showed consistent pore Hg(0)g concentrations below levels measured in the atmosphere (termed Hg(0)g immobilization), ranging from 66 to 94% below atmospheric concentrations throughout multiple seasons. The lowest pore Hg(0)g concentrations were observed in the deepest soil layers (40 cm), but significant immobilization was already present in the top 7 cm. In the absence of sinks or sources, pore Hg(0)g levels would be in equilibrium with atmospheric concentrations due to the porous nature of the soil matrix and gas diffusion. Therefore, we explain decreases in pore Hg(0)g in mineral soils below atmospheric concentrations--or below levels found in upper soils as observed in previous studies--with the presence of an Hg(0)g sink in mineral soils possibly related to Hg(0)g oxidation or other processes such as sorption or dissolution in soil water. Surface chamber measurements showing daytime Hg(0)g emissions and nighttime Hg(0)g deposition indicate that near-surface layers likely dominate net atmospheric Hg(0)g exchange resulting in typical diurnal cycles due to photochemcial reduction at the surface and possibly Hg(0)g evasion from litter layers. In contrast, mineral soils seem to be decoupled from this surface exchange, showing consistent Hg(0)g uptake and downward redistribution--although our calculations indicate these fluxes to be minor compared to other mass fluxes. A major implication is that once Hg is incorporated into mineral soils, it may be unlikely subjected to renewed Hg(0)g re-emission from undisturbed, background soils emphasizing the important role of soils in sequestering past and current Hg pollution loads.

Constraints from observations and modeling on atmosphere–surface exchange of mercury in eastern North America

“…Unusually high water discharges were measured in July 2013 for rivers in South Carolina (see Figure S3a for an example). Additionally, measurements within the NADP Mercury Deposition Network (MDN; Prestbo and Gay, 2009) showed heavy rainfall and high ) over background bare soil, which is in agreement with measured vertical profiles of Hg 0 in soil pores (Obrist et al, 2014). However, forest is the most important land use type in the eastern United States, especially during summer when the leaf area index is maximum (thus leaf surface areas are several times larger than underlying soil surface areas) (Drummond and Loveland, 2010;Buermann et al, 2001).…”

Investigation of factors affecting mercury emission from subtropical forest soil: A field controlled study in southwestern China

“…In addition, particulate-bound mercury in solid phase was dissolved to aqueous phase, then the reduction by biotic and abiotic actions enhanced the conversion of Hg 2+ to Hg 0 as we discussed above. Obrist et al (2014) showed that heavy rainfall led an increment in pore Hg 0 concentrations in two forests of California, USA, and they attributed this to the decrease of soil redox potential which enhanced the conversion of Hg 2+ to Hg 0 and favorable oxidation leading to Hg 0 immobilization under unsaturated soil moisture levels. Although Moore and Castro (2012) did not observed soil moisture correlated with soil TGM concentrations in their field study, they showed the negative correlation between pore Hg 0 levels and soil redox potential in forest soils.…”

Mercury isotope compositions across North American forests

“…Hg(0) is delivered to forest ecosystems largely by stomatal uptake in foliage, which subsequently delivers Hg to forest floor as litterfall [ Rea et al ., ; Ericksen et al ., ; Stamenkovic and Gustin , ; Rutter et al ., ]. Additionally, Hg(0) may be directly immobilized in soils [ Obrist et al ., ], although the importance of this deposition pathway is not quantified. Hg(ox) deposits directly onto soil surfaces via wet and dry deposition or can be intercepted by forest canopies and then deposited by throughfall and litterfall [ Iverfeldt , ; Rea et al ., ; Graydon et al ., ; Juillerat et al ., ].…”