Environ. Sci. Techno/. 1995, 29, 126-135
Micrometeamlogical Gradient Approach for hantifyinfl Ai$Surface Exchange of- Mercury Vapor: Tests over -Contaminated STEVEN E . L I N D B E R G , * , ' KI-HYUN K I M , ' TILDEN P . M E Y E R S , § A N D J I M G . OWENS' Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennesee 37831-6038, and Atmospheric Turbulence and Diffusion Division, National Oceanic and Atmospheric Administration, P.O. Box 2456, Oak Ridge, Tennesee 37831
The micrometeorological modified Bowen ratio method was used for the first time t o quantify fluxes of elemental mercury vapor (HgO) over contaminated soils during the spring and fall of 1993. W e determined fluxes using relationships among the concentration graiients and fluxes of water vapor (measured with fast-response sensors and eddy correlation) and measured gradients in Hgo (sampled with a multireplicate, high-precision device). The contaminated soils consistently exhibited significant emission of Hgo to the atmosphere. Fluxes associated with winds from the primary source area ranged from -10 to -200 ng mb2 h-l, up to several orders of magnitude above background. Fluxes increased exponentially with soil temperature, yielding a temperature coefficient similar to nitrogen oxide (exp[(0.065 f 0.01 l)Tsoi~l, for Ts0,l= 7-29 "C). These results provided confidence in the method for HgO because the gradients and fluxes behaved predictably. Fluxes appear to be controlled by volatilization of existing contaminant elemental Hg plus reduction of Hg2+ t o HgO in soil solution. W e modeled the emission from an area of contaminated soils within -20-50 m of our site using the fluxlsoil temperature relationship and annual temperature data. The predicted hourly fluxes were within 30-40% of our measurements and scaled up to an annual Hg emission rate of -800 pg m-2 y-'. W e estimate the magnitude of the HgO emission from the 100-ha contaminated area to be on the order of 1 -10 kg/y.
Introduction The bioaccumulation of toxic Hg is widelyreported to result from long-range transport and deposition of atmospheric emissions of vapor-phase Hg (1-3). There has been progress in understanding the cycling of Hgwithin aquatic systems (e.g., refs 3-51, but less is known about the air/ surface exchange of Hg. Mercury appears to be readily re-emitted to the atmosphere (6, 3 , and estimates of the global Hg budget suggest that re-emission from both water and soil surfaces could be a significant source of Hg to the troposphere (3, 7-9). However, there are few direct flux measurements for Hg in the field (10, I ] ) , and reliable measurement methods are needed to reduce the uncertainty of global budget estimates (3). Mercury emissions have been quantified from soils using laboratoryandfield chambers (11-14). However,chambers suffer from blank problems and their potential to influence the limited surface area measured (11). Micrometeorological techniques are advantageous because the surface is not disrupted by these "in-air'' methods, and the measurements provide an areal average of the flux (15). Fastresponse Hg sensors are not available for eddy correlation, but measured gradients of gas concentrations can be used to infer fluxes from simultaneous measurements of turbulent mixing parameters (e.g.,ref 16). Serious tests of the gradient method for Hg fluxes have not been reported in the open literature, and previous gradient measurements have lacked the precision and time resolution necessary to accurately quantify fluxes (17-19). We describe here the first extensive application and testing of a micrometeorological gradient approach for Hg. We used the modified Bowen ratio (MBR) method to estimate Hg vapor fluxes over contaminated forest soils during 1993 using a high-precision sampling system (20). Our primary objective was to demonstrate the capability of our approach at a known source area before beginning extensive background flux studies (21,22). We also made order of magnitude estimates of the total Hg emissions from the contaminated area.
Research Site and Measurement Techniques Site Description and Sampling Design. The forested flood
plain of East Fork Poplar Creek (EFPC) in Oak Ridge, TN, was contaminatedwithHg duringthe 1950sby anupstream nuclear weapons plant which released -lo5 kg of Hg in both reduced (metallic) and oxidized forms (23). Recent estimates suggest that nearly 80 000 kg of Hg remains in the -250 ha of flood plain soils (241,predominantly as HgS but with a notable fraction (-6%) in the elemental form (25). Figure 1 illustrates the distribution of soils containing ?50pg/gofHginthevicinityofoursamplingsite. Although the ideal sampling design would require a centralized sampling location or multiple transects across the area, the threat of flooding and serious personnel and equipment contamination prevented direct access to the center of the flood plain. In March 1993,we established a sampling site Environmental Sciences Division Publication No. 4316, ORNL. * Corresponding author; e-mail address:
[email protected]. Oak Ridge National Laboratory.
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126 1 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 1. 1995
National Oceanic and Atmospheric Administration.
0013-936)(/95/0929-0126$09.00/0 0 1994 American Chemical Society
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at the nonbem edge ofthe most highlycontaminated area. To avoid contact with contaminated material, we built an elevatedwalkway and sampling platform extending -50 m into the forest to our sampling site. Gradients were measured directly over an undisturbed area of contaminated soil -2 m from the walkway. The surface soils (0-3 cm) surrounding the platform were moderately contaminated (4.8 0.4pg Hg/gl and in a transect to the Southeast increased from -20 pg/g at 10 m, to -40 pglg at 50 m. to - 6 O ~ g kat 75 m. A fluxfootprint analysisusing measured turbulence data suggested that those soilswithin 20-50 m of our site contributed most significantlyto the measured gradients (discussed below). Analytical and Sampling Methods. We collected gaseous Hg in air on reusable gold-coated quartz sand
*
absorbers (termed gold traps), which were blanked by heating to -450 "C prior to use and sealed with Teflon plugs. The traps efficientlycollectknown forms of gaseous mercury by amalgamation and adsorption on the gold coating (26). Gaseous Hg may exist in the elemental, methylated, and oxidized forms, but Hg in air is dominated (295-9996) by elemental Hg vapor (26, 23, and we will refer to the Hg collected hereafter as HgO. The gold traps were analyzed for HgO by dual-amalgamation cold vapor atomic fluorescencespectrometry (CVAFS) using published (26). The system was using a gas.tight microsyringe to inject Hgxsamated air from a constant temperature bath onto a gold trap (e.g., ref 28). This procedure yieldedveryhigh precision (
