ASSESSING THE POTENTIAL FOR RE-EMISSION OF MERCURY DEPOSITED IN PRECIPITATION FROM ARID SOILS USING A STABLE ISOTOPE

Mercury (Hg) can be emitted from terrestrial surfaces to the atmosphere, and deposited to terrestrial surfaces from the atmosphere by wet and dry processes. After deposition Hg can be subsequently re-emitted back to the atmosphere. One of the large uncertainties in quantifying Hg emissions from terrestrial surfaces is distinguishing between Hg originally present in the substrate and Hg deposited from the atmosphere and then re-emitted.

This study characterized the potential for re-emission of Hg deposited in precipitation to desert soils. Non-polar deserts comprise roughly 36% of the earth's land surface so an understanding the process of air-soil exchange of Hg in these settings is important for development of global mass balance estimates. The study site was located 30 km from Reno, NV, on barren soil with low Hg concentration (0.01 mg Hg/kg). The natural abundance of Hg isotopes in air measured at the site agreed quite well with published values.

A solution of 0.1 mg/L ¹⁹⁸Hg in the form of HgCl₂ was added to a 4 m² area of soil in a simulated light rain event. During the night, 8.2 ± 3.6 µg ¹⁹⁸Hg was applied to each m² of soil, and immediately after soil Hg fluxes were measured using three dynamic flux chambers, and isotope concentrations were determined using ICP-MS.

After ¹⁹⁸Hg was applied to the soil, there was an immediate release of the isotope, and then emissions decreased exponentially. Within the first six hours after the isotope was added to the soil, ~12 ng/m² of ¹⁹⁸Hg was emitted to the atmosphere, followed by a relatively steady flux of the isotope at 0.2 ± 0.2 ng/m²/h for the remainder of the experiment (62 days). Over this time, ~180 ng/m² or 2.2% of the ¹⁹⁸Hg isotope was emitted from the soil. During the experiment, dry deposition of elemental Hg from the atmosphere was measured with an average deposition rate of 0.2 ± 0.1 ng/m²/h. Emission of Hg from the soil was observed after soil wetting with the isotope solution and after a storm event. The added moisture from the storm event did not affect ¹⁹⁸Hg flux. Results suggest that in this desert environment where there is limited precipitation, Hg deposited by wet processes is not readily re-emitted and that dry deposition of elemental Hg may be an important process.