1396 Part 2 - Biogeochemistry Correlation of dissolved gaseous mercury and radon profiles in the Mediterranean Basin seawater Janja Vaupotič1, Jož e Kotnik1, Milena Horvat1 & Nicola Pirrone2 'Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia; E-mail: janja.vaupotic@ijs.si. 2CNR, Institute for Atmospheric Pollution, Rende, Italy. Abstract: Vertical profiles of dissolved gaseous mercury (DGM) and radon gas (222Rn) in seawater in the Mediterranean Basin have been measured. Preliminary results indicate similar trends of both gases at some locations, but not at others. Several factors should be considered in order to better understand the relationship between levels of these two gases. The profiles should be interpreted on the basis of the different mechanisms of origin and movement of the gases, as well as their different properties. Key words: dissolved gaseous mercury (DGM), radon (222Rn), seawater profiles, correlation Introduction The main goal of the MERCYMS (Mercury) project is to improve our knowledge of mercury cycling in the Mediterranean Sea. Other constituents of the seawater, which either affect the level of mercury or only accompany it are of interest because they may help better understand the transport and specia-tion of mercury. Thus, dissolved gaseous mercury (DGM, Hg°) and radioactive gas radon (222Rn) may both originate in the bottom sediment and be transported to the water surface by carrier gases. Therefore a relationship between their profiles in the water column could be expected. In oceanic waters, 10-30 % of the Hg may be present in the dissolved gaseous form. While reduction of Hg(II) by aquatic microorganisms was believed to be the predominant source of DGM, recent studies show that photoreduction of Hg may be another important source.[1] Most surface waters are supersaturated with DGM relative to the atmosphere and therefore elemental Hg is readily lost from water to the atmosphere.'21 Radon is produced by radioactive decay of radium (22TRa) in bottom sediments and of dissolved radium, and may originate from submarine springs and release of fresh waters generally having much higher radon levels than the sea water.[3] In contrast to mercury, radon is a noble gas and does not undergo chemical changes on its travel to the surface, but alpha - decays into its short-lived progeny. In seawater, Mn - 222Rn, CH4 - 222Rn and 3He - 222Rn correlations have been obtained when studying heat and chemical flux from the sea bottom.[4] Hg - Rn correlation has been studied in soil gas at eruptive sites'5" but not in seawater. Results on DGM and 222Rn in seawater taken and analysed during the Urania cruises in the Mediterranean Sea in summer 2003 and spring 2004 are re- RMZ-M&G 2004, SI 7th International Conference on Mercury as a Global Pollutant 1431 ported, and the correlation between the profiles is discussed. Experimental Sampling locations are shown in Fig. I and their coordinates given in Table I. Water samples were taken by the rosette at the de- sired depths. In the laboratory, DGM was purged by N2 from 0.5 dm3 water samples and collected in a gold trap, which was then transferred to a CV AFS analyser system. Radon was degassed from 1.7 dm3 water samples by bubbling in a closed loop and then measured with an 0.7 dm3 evacuated alpha scintillation cell on an alpha scintillation counter. Figure I. Sampling locations. Results and discussion Table I summarises the experimental parameters and shows the DGM - 222Rn correlation. Four typical depth profiles are shown in Figs. 2-5. The following DGM - 222Rn concentration relationship has been observed: 3 strong (Figs. 2 and 3), 3 moderate and 4 weak positive correlations, and I strong and 4 weak (Figs. 4 and 5) negative correlations. No final conclusions can be drawn on the basis of these preliminary results, but we do hope that after our DGM and 222Rn data have been further elaborated together with other data collected during the two MERCYMS cruises, we will be able to better interpret the DGM and 222Rn profiles observed. In addition, water samples were collected and will be analysed for 226Ra, in order to provide additional information on 222Rn balance.'61 RMZ-M&G 2004, SI 1396 Part 2 - Biogeochemistry Table I. Description of sampling locations and DGM - 222Rn correlation coefficients. Location Latitude Longitude Date of Number of Bottom DGM- 222Rn (N) (E) measurement sampling levels m correlation coefficient WPT 3 37°16' 11°53' 6.8.2003 7 95 -0.17 WPT 4 35°45' 17°55' 8.8.2003 10 4060 0.35 WPT 5 34° 19' 24°20' 10.8.2003 9 2235 -0.25 WPT 11 37°37' 15°16' 15.8.2003 8 728 0.73 WPT 12 40°34' 14°17' 16.8.2003 5 940 -0.92 ST. 1 40°29' 11°18' 19.3.2004 8 2883 0.85 ST. 2 41°25' 7°59' 21.3.2004 10 2600 -0.03 ST. 3 37°52' 5°21' 25.3.2004 9 2816 0.08 ST. 4 37°29' 11°34' 27.3.2004 10 945 0.30 ST. 5 35°45' 17°55' 29.3.2004 12 4040 0.49 ST. 6 39°59' 19°00' 30.3.2004 10 919 0.09 ST. 7 37°37' 15°15' 1.4.2004 11 683 0.41 ST. 8a 38°39' 15°05' 2.4.2004 6 40 -0.25 ST. 8b 38°39' 15°06' 2.4.2004 7 73 0.86 ST. 9 39°55' 14°00' 3.4.2004 10 2380 0.40 Rn-222 / Bqm3 Rn-222/Bqm3 0 5 10 15 20 25 0 2 0 - -■-1-- 4 6 8 100 150 DGM / ngm3 Figure 2. DGM - 222Rn profile at ST. 1. Figure 3. DGM - 222Rn profile at WPT. 11. 60 90 DGM/ngm3 222- Figure 4. DGM - ^Rn profile at ST. 3. 01000 40 60 DGM /ngm 222t Figure 5. DGM - //zRn profile at WPT. 5. RMZ-M&G 2004, SI 7th International Conference on Mercury as a Global Pollutant_ 1433 Conclusions Our preliminary results indicate that at some locations, DGM and 222Rn concentrations in the seawater column are well correlated, while such a correlation has not been found at others. No general rules have been eluci- dated either from data obtained for various locations in the same season, or from comparing data obtained in summer and spring at the same station. Final conclusion will be possible after the data have been further elaborated and results of 22TRa analyses will be included in the interpretation. References [1] Costa, M. & Liss, P. (2000): Photoreduction and evaluation of mercury from seawater; Science of the Total Environment 261, pp. 12S-13S. [2] Horvat, M., Kotnik, J., Logar, M., Fajon, V., Zvonaric, T. & Pirrone, N. (2003): Speciation of mercury in surface and deep-sea waters in the Mediterranean Sea; Atmospheric Environment 37, pp. S93-S108. [3] Corbett, D. R., Dillon, K., Burnett, W. & Chanton, J. (2000): Estimating the groundwater contribution into Florida Bay via natural tracers 222Rn and CHr; Limnology and Oceanography 4S, pp. 1S46-1SS7. [4] Rosenberg, D. N., Lupton, J. E., Kadko, D., Collier, R., Lilley, M. D. & Pak, H. (1988): Estimation of heat and chemical fluxes from a seafloor hydrothermal vent field using radon measurements; Nature 334, pp. 604-607. [5] Thomas, D. M. (1986): Geothermal resources in Hawaii; Geothermics IS, pp. 43S-S14. [6] Sarmiento, J. L., Broecker, W. S. & Biacaye, P. E. (1978): Excess bottom 222Rn distribution in deep ocean passages; Journal of Geophysical Research 83, pp. S069-S076. RMZ-M&G 2004, SI