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Case study : 137 Cs as tracer for sedimentation processes in an Italian coastal area

IAEA Regional Training Course on Sediment Core Dating Techniques. RAF7/008 Project CNESTEN, Rabat, 5 – 9 July 2010. Case study : 137 Cs as tracer for sedimentation processes in an Italian coastal area. Roberta Delfanti , ENEA - La Spezia, Italy.

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Case study : 137 Cs as tracer for sedimentation processes in an Italian coastal area

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  1. IAEA Regional Training Course on Sediment Core Dating Techniques. RAF7/008 Project CNESTEN, Rabat, 5 – 9 July 2010 Case study:137Cs astracerforsedimentationprocesses in anItaliancoastal area Roberta Delfanti, ENEA - La Spezia, Italy

  2. Sedimentationprocessesin the N-AdriaticSea Why the N- Adriatic? Significant input of 137Cs (Chernobyl, 1986) Different depositional environments, useful for discussion

  3. Whythe N-AdriaticSea? The Chernobyl fallout in Italy

  4. Friuli Venezia Giulia Region (NE Italy) Wet deposition of 137Cs (kBq m-2) 30 Apr – 9 May 1986 Global fallout: 4 kBq m-2 Trieste

  5. The Chernobyl falloutin the MedRegion Itcontained a varietyofradionuclides produced in the nuclearreactor. Amongthese: Short lived: 131I, 106Ru, 134Cs…. Long lived: mainly137Cs In the MedRegion no 90Sr, no Pu isotopes (depositedclose, 100 km, to the reactor)

  6. Afteraccidentmonitoring In June 1986 137Cs conc. in seawater 100 Bq m-3, decreased to 65 in October. Max conc of 137Cs in mussels and fish < 10 Bq kg-1 ww.

  7. For the followingyears, expecteddelayed input ofradionuclidesfromrivers and accumulationof137Cs in coastalsediments. Area monitoredstarting in 1987. Sampling in key areas up to 2001, tofollowtime trend.

  8. The study area

  9. Factors controlling coastalcurrents: The prevailing winds (NE “Bora” and SE “Scirocco”) Planning ofsedimentstudy

  10. Bora Surfacecurrents

  11. Tagliamento rivermouth

  12. Distribution of fine sediments expoted by rivers Weak currents Strong currents Shoal Pelite Brambati et al., 1983

  13. 30-40 150-300 10-20 10-20 3-10 60-200 0.5 -3 Radionuclidesastracersforpollutantsassociatedtoparticlesexportedbyrivers 137Cs conc. in surface sediments (Bqkg-1), 1987

  14. Minimum 137Cs surface activities were found in the sandy areas offshore (relict sands) where the balance between deposition and re-suspension results in no net accumulation of recent material. Relatively high concentrations charatcterised fine sediments. Highest conc. (> 150 Bq kg-1) were found in the submerged delta of the rivers, first depositional front of particles exported.

  15. The Po (mainItalianriver) delta

  16. The Po river area Same charachteristics Sedimentological map 137Cs surface concentrations

  17. But surface concentrationsjust tell one partof the story……

  18. Tracer profiles also strongly depend on sedimentary regime Weak currents Strong currents Shoal Tagliamento R. Pelite Brambati et al., 1983

  19. Isonzo River area Isonzo area ofinfluenceGulfof Trieste Classical profile: Chernobyl Cs over a layer containing global fallout Cs 1963 Mass depth (g cm-2) Regular sediment accumulation 0.5 g cm-2 y-1

  20. Tagliamento River area Tagliamento Prodelta Only Chernobyl Cs Only sediment deposited after 1986 over a layer more than 40 years old. Mass depth (g cm-2) Erosion events so effective to remove most of the sediment deposited

  21. Time trend • Studies in the area were repeated in 1989 and 2001. • Sediment cores. • Focus on the areas of max radionuclide • accumulation.

  22. Tagliamento prodelta: time trend From 1987 to 1989 sediment slowly accumulates: the Cs peak deepens and smoothens. In 2001 the Chernobyl sediment has been re-suspended and substituted by a thick layer with max conc. of 50 Bq kg-1, much less than in the original sediment, where 137Cs conc were at least double.

  23. 1963 1986 Gulfof Trieste: time trend Mass depth (g cm-2) Mass depth (g cm-2) Sediment regularly accumulates, surface concentrations were in 2001 about 2/3 those measured in 1987. Sed accum. rate: ~ 0.5 g cm-2 y-1

  24. Chernobyl radionuclides are good tracers to derive information on sediment dynamics But 137Cs profiles can be influenced by a series of post-depositional processes (mobility through pore water, bioturbation…). • 210Pb has different input function and geochemical behaviour, similar time scale (100 y): • good for independent estimate of sed. accum. rate.

  25. Gulf of Trieste 1945 210Pb shows exponential decay: regular sed accumulation rate

  26. Tagliamentoprodelta 210Pb indicates deposition of a thick layer of sediment with the same age.

  27. The combineduseof137Cs and 210Pb verticalprofiles in sedimentallowed: • a goodcharacterizationof the sedimentation regime in the study area; • identificationof the areasofpreferentialaccumulationofpollutantstransportedby the rivers. • definitionof the accumulationrates.

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