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

Sedimentation processes in the N-Adriatic Sea Why the N- Adriatic? Significant input of 137 Cs (Chernobyl, 1986) Different depositional environments, useful for discussion 2

Why the N-Adriatic Sea? The Chernobyl fallout in Italy 3

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

The Chernobyl fallout in the Med Region It contained a variety of radionuclides produced in the nuclear reactor. Among these: Short lived: 131 I, 106 Ru, 134 Cs…. Long lived: mainly 137 Cs In the Med Region no 90 Sr, no Pu isotopes (deposited close, 100 km, to the reactor) 5

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

For the following years, expected delayed input of radionuclides from rivers and accumulation of 137 Cs in coastal sediments. Area monitored starting in Sampling in key areas up to 2001, to follow time trend. 7

The study area 8

Planning of sediment study Factorscontrolling coastal currents: The prevailing winds (NE “Bora” and SE “Scirocco”) 9

Surface currents Bora 10

Tagliamento river mouth 11

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

Cs conc. in surface sediments (Bqkg -1 ), 1987 Radionuclides as tracers for pollutants associated to particles exported by rivers 13

Relatively high concentrations charatcterised fine sediments. Minimum 137 Cs 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. Highest conc. (> 150 Bq kg-1) were found in the submerged delta of the rivers, first depositional front of particles exported. 14

The Po (main Italian river) delta 15

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

But surface concentrations just tell one part of the story…… 17

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

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

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

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

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 137 Cs conc were at least double. 22

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

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

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

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

137 Cs and vertical profiles in sediment allowed: The combined use of 137 Cs and 210 Pb vertical profiles in sediment allowed:  a good characterization of the sedimentation regime in the study area;  identification of the areas of preferential accumulation of pollutants transported by the rivers.  definition of the accumulation rates. 27