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

Case study:Case study:137137Cs as tracer for sedimentation Cs as tracer for sedimentation

processes in an Italian coastal areaprocesses in an Italian coastal area

Roberta Delfanti, ENEA - La Spezia, Italy

IAEA Regional Training Course on Sediment Core Dating Techniques. IAEA Regional Training Course on Sediment Core Dating Techniques. RAF7/008 ProjectRAF7/008 Project

CNESTEN, Rabat, 5 – 9 July 2010

1

Sedimentation processes Sedimentation processes in the N-Adriatic Seain the N-Adriatic Sea

Why the N- Adriatic?

Significant input of 137Cs (Chernobyl, 1986)

Different depositional environments, useful for discussion

2

Why Why the N-Adriatic the N-Adriatic

Sea?Sea?

The Chernobyl fallout in Italy

3

Wet depositionof 137Cs (kBq m-2)30 Apr – 9 May1986Global fallout:4 kBq m-2

Friuli Venezia Giulia Region (NE Italy)

Trieste

4

The Chernobyl falloutThe Chernobyl falloutin the Med Regionin the Med Region

It contained a variety of radionuclidesIt contained a variety of radionuclidesproduced in the nuclear reactor.produced in the nuclear reactor.

Among these: Among these: Short lived: Short lived: 131131I, I, 106106Ru, Ru, 134134Cs….Cs….Long lived: mainly Long lived: mainly 137137Cs Cs

In the Med Region no In the Med Region no 9090Sr, no Pu isotopesSr, no Pu isotopes(deposited close, 100 km, to the reactor)(deposited close, 100 km, to the reactor)

5

In June 1986 137Cs conc. in seawater100 Bq m-3, decreased to 65 in October.

1985 1990 19950

20

40

60

80

100

120

C

s-13

7 in

sea

wat

er

(Bq

m-3)

Year

0

2

4

6

Cs-

137

in m

uss

el s

oft

par

ts (

Bq

kg

-1 w

.w.)

Max conc of 137Csin mussels and fish< 10 Bq kg-1 ww.

After accident monitoringAfter accident monitoring

6

For the following years, expected delayed For the following years, expected delayed input of radionuclides from rivers and input of radionuclides from rivers and accumulation of accumulation of 137137Cs in coastal sediments.Cs in coastal sediments.

Area monitored starting in 1987.Area monitored starting in 1987.Sampling in key areas up to 2001, Sampling in key areas up to 2001, to follow time trend.to follow time trend.

7

The study areaThe study area

8

Planning of sediment study

Factors Factors controlling controlling coastal currents:coastal currents:

The prevailing winds(NE “Bora”and SE “Scirocco”)

9

Surface currents

Bora

10

Tagliamento river mouthTagliamento river mouth

11

Distribution of fine sediments expoted by rivers

Pelite

Brambati et al., 1983

Weak currents

Strong currentsShoal

12

0.5 -3

60-200

10-20

150-300

30-40

10-20

3-10

137Cs conc. in surface sediments (Bqkg-1), 1987

Radionuclides as tracers for pollutants Radionuclides as tracers for pollutants associated to particles exported by associated to particles exported by

riversrivers

13

Relatively high concentrations charatcterised fine sediments.

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.

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) deltaThe Po (main Italian river) delta

15

The Po river area

Sedimentological map

137Cs surface concentrations

Same charachteristics

16

But surface concentrationsjust tell one partof the story……

17

Tracer profiles also strongly depend onsedimentary regime

Pelite

Brambati et al., 1983

Weak currents

Strong currentsShoal

Tagliamento R.

18

Isonzo area of influenceGulf of Trieste

Classical profile: Chernobyl Cs over a layer containing global fallout Cs

Regular sediment accumulation

1963

0.5 g cm-2 y-1

0

5

10

15

20

25

30

35

0 10 20 30 40 50 60

June 1987

Cs-134 Cs-137 (Bq/kg)

Dep

th (

cm)

Isonzo River area

Mas

s de

pth

(g c

m-2

)

19

Tagliamento ProdeltaTagliamento Prodelta

Only Chernobyl Cs

Only sediment deposited after 1986 over a layer more than 40 years old.

Erosion events soeffective to removemost of the sediment deposited35

30

25

20

15

10

5

00 50 100 150 200 250 300

Dep

th (

cm)

0 50 100 150 200 250 300

June 87

Cs-134 Cs-137 (Bq/kg)

0 50 100 150 200 250 300

Tagliamento River area

Mas

s de

pth

(g c

m-2

)

20

Time trendTime trend

• Studies in the area were repeated in 1989 and 2001.

• Sediment cores.

• Focus on the areas of max radionuclide accumulation.

21

Tagliamento prodelta: time trendTagliamento prodelta: time trend

35

30

25

20

15

10

5

00 50 100 150 200 250 300

June 2001

Cs-137

Dep

th (cm

)

134,137Cs vertical profiles at Station 527 1987 - 2001

35

30

25

20

15

10

5

00 50 100 150 200 250 300

June 1989

Cs-134 Cs-137 (Bq/kg)

Dep

th (cm

)

0 50 100 150 200 250 300

0 50 100 150 200 250 300

35

30

25

20

15

10

5

00 50 100 150 200 250 300

Dep

th (c

m)

0 50 100 150 200 250 300

0 50 100 150 200 250 300

November 1987

Cs-134 Cs-137 (Bq/kg)

35

30

25

20

15

10

5

00 50 100 150 200 250 300

Dep

th (c

m)

0 50 100 150 200 250 300

June 87

Cs-134 Cs-137 (Bq/kg)

0 50 100 150 200 250 300

From 1987 to 1989 sediment slowly accumulates: the Cs peak deepens and smoothens.

In 2001 the Chernobyl sediment has been re-suspended and substitutedby a thick layer with max conc. of 50 Bq kg-1, much less than in the originalsediment, where 137Cs conc were at least double.

22

Gulf of Trieste: time trendGulf of Trieste: time trend

Sediment regularly accumulates, surface concentrations were in 2001about 2/3 those measured in 1987.

1963

1986

Sed accum. rate: ~ 0.5 g cm-2 y-1

137Cs vertical profile at Station 567 1987 and 2001

60

50

40

30

20

10

00 10 20 30 40 50 60

June 2001

Cs-137 (Bq/kg)

De

pth

(c

m)

60

50

40

30

20

10

00 10 20 30 40 50 60

November 1987

Cs-134 Cs-137 (Bq/kg)D

epth

(cm

)M

ass

dept

h (g

cm

-2)

Mas

s de

pth

(g c

m-2

)

23

Chernobyl radionuclides are good tracers to derive information on sediment dynamics

But 137Cs profiles can be influenced by a series ofpost-depositional processes (mobility throughpore water, bioturbation…).

210Pb has different input function and geochemical behaviour, similar time scale (100 y): good for independent estimate of sed. accum. rate.

24

Gulf of TriesteGulf of Trieste

210Pb shows exponential decay: regular sed accumulation rate

60

50

40

30

20

10

00 10 20 30 40 50 60 70

St 567, june 2001

Sed rate, CF CS, 0.53 g cm-2y-1

Pb-210 ex

Mas

s d

epth

(g

/cm

2 )

60

50

40

30

20

10

00 10 20 30

1988

1965

Core 567 June 2001

sed rate: 0.55 g cm-2 y-1

Cs-137 (Bq/kg)

Ma

ss

de

pth

(g

/cm

2 )

25

1945

35

30

25

20

15

10

5

00 50

June 2001

Pb-210ex

Dep

th (

cm)

0 50

0 20 40 60 80

Tagliamento prodeltaTagliamento prodelta

35

30

25

20

15

10

5

00 20 40 60 80 100

June 2001

Cs-137

Dep

th (

cm)

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

26

• The combined use of 137137Cs and Cs and 210Pb vertical vertical profiles in sediment allowed:profiles in sediment allowed:

a good characterization of the sedimentation a good characterization of the sedimentation regime in the study area;regime in the study area;

identification of the areas of preferential identification of the areas of preferential accumulation of pollutants transported by the accumulation of pollutants transported by the rivers.rivers.

definition of the accumulation rates.definition of the accumulation rates.

27