case study : 137 cs as tracer for sedimentation processes in an italian coastal area
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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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
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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
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Why Why the N-Adriatic the N-Adriatic
Sea?Sea?
The Chernobyl fallout in Italy
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Wet depositionof 137Cs (kBq m-2)30 Apr – 9 May1986Global fallout:4 kBq m-2
Friuli Venezia Giulia Region (NE Italy)
Trieste
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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)
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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
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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.
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The study areaThe study area
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Planning of sediment study
Factors Factors controlling controlling coastal currents:coastal currents:
The prevailing winds(NE “Bora”and SE “Scirocco”)
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Surface currents
Bora
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Tagliamento river mouthTagliamento river mouth
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Distribution of fine sediments expoted by rivers
Pelite
Brambati et al., 1983
Weak currents
Strong currentsShoal
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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
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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.
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The Po (main Italian river) deltaThe Po (main Italian river) delta
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The Po river area
Sedimentological map
137Cs surface concentrations
Same charachteristics
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But surface concentrationsjust tell one partof the story……
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Tracer profiles also strongly depend onsedimentary regime
Pelite
Brambati et al., 1983
Weak currents
Strong currentsShoal
Tagliamento R.
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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
)
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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
)
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Time trendTime trend
• Studies in the area were repeated in 1989 and 2001.
• Sediment cores.
• Focus on the areas of max radionuclide accumulation.
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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.
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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
)
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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.
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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
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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.
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• 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.
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