correlate the sediment archives with the evolution of the river basin upstream of the
coring site, and its complex interactions with societies.
The interdisciplinary approach, developed by the PIREN-Seine to go further than
a geochemical description of the contamination trends, is presented in Sect. 2.
Section 3 briefly presents the Seine River basin, our coring strategy and core
analysis. Greater detail on the basin and its pollutant sources can be found in other
chapters of this volume and in original papers for coring, dating and analysis details.
Sections 4–7 present the different steps of the historical reconstruction of the basin
contamination to allow for intra- and interbasin comparisons, as conceptualised in
Fig. 1, illustrated with different particulate contaminants. The final section links the
sediment archives with the basin history (past pressures, awareness, conflicts,
societal responses and regulations, creation of institutions), i.e. defining the contamination trajectory, taking PCB as an example.
2 Long-Term Reconstruction of Past Contamination
Trajectories Based on River Sedimentary Archives
2.1 Conceptual Steps for the Study of Contaminants in Cores
These studies can be schematised by a succession of steps, from the most common
ones to those developed on the Seine River by the PIREN-Seine programme, as
follows.
Step A. Depth contaminant profile: cored fine sediments – clay and silt deposits –
are sliced, and then targeted contaminants are analysed on each slice. To minimise
the effect of grain size on the contents of pollutants, samples can be sieved at the
clay–silt fraction, typically <63 μm. Al, Th or Sc should also be measured as
quantitative tracers of this fraction. Particulate organic pollutant contents are often
normalised to the particulate organic carbon (POC). When the sedimentation rate is
regular, the slices are averaged temporal windows, e.g. yearly averages. The best
cores (e.g. lakes and reservoirs) provide an annual resolution. Cores taken in alluvial
plains record the deposits corresponding to high flows and floods: the temporal
record is more fragmented. Sometimes a single flood event corresponds to a multicentimetre layer.
Step B. Core dating: generally using the measurement of Cs-137, an artificial
radioisotope which provides three temporal markers: (1) the Cs-137 emergence in
1945, (2) a first maximum in 1962 originating from atmospheric nuclear test bombs
and (3) a second and sharper maximum generated by the Chernobyl nuclear accident
in 1986; Pb-210 decay can be used as well [22]. The evolution of the sedimentation
rate is then used to convert the depth profile into a temporal evolution of contamination. Reliable geochemical tracers for dating before 1945 are lacking.
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S. Ayrault et al.
coring site, and its complex interactions with societies.
The interdisciplinary approach, developed by the PIREN-Seine to go further than
a geochemical description of the contamination trends, is presented in Sect. 2.
Section 3 briefly presents the Seine River basin, our coring strategy and core
analysis. Greater detail on the basin and its pollutant sources can be found in other
chapters of this volume and in original papers for coring, dating and analysis details.
Sections 4–7 present the different steps of the historical reconstruction of the basin
contamination to allow for intra- and interbasin comparisons, as conceptualised in
Fig. 1, illustrated with different particulate contaminants. The final section links the
sediment archives with the basin history (past pressures, awareness, conflicts,
societal responses and regulations, creation of institutions), i.e. defining the contamination trajectory, taking PCB as an example.
2 Long-Term Reconstruction of Past Contamination
Trajectories Based on River Sedimentary Archives
2.1 Conceptual Steps for the Study of Contaminants in Cores
These studies can be schematised by a succession of steps, from the most common
ones to those developed on the Seine River by the PIREN-Seine programme, as
follows.
Step A. Depth contaminant profile: cored fine sediments – clay and silt deposits –
are sliced, and then targeted contaminants are analysed on each slice. To minimise
the effect of grain size on the contents of pollutants, samples can be sieved at the
clay–silt fraction, typically <63 μm. Al, Th or Sc should also be measured as
quantitative tracers of this fraction. Particulate organic pollutant contents are often
normalised to the particulate organic carbon (POC). When the sedimentation rate is
regular, the slices are averaged temporal windows, e.g. yearly averages. The best
cores (e.g. lakes and reservoirs) provide an annual resolution. Cores taken in alluvial
plains record the deposits corresponding to high flows and floods: the temporal
record is more fragmented. Sometimes a single flood event corresponds to a multicentimetre layer.
Step B. Core dating: generally using the measurement of Cs-137, an artificial
radioisotope which provides three temporal markers: (1) the Cs-137 emergence in
1945, (2) a first maximum in 1962 originating from atmospheric nuclear test bombs
and (3) a second and sharper maximum generated by the Chernobyl nuclear accident
in 1986; Pb-210 decay can be used as well [22]. The evolution of the sedimentation
rate is then used to convert the depth profile into a temporal evolution of contamination. Reliable geochemical tracers for dating before 1945 are lacking.
272
S. Ayrault et al.