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| Titel |
Tracking selenium behaviour in chalk aquifer (northern France): Sr and 34S-sulphates isotopes constraints. |
| VerfasserIn |
Lise Cary, Hind Benabderraziq, Jamal Elkhattabi, Marc Parmentier, Laurence Gourcy, Philippe Négrel |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250089541
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| Publikation (Nr.) |
 EGU/EGU2014-3746.pdf |
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| Zusammenfassung |
| Groundwaters in parts of the Paris Basin (France) are facing increasing selenium
(Se) contents that can exceed the drinking water limit of 10 μg/L according to the
European Framework Directive in the field of water policy (2000/60/EC). To better
understand the groundwater origins and the selenium dynamics, the water chemistry of
the Chalk aquifer supplying drinkable water to Lille city was studied. This area is
submitted to quantitative and qualitative pressure from industrial, urban and agriculture
origins.
An integrated study was settled to determine the water sources and dynamics of elements,
with a focus on Se. After a large chemical characterisation of the groundwater chemistry in
the four field wells, a monthly monitoring was held in four wells and in the Deûle
channel. Chemical analysis of major and trace elements, stable isotopes (δ18O, δ2H),
strontium isotopes, and δ34S and δ18O of sulphates were realised. The chemical
composition of solids sampled at various depths at vicinity of the four wells was also
analysed.
The specific geochemical signature of groundwater as revealed by Sr isotopes, in addition
to element concentrations ratios like Mg/Sr and Se/Sr, highlighted mixture of three main
groundwaters bodies: (1) the upstream groundwaters in the recharge area with the most
radiogenic 87Sr/86Sr isotopic signature; (2) the confined groundwaters with high Sr
concentrations due to water-rock interactions and the lowest 87Sr/86Sr isotopic signature
close to the one of the chalk in Paris and London basins; (3) the Se-rich formations of
Tertiary and Quaternary.
The contents of Se, mainly present as SeV I (and locally as SeIV ), displayed spatial and
temporal disparities that can be explained by geological and hydrogeological conditions.
Se-rich clayed sediments originating from the dismantling of Se-rich tertiary formations (i.e.
Ypresian) overlay the chalk formation and can be found in saturated conditions depending of
the water table level. Oxidation of Se0, Se-pyrite and Se linked to organic matters
happens according to two pathways. The first one is oxidation of Se species present in
the reductive clayed sediments by oxidizing groundwaters during periods of high
piezometric levels. The decrease of the piezometric level induces a decrease of the Se
contents in groundwaters. Negative δ34S (0 to -28 ) coherent with pyrite oxidation are
observed in groundwater. The second one is linked to the infiltration of nitrate-rich
recharge waters through quaternary loess which also contain Se. In this case, δ34S of
groundwaters are slightly positive and close to the signature of fertilizers. Denitrification by
pyrite or Se0 oxidation plays a great role in enhancing Se mobility in a very reactive
system.
To limit the Se content in groundwaters, a precise water management is needed, e.g.
mainly maintaining low piezometric levels in the chalk aquifer or avoiding mixings between
contaminated wells, together with a precise knowledge of the geology of the quaternary
clayed sediments. |
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