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| Titel |
The hydrogeochemical evolution of a barrier island freshwater reservoir: Conceptual understanding and identification of key processes |
| VerfasserIn |
Stephan Seibert, Tobias Holt, Janek Greskowiak, Holger Freund, Michael E. Böttcher, Gudrun Massmann |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250140184
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| Publikation (Nr.) |
 EGU/EGU2017-3536.pdf |
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| Zusammenfassung |
| Coastal aquifers play an important role in satisfying the water demands for many people in
the world. However, exposition to storm surges, climate change and extensive abstraction
pose a threat to current and future use of these valuable water resources in many cases.
To mitigate water quality constraints and ensure safe water supply applications,
an in-depth understanding of relevant process that determine the water quality is
required.
We investigated two freshwater reservoirs below the barrier island Spiekeroog, Germany.
The main freshwater reservoir is located at the western part of the island, ∼350 years old and
has a vertical extension of ∼45m. The other investigated freshwater reservoir is located at the
east of Spiekeroog, only a few decades old and has a vertical extension <7m. The ultimate
goal of our work is to identify, quantify and model the processes governing the water quality
evolution of freshwater reservoirs below barrier islands. During sampling campaigns in 2011,
2014 and 2016, field parameters were measured and samples for major cation and anion
analysis were taken from 16 freshwater wells. Additional samples were taken in 2011
(3H-3He, δ18O, δD), 2014 (DOC, HS−, NH4+, phosphate) and 2016 (DOC, HS−,
NH4+, phosphate, 3H-3He, δ18O, δD, δ13C-DIC). Based on the collected data, we
conceptualized some important hydrogeochemical processes that are potentially
relevant at Spiekeroog. This includes cation exchange, calcite dissolution and redox
reactions.
The expected freshening time of the main aquifer was estimated by means of the
retardation formula for sharp fronts and corresponds to ∼880 years. A 1D PHREEQC
simulation showed, however, that the effects of ongoing freshening on the present water
quality (after ∼350 years) are negligible. Calculated decalcification rates amount to ∼4.4
mm/year which corresponds to ∼1.5 m downward movement of the decalcification front
within the last 350 years. pH values ranging between 7.5-8.5 confirm that groundwater at
Spiekeroog is in equilibrium with calcite and underline that calcite dissolution is an important
process. With respect to the redox system, the data indicates oxygen and nitrate reduction
within the first meters of the saturated zone but Mn-Oxide and Fe-Oxide reduction rates seem
to be low in the aquifer based on measured dissolved Mn(2+) and Fe(2+) concentrations. The
absence of dissolved Fe(2+) could be explained by the formation of iron sulfide
minerals which is in agreement with observed sulfate reduction at greater depth
indicated by elevated H2S concentrations and PHREEQC speciation calculations. |
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