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| Titel |
Integration of major ion chemistry and stable isotope data with hydrogeological modelling to infer groundwater pathways |
| VerfasserIn |
Dirk Mallants, Koen Beerten, Isabelle Wemaere, Matej Gedeon, Bart Rogiers, Serge Labat |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052034
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| Zusammenfassung |
| Determination of groundwater pathways and groundwater residence time is important in
groundwater resources management. A variety of methods exist to differentiatie infiltration
zones from seepage zones and to quantify their relationship. For this purpose we combined
data on major ion chemistry and stable isotopes with calculations from a hydrogeological
model whose detailed hydrostratigraphic units were derived from a dense network of cone
penetration tests.
The analysis was done to quantify the connectivity between two aquifers separated by a
distinct clay layer several meters thick. The clay layer plays an important role in the
relationship between infiltration and seepage zones The upper aquifer has developed in a
sequence comprising the sandy Formations of Mol and Kasterlee, while the lower aquifer
mainly consisted of the sandy glauconite-rich Diest Formation.
The detection of isotopes as they occur in nature is an invaluable tool in studying the
behaviour of water in the hydrological cycle. Owing to the difference in mass, stable isotopes
behave slightly different in physical, chemical, and biological processes. For instance, due to
evaporation and exchange processes in the atmosphere, the stable isotopes 2H/1H and
18O/16O become fractionated. The resulting small changes in isotopic concentrations may
yield information on the circulation of water, e.g. from the transition of precipitation to soil
infiltration and recharge to groundwater, with further variations in young shallow
groundwater in infiltration zones and older groundwater from deeper aquifers that drain into
the seepage zones.
In the present study, 18O and 2H were measured in rainwater, surface water, and
groundwaters to reveal interactions among them. Rainwater was included in the analysis for
tracing groundwater recharge, while the analysis of surface water in terms of stable
isotopes was necessary to determine the contribution of deep groundwater to river base
flow.
The hydrochemical analyses showed existence of different groundwater compositions for
the upper and lower aquifers. All groundwaters seem to be meteoric in origin, whereby the
alkalinity increases with depth while the cations shift from divalent Ca to monovalent Na. A
distinct cluster of data points for the upper aquifer was evident, characterised by a strong
sulphate component (between 60-80%). Based on the cation analysis, most samples are of
the sodium or potassium type, where in general K and Na are dominant, while
fewer are from the calcium type. The water composition in the lower aquifer (Diest
Formation) is quite different compared to the upper aquifer. The anions are dominated by
bicarbonate (bicarbonate type groundwater), while Ca becomes the most important cation
(calcium type groundwater). This could be related to the higher Ca-concentrations
observed in the Diest mineral (i.e. carbonate) phase. The river water has a composition
inbetween that of both aquifers, suggesting mixing of both waters in the seepage
zone.
Analysis of stable isotope data reveals the existence of two clusters for the hydrostratigraphic
units that define the upper and lower aquifer: one for the units above the Kasterlee clay (i.e.,
Mol and Kasterlee Sands), and one for the Diest Sands. It appears that the shallower and
younger water from the Mol Sands is slightly less depleted in 2H and 18O in comparison with
water from the deeper Diest Sands (for the latter aquifer deviations from the meteoric lines
are larger both for 18δ and2δ). These two clusters thus confirm the geochemical
analysis.
The groundwater geochemistry and stable isotope data confirmed the general flow paths
obtained from running the groundwater flow model: infiltration zones that are mainly
connected with the seepage zones along the Kleine Nete river via shallow groundwater
pathways. There is also confirmation from the groundwater geochemistry that the upper and
lower aquifers are well separated by the clay aquitard, although in the seepage zone strong
upward gradients across the clay exist. |
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