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| Titel |
Parameter sensitivities for helium-4 accumulation in groundwater due to in-situ production in dual porosity aquifers |
| VerfasserIn |
Torsten Lange, Thomas Graf, Martin Sauter |
| 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 |
250058053
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| Zusammenfassung |
| The accumulation of 4He in groundwater is a potential water age indicator if all sources and
fluxes can be quantified sufficiently accurately. However, multi-porosity characteristics of
aquifers like fractured sandstone or karstic limestone are well-known phenomena that
distinctively affect the spatial distribution of flow velocities, the variation of flow paths, as
well as the transport of 4He. The impacts on the distribution of any tracer used for
groundwater age estimation are crucial and were discovered by researchers very early. A
good example for dual-porosity behaviour is chalk as it usually has a high primary porosity
with low pore diameters, and minor secondary porosities like fractures and others. It is
obvious that a tracer is being transported differently in a dual-porosity material than in
unconsolidated aquifer material.
Due to their chemical inertness, noble gases like helium are valuable complementary
environmental tracers in hydrogeological studies including processes of mixing and
salinization, estimation of paleo-infiltration temperatures, characterization of deep circulating
systems, detection of mantle degassing along fracture systems, or estimation of groundwater
age. The most frequently applied noble gas tracer is helium with its two isotopes 3He and
4He. While 3He is more related to a degassing primordial mantle source, 4He is a stable
daughter isotope from uranium and thorium decay taking place mainly within crustal rocks,
and it accumulates in groundwater as a function of time. Because of the accumulation
mechanism, 4He is a potential tracer for estimation of groundwater age if the uranium
and thorium concentration in the aquifer material is known. However, those two
elements are common in most types of crustal rocks. The resulting crustal 4He flux
must be considered as potential second source that may overlay the in-situ aquifer
production.
To better understand the mechanisms of in-situ 4He accumulation in dual porosity
aquifers depending on five aquifer parameters, a sensitivity analysis was conducted. From a
geological 3D GoCAD model, a 2D cross section through a well field southwest of
Jerusalem, Israel, was extracted, where helium isotope data already exist for several deep
wells. The 2D meshes were also created in GoCAD and reformatted for HydroGeoSphere,
the applied flow and transport simulator. Due to the long individual model run times and the
expected monitoring demand, the process was automated in terms of batch software creating
all input files, running the cases, and selecting the appropriate mesh (Peclet-criterion) and
maximum time step (Courant- and Neumann-criterion). The sensitive parameters are matrix
and fracture porosities, fracture space hydraulic conductivity and dispersivity, as well as the
head gradient.
Within the chosen parameter ranges, the 4He accumulation is dominantly sensitive to the
head gradient. However, the chosen parameters have different units and scales, which has to
be considered for the interpretation. |
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