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Titel |
A multitracer approach to estimate groundwater residence time distributions at a managed aquifer recharge site |
VerfasserIn |
Andrea Popp, Rolf Kipfer |
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 |
250146374
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Publikation (Nr.) |
EGU/EGU2017-10398.pdf |
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Zusammenfassung |
Managed aquifer recharge (MAR) has become a common water management tool and serves
various purposes such as improving the quality of groundwater (GW). At the study site, the
Hardwald in Muttenz (Switzerland), MAR has been implemented in the mid-1950s to
overcome increasing water demands. GW is artificially recharged with water from the river
Rhine through a system of channels and ponds. The area is surrounded by potential
contamination sites such as chemical industry, former landfills, a highway and a freight depot.
Furthermore, the area shows a complex hydrogeologic setting with several fault zones and
two main aquifers, the Quaternary Rhine gravel aquifer overlying a karstified Upper
Muschelkalk limestone aquifer. Water from the deeper limestone aquifer is suspected to
contain contaminants originating from the landfills. The fractures might serve as a
hydraulic connection between the upper and lower aquifer. Further, groundwater
pumping might enhance the mixing of recently infiltrated water with older water from
the lower aquifer. Hence, the proximity to potential contamination sites and the
complex geologic setting both pose risks for GW pollution and challenge the drinking
water production in this area. To guarantee a safe drinking water supply, it is crucial
to know the mixing patterns of young and old GW abstracted from the pumping
wells.
With this study we aim to determine the spatial variability of GW residence time
distributions to differentiate between recently infiltrated river water and older groundwater.
To reach our objectives, we use a combination of the following tracers to cover a wide
range of possible GW ages: (1) radiogenic 222Rn (young water := <3 weeks);
(2) tritium (3H) in combination with its tritiogenic decay product 3He (old water
:= 0.5–50 years); and (3) radiogenic 4He (very old water := 100–1000 years).
Additionally, we analysed other dissolved (noble) gases (O2, N2, Ar, Kr) to estimate the
amount of excess air and to derive the equilibration temperature. We also sampled for
physico–chemical parameters such as water temperature, electrical conductivity, alkalinity,
total hardness, DOC, Na+, Mg2+, Ca2+, K+, Cl−, NO3−, SO42− and H4SiO4
concentrations to complement the interpretation of the age tracers. All parameters were
analysed at 20 observation and pumping wells distributed throughout the study area.
First results indicate that GW abstracted in the vicinity to the area with the highest
recharge rates exhibits young water only. Wells further away from these high recharge
areas contain a mixture of young and old GW. However, only one pumping well
showed a mixture of very young, old and very old GW. Some wells within the
fracture zones show higher 3H/3He ages which supports our hypothesis of a hydraulic
connection between the deeper and the upper aquifer. As a next step, we plan on using
a mixing model to quantify the fractions of young and (very) old groundwater. |
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