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| Titel |
Carbon-14 as a tracer of groundwater discharge to streams |
| VerfasserIn |
Sarah Bourke, Glenn Harrington, Peter Cook, Vincent Post, Shawan Dogramaci |
| 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 |
250087138
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| Publikation (Nr.) |
 EGU/EGU2014-1169.pdf |
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| Zusammenfassung |
| The provenance of groundwater discharge to a stream can be determined by measuring the
response of multiple groundwater age tracers within the stream across the discharge
zone. The sampling interval required to detect groundwater discharge is limited by
the rate of equilibration with the atmosphere downstream of the discharge zone,
which is determined by the gas transfer velocity. Carbon-14 (14C) equilibration is
driven by CO2 exchange, which is a small component of the dissolved inorganic
carbon in most stream systems, and therefore the rate of equilibration is slower
than for other gaseous age tracers. In this paper we use a step-wise approach to
develop and demonstrate the use of 14C as a tracer in streams receiving groundwater
discharge.
Excess carbon dioxide (CO2) in the emerging groundwater degasses until equilibrium
with atmospheric CO2 is reached; increasing pH and enriching the residual 14C by
fractionation. In addition, the 14C gradient between groundwater and the atmosphere drives a
slower process of isotopic equilibration. We have measured the rates of this chemical and
isotopic equilibration experimentally by exposing 250L of old groundwater to the
atmosphere in an evaporation pan. Chemical equilibrium was achieved within 2 days, during
which the 14C increased from 6 to 16 pMC. The influence of fractionation during the initial
CO2 degassing on isotopic equilibrium rates was negligible. Isotopic equilibrium took over 2
months, with 14C in the evaporation pan increasing to 108 pMC over 71 days. This increase
in 14C was simulated using a mass balance model with an effective 14C gas transfer velocity
of 0.013 m d-1.
Field testing of the method was conducted at two sites. Firstly, we measured the evolution
of 14C in dewatering discharge as it flows along an ephemeral creek channel in the Pilbara,
Western Australia. Measured 14C increased from 11 to 31 pMC along the 10km reach, which
corresponds to a travel time of about 2 days. The measured increase was simulated using a
mass balance model with an effective 14C gas transfer velocity of 0.025 m d-1. Secondly, we
measured 14C at six sites along the Daly River, Northern Territory. Here we observed a
decrease of 7 pMC across a major groundwater discharge zone and were able to
simulate the observed 14C using a gas transfer velocity of 0.14 m d-1. We quantify the
total groundwater influx using measurements of stream discharge and radon-222,
which allows us to estimate the 14C activity of the groundwater discharge at 64
pMC.
The potential for groundwater discharge to be detectable in other streams using 14C
depends on the magnitudes of groundwater discharge, dissolved inorganic carbon, and 14C
relative to stream values. |
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