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Time-Dependent Factors Inherent in the Age Equation for Determining Residence Times of Groundwater Using 14C: A procedure to Compensate for the Past Variability of 14C in Atmospheric Carbon Dioxide, with Application to the Wairau Deep Aquifer, Marlborough, New Zealand
Author
Taylor, Claude B.Issue Date
2004-01-01Keywords
absolute agealluvium aquifers
aquifers
Australasia
C 13 C 12
C 14
carbon
case studies
Cenozoic
concentration
corrections
dates
geochemistry
ground water
hydrochemistry
hydrogen
isotope ratios
isotopes
Marlborough New Zealand
New Zealand
O 18 O 16
oxygen
principles
Quaternary
radioactive isotopes
recharge
residence time
South Island
stable isotopes
tritium
upper Quaternary
Wairau Plain
Metadata
Show full item recordCitation
Taylor, C. B. (2004). Time-dependent factors inherent in the age equation for determining residence times of groundwater using 14C: a procedure to compensate for the past variability of 14C in atmospheric carbon dioxide, with application to the Wairau deep aquifer, Marlborough, New Zealand. Radiocarbon, 46(2), 501-515.Journal
RadiocarbonDescription
From the 18th International Radiocarbon Conference held in Wellington, New Zealand, September 1-5, 2003.Additional Links
http://radiocarbon.webhost.uits.arizona.edu/Abstract
The radiocarbon concentration of dissolved inorganic carbon in groundwater is most logically and completely represented as the product of 5 time-variable factors; these are mutually independent, and all must be considered and evaluated to determine a groundwater residence time. In the case of one factor, the 14C/(12C+13C) ratio of atmospheric CO2, its time variability can be side-stepped by assuming it to be constant at the pre-bomb 1950 value, and assigning an apparent half-life in the radioactive decay term. Apparent half-lives are calculated here for 5 separate periods extending back to 24,000 BP, working from the INTCAL98 atmospheric calibration. This approach can be extended further back in time when the necessary atmospheric calibrations are updated with greater certainty. The procedure is applied to the recently-explored Wairau Deep Aquifer, underlying central areas of the coastal Wairau Plain, Marlborough. The evolution of dissolved inorganic carbon concentration for this river-recharged groundwater is apparent from distinct trends in 13C, and is confirmed by hydrochemical modelling. Extension to 14C concentrations yields minimum/maximum limits for groundwater residence times to 3 wells. In all 3 cases, the maximum is uncertain due to present uncertainty of the apparent half-life applicable before 24,000 BP. Residence times for the 2 wells closest to the recharge area are at least 17,400 yr, while that for a well further down the aquifer is at least 38,500 yr. Recharge, therefore, occurred during the Otiran glaciation, while the present-day near-surface fluvioglacial deposits of the Wairau Plain were accumulating. Drawdown-recovery records over 3 yr indicate a permeable connection to compensating recharge, enabling limited exploitation for vineyard irrigation.Type
Proceedingstext
Language
enISSN
0033-8222ae974a485f413a2113503eed53cd6c53
10.1017/S0033822200035566