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Fritz, S.J.* ; Drimmie, R.J.* ; Fritz, P.J.

Characterizing shallow aquifers using tritium and 14C: Periodic sampling based on tritium half-life.

Appl. Geochem. 6, 17-33 (1991)
DOI
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Thirteen water-production wells in glacial sediments in and around Waterloo, Ontario, (Canada) were sampled for major ions, 3H, 13C, 14C and 18O in 1976 and again in 1988 to gauge the movement of bomb-spike tritium and 14C through the Waterloo aquifer and also to assess the reliability of a hydrogeochemical interpretation of the dynamic character of an aquifer based only on single sampling episode. The suite of samples collected in 1976 was not able to indicate how fast the tritium spike was moving or even if each well-screen was intercepting the front end or tail end of the bomb-spike peak corresponding to infiltration of mid-1960s precipitation. The second sampling event portrayed movement which, qualitatively at least, gave information relating to the more permeable zones of the aquifer and direction of water movement. Water from all but one of the 13 wells sampled in 1988 registered a drop in tritium relative to the 1976 sampling. A well is deemed to be intercepting the front end of the peak of the bomb-spike if its 1988 tritium activity (±2 TU analytical uncertainty) is greater than one half that of the tritium activity of well water measured in 1976 (±8 TU uncertainty). Only one well and two shallow piezometers met this criterion. Ten wells had TU1988/TU1976 ratios which could be interpreted as either greater than or less than 0.5, if the analytical uncertainties of both analyses in this ratio are rigorously applied. Screens of two wells intercept the tail end of the spike because both their uncertainty-adjusted TU1988/TU1976 ratios range from 0.29 to 0.41.Carbon-14 activity for individual wells varied by no more than 6 Per cent Modern Carbon (PMC) between 1976 and 1988. Bomb-spike 14C is not so easily detected as tritium in passage through aquifers because the thermonuclear input of 14C into the atmosphere was much less intense (relative to pre-bomb, background levels) than that for tritium. Also, incongruent dissolution of dolomite, coupled with differing dissolution kinetics between dolomite and calcite, precipitates 14C-bearing calcite in the saturated zone. Although the chemistry of an aquifer can be reasonably characterized by a single sampling episode, the recharge rate and groundwater flow paths are best delineated from a geochemical perspective by multiple samplings in which a persistent chemical and/or isotopic tracer is sought out and repeatedly analyzed. In the Waterloo aquifer, these are tritium and Cl. The sameness of 14C activity in waters sampled 12 a apart makes this a poor candidate to use in this scheme.
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Publication type Article: Journal article
Document type Scientific Article
Corresponding Author
ISSN (print) / ISBN 0883-2927
e-ISSN 0883-2927
Journal Applied Geochemistry
Quellenangaben Volume: 6, Issue: 1, Pages: 17-33 Article Number: , Supplement: ,
Publisher Elsevier
Non-patent literature Publications
Reviewing status Peer reviewed
Institute(s) Institut für Hydrologie