Dating groundwater in a limestone aquifer
Final Report Abstract
To better understand contamination with geogenic arsenic (As) in private supply wells in a rural area in central Florida, 48 groundwater samples were dated using the tritiumhelium method. Of those, 20 were from private supply wells, while the remaining 28 were from monitoring wells, which were screened at 5 different depths, approximately 10 m, 50 m, 65 m, 80 m and 100 m. The ages for the monitoring wells showed a clear separation between shallow and deep samples. To a depth of 50 m the water was generally younger than 50 years, while below that depth all samples were older than 50 years. This would indicate rapid infiltration to a depth of 50 m and the likely presence of an aquiclude between 50 m and 65 m. The ages for the private supply wells varied, but high 4He concentrations indicated that they are a mixture of very old and younger water, which lead to the conclusion that those wells have very long screens allowing for the mixing between shallow and deep groundwater. Thus, the likely scenario for As release from the aquifer matrix is the rapid introduction of near surface oxygenated groundwater into the deeper regions of the aquifer, followed by oxidation of pyrite and the release of As bound in the pyrite structure.
Publications
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„A stable and radioactive isotope study of geogenic arsenic contamination in a limestone aquifer”, FH-DGG-Tagung „Grundwasser für die Zukunft“, May 2010
Thomas Pichler, Jürgen Sültenfuß
