W.D. Robertson
Eurybiades Busenberg
Niel Plummer
M.C. Ryan
S.L. Schiff
L.A. Sebol
2007
<p><span>A detailed local-scale monitoring network was used to assess CFC distribution in an unconfined sand aquifer in southwestern Ontario where the zone of 1–5-year-old groundwater was known with certainty because of prior use of a bromide tracer. Groundwater ⩽5 years old was confined to an aerobic zone at ⩽5</span><span> </span><span>m depth and had CFC concentrations consistent with modern atmospheric mixing ratios at recharge temperatures of 7–11</span><span> </span><span>°C, as was observed in the 3-m thick vadose zone at the site. At depths below 6</span><span> </span><span>m, the groundwater became progressively more reducing, however, with a denitrifying horizon at 6–7</span><span> </span><span>m depth, and a Mn and Fe reducing zone below 7</span><span> </span><span>m depth. In the anaerobic zone, </span><sup>3</sup><span>H/</span><sup>3</sup><span>He ratios indicated that groundwater-age continued to increase uniformly with depth, to a maximum value of 27 years at 13</span><span> </span><span>m depth. CFC concentrations, however, decreased abruptly within the denitrifying zone, leading to substantial age overestimation compared to the </span><sup>3</sup><span>H/</span><sup>3</sup><span>He ages. Noble gas data indicated that the apparent CFC mass loss was not likely the result of gas stripping from possible bubble formation; thus, CFC degradation was indicated in the anoxic zone. The field data are consistent with first-order degradation rates of 0.3</span><span> </span><span>yr</span><sup>−1</sup><span> for CFC-12, 0.7</span><span> </span><span>yr</span><sup>−1</sup><span> for CFC-11, and 1.6</span><span> </span><span>yr</span><sup>−1</sup><span> for CFC-113. CFC attenuation at this site coincides with a zone where reduced S (pyrite) is actively oxidized by NO</span><sub>3</sub><span> and dissolved oxygen (DO). Similar behavior has been observed at other sites [Tesoriero, A.J., Liebscher, H., Cox, S.E., 2000. Mechanism and rate of denitrification in an agricultural watershed: electron and mass balance along groundwater flow path. Water Resour. Res. 36 (6), 1545–1559; Hinsby, K., Hojberg, A.L., Engesgaard, P., Jensen, K.H., Larsen, F., Plummer, L.N., Busenberg, E., Accepted for publication. Transport and degradation of chlorofluorocarbons (CFCs) in a pyritic aquifer, Rabis Creek, Denmark. Water Resour. Res.], further demonstrating that the use of CFCs for age-dating anaerobic groundwater should be approached with caution, particularly if the sediment contains pyrite.</span></p>
application/pdf
10.1016/j.jhydrol.2007.08.009
en
Elsevier
Evidence of CFC degradation in groundwater under pyrite-oxidizing conditions
article