| The use of chlorofluorocarbons (CFCs), tritium/helium-3 (3H/ 3He), and/or sulfur hexafluoride (SF6) to determine groundwater age was examined at three field sites in Ontario (Strathroy, Woodstock, and Sturgeon Falls). Also, a new set of potential tracers (HCFCs and HFCs) was evaluated because of the current difficulty in obtaining accurate CFC ages for waters recharged after 1990.; Strathroy, the primary site for this study, has an unconfined fine-sand aquifer with a predominately horizontal flow regime, which was evaluated to a depth of 15 m using a very detailed monitoring network of 67 small-screened piezometers. The aquifer is oxic but becomes NO3 reducing at a depth of ∼6 m, Mn reducing at ∼6.7 m, and then Fe reducing at ∼7.5 m. Based on the geochemistry and isotope data, it appears that pyrite oxidation is the primary source of electrons for the denitrification and Mn/Fe-reduction at depths below 6 m. CFCs, 3H/3He, and SF 6 have been evaluated at Strathroy in relation to the different redox environments.; The depth position of groundwater recharged during the past five years was known with certainty due to a concurrent bromide tracer study (Ryan et al., 1999). 3H/3He ages closely match the bromide derived ages in the shallow zone, and range from 0 at the water table to 27 years at the 12 to 15 m zone. CFC ages get older with depth as expected; however, all CFC ages, which were derived using three different sampling procedures (copper tube, glass ampoule, and glass bottle) and were analyzed at three different laboratories, indicate ages in the NO3 and Mn/Fe reducing zones that were up to 30 years older compared to the 3H/ 3He ages. In this zone the CFCs decline rapidly, suggesting degradation. Apparent first order degradation rates of 0.31, 0.66, and 1.18 yr-1 were derived for CFC-12, CFC-11, and CFC-113, respectively. Complete degradation of all three CFCs was observed in the Fe/Mn reducing zone. Concentrations of CFC-12, CFC-11, CFC-113 and SF6 were elevated in the vadose zone compared to expected North American air concentrations. Corrections of 1.1, 4.8, 4.2, and 46.3% were applied to CFC-12, CFC-11, CFC-113, and SF 6 ages, respectively, based on measured vadose zone excesses. These corrected SF6 data yielded ages that were a good match (within 2 years) to the 3H/3He derived ages to a depth of 7 m, but below this SF6 was apparently affected by degassing during sampling. (Abstract shortened by UMI.)... |
