IAMG 2001--Cancún Technical Program--Session A

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CFCs and SF6 in Aquifer Investigations--The Significance of the Unsaturated Zone and the Origin of Groundwater by Jürgen Mahlknecht, University of Guanajuato, Mexico, Jean Friedrich Schneider, University of Agricultural and Soil Sciences BOKU, Austria, and Harald Oster, Spurenstofflabor, Germany, Chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF6) were used as age-dating tools and environmental tracers for groundwater flow in a field study in Central Mexico. These gaseous tracers are used because of their relatively well-known global input function. The interpretation of the CFC data led to variable residence times: in most of the investigated water the existence of young water components could be proved; whereas in only a small percentage, the CFC content is lower than the detection limit. In general, the measured contents of CFC-11 (CFCl3), CFC-12 (CF2Cl2), and CFC-113 (C2Cl3F3) are nearly ideally correlated and allowed unambiguous interpretations. As the unsaturated soil zone is some tens of meters thick, the significance of this part has been investigated. It could be concluded that the delay time due to transport in the unsaturated zone is relatively low, i.e. less than 20 years. A two-component mixture-model was used. It shows that the young water component ( <50 years old) of all sample sites lies between less than 1 % and about 90 %. Assuming an exponential distribution function of the travel time between input and arrival at the well, the mean residence time in each site was calculated, based on the CFC-11 and CFC-12 concentrations. The CFC calculated ages were checked for consistency. Finally, a comparison between CFCs and SF6 results revealed the existence of two different groundwater groups: one as a result from atmospheric input; and the second derivative from (deep) natural leaching processes of SF6 in the aquifer. Paper in PDF format Mahlknecht and others, Acrobat PDF, 928k.