Transport and Time Lag of Chlorofluorocarbon Gases in the Unsaturated Zone, Rabis Creek, Denmark

et al., 1995, 1996; Szabo et al., 1996), flow and groundwater quality (Johnston et al., 1998; Böhlke and Denver, Transport of chlorofluorocarbon (CFC) gases through the unsatu1995), and groundwater–surface water interactions (Katz rated zone to the water table is affected by gas diffusion, air–water et al., 1995). A critical component in such analyses is the exchange (solubility), sorption to the soil matrix, advective–dispersive transport in the water phase, and, in some cases, anaerobic degradaconsideration of the transport and fate of the CFC tracers tion. In deep unsaturated zones, this may lead to a time lag between in the overlying unsaturated zone. For example, what entry of gases at the land surface and recharge to groundwater. Data are the CFC concentrations in recharge water and how from a Danish field site were used to investigate how time lag is aflong were the CFC gases in the unsaturated zone before fected by variations in water content and to explore the use of simple reaching the groundwater system? The residence time analytical solutions to calculate time lag. Numerical simulations demof a CFC tracer in the unsaturated zone is also called onstrate that either degradation or sorption of CFC-11 takes place, the time lag (Cook and Solomon, 1995). An accurate whereas CFC-12 and CFC-113 are nonreactive. Water flow did not estimate of the age of a groundwater sample also relies appreciably affect transport. An analytical solution for the period with on an accurate estimate of the time lag. a linear increase in atmospheric CFC concentrations (approximately For very shallow unsaturated zones of only a few early 1970s to early 1990s) was used to calculate CFC profiles and time lags. We compared the analytical results with numerical simulations. meters thickness, diffusion and barometric pumping sufThe time lags in the 15-m-deep unsaturated zone increase from 4.2 to ficiently mix the gases in the unsaturated zone so soilbetween 5.2 and 6.1 yr and from 3.4 to 3.9 yr for CFC-11 and CFC-12, gas CFC concentrations are similar to those in the trorespectively, when simulations change from use of an exponential to posphere. The input to the groundwater system is thus a linear increase in atmospheric concentrations. The CFC concentravery close to the atmospheric changes in CFC concentions at the water table before the early 1990s can be estimated by trations. This simple approach often has been used for displacing the atmospheric input function by these fixed time lags. A dating groundwater, where the concentrations measured sensitivity study demonstrates conditions under which a time lag in in groundwater are used directly together with the atmothe unsaturated zone becomes important. The most critical parameter spheric concentrations to estimate the time of recharge, is the tortuosity coefficient. The analytical approach is valid for the thus neglecting any time lag of the CFCs in the unsatulow range of tortuosity coefficients ( 0.1–0.4) and unsaturated zones greater than approximately 20 m in thickness. In these cases rated zone. the CFC distribution may still be from either the exponential or linear In deeper unsaturated zones the time lag can be imphase. In other cases, the use of numerical models, as described in portant, and various processes affecting CFC transport our work and elsewhere, is an option. become important. Gas diffusion and air–water exchange (solubility) are especially important in controlling the migration and attenuation rate. Both processes are a C hlorofluorocarbons are volatile organic comfunction of the water content and, thus, seasonal and pounds used, for example, as aerosol propellants year-to-year changes in infiltration and depth to the and refrigerants since the 1930s (Plummer and Busenwater table. Cook and Solomon (1995) investigated nuberg, 1999) and now found in the subsurface because of merically the conditions under which the time lag of the release to the atmosphere. There are numerous exCFCs in the unsaturated zone is of importance by examamples of the use of CFCs as tracers in groundwater ining the relative effects of various soil parameters on studies, including studies of dating recharge water (Ekthe distribution of gases in the unsaturated zone. Buwurzel et al., 1994; Plummer et al., 2000), dating young senberg and Plummer (2000) used the same model as groundwater ( 50 yr) (Busenberg and Plummer, 1992; Cook and Solomon (1995) to study lag times of SF6 in Oster et al., 1996; Plummer et al., 2001), groundwater unsaturated zones and found that the lag times of SF6 flow and transport processes (Reilly et al., 1994; Cook are smaller than for CFCs because of its low solubility. Oster et al. (1996) measured 57 profiles of both CFC-11 and CFC-12 in a 4.5-m-thick unsaturated zone in a forest P. Engesgaard and K.H. Jensen, Geological Institute, Univ. of Copensoil in Germany. A clear damping of the annual changes hagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark; A.L. in CFC atmospheric concentrations were found with a Højberg, K. Hinsby, and T. Laier, Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen K, Denmark; relaxation time (time lag) of 30 d for 4 m. Weeks et al. F. Larsen, Environment and Resources, Technical Univ. of Denmark, (1982) used analytical and numerical transport models Building 204, 2800 Lyngby, Denmark; E. Busenberg and L.N. Plumto simulate the observed distribution of CFC-11 and mer, USGS, 12201 Sunrise Valley Drive, Reston, VA 20192, USA. CFC-12 in a 50-m-deep unsaturated zone. The analytiReceived 4 July 2003. Original Research Paper. *Corresponding author ([email protected]). cal model was a pure diffusion model, whereas the nuPublished in Vadose Zone Journal 3:1249–1261 (2004). © Soil Science Society of America Abbreviations: CFC, chlorofluorocarbon; GC, gas chromatography; GEUS, Geological Survey of Denmark and Greenland. 677 S. Segoe Rd., Madison, WI 53711 USA