Knowledge of groundwater residence time is important in understanding key issues in the evolution of water quality, whether this occurs due to water-rock interaction or simply by mixing or contamination. The build-up in the atmosphere of the trace gases chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF 6 ) from the middle of the last century offers a convenient way of dating waters up to ~60 yrs old. The gases are well-mixed in the atmosphere so their input functions are not area-specific as is the case with tritium. While any one of these trace gases can in principle provide a groundwater age, when two or more are measured on water samples the potential exists to distinguish between different modes of flow including piston flow, exponential flow and simple end-member mixing. As with all groundwater dating methods, caveats apply. Factors such as recharge temperature and elevation must be reasonably well-constrained. Primarily for SF 6 , the phenomenon of 'excess air' also requires consideration. Primarily for the CFCs, local sources of contamination need to be considered, as do redox conditions. For both SF 6 and the CFCs, the nature and thickness of the unsaturated zone need to be factored into residence time calculations. However, as an inexpensive dating method, the trace gases can be applied to a wide range of groundwater problems where traditional age indicators might once have been used more sparingly. Examples include tracing flowlines, detecting small modern inputs in 'old' waters, and pollution risk assessment. In the future, with the main CFCs are already declining in the atmosphere, new anthropogenic trace gases are likely to take their place.

The fluorescence properties of groundwaters from sites in two UK aquifers, the Penrith

A major water quality issue in urban areas underlain by a productive aquifer is the impact of modern recharge. Using a variety of sample sources including multi-level boreholes, this study has found detectable CFCs and SF 6 throughout the upper 50 m of the saturated aquifer beneath a suburb of Doncaster, indicating that modern (<50 year old) recharge has penetrated to at least this depth. Additional support for this deep penetration is provided by the detection of sulphite-reducing clostridia and faecal streptococci. Despite the upper aquifer being a poorly cemented sandstone, the residence time indicators suggest that some modern recharge is travelling via fracture systems in addition to that moving down by simple piston flow. However, the overall impact of 80 years of steady urbanisation on water quality in the aquifer beneath this suburb has in general been limited. This is attributed to a combination of factors including previous land use, dilution by direct recharge of rainfall through green-space areas including gardens, and locally high storage in the friable upper aquifer.

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