ASSESSING THE APPLICABILITY OF GLOBAL CFCAND SF6 INPUT FUNCTIONS TO GROUNDWATERDATING IN BRITAIN

摘要

Chlorofl uorocarbons (CFCs) and sulphur hexafl uoride (SF6 ) are increasinglybeing used to date groundwaters. While these trace gases are generally well-mixed inthe atmosphere, local atmospheric excesses (LAEs) have been found in some parts ofthe world, thereby affecting the interpretation of data from groundwater studies. Toinvestigate the possibility of LAEs in Britain, mixing ratios of CFC-11, CFC-12 and SF6were measured in soil gases from two large conurbations (London and Birmingham)and a smaller urban area, Bristol. Most mixing ratios for CFC-12 and SF 6 were <10%above the current northern hemisphere atmospheric mixing ratio (NH-AMR) values.CFC-11 was more variable, but usually <20% above the NH-AMR value. Given theerrors associated with trace-gas groundwater dating, there is little justifi cation forfactoring in the minor LAEs found in this study to urban groundwater investigations,and accordingly the use of CFC and SF6 input functions based on the NH-AMR curvesappears justifi ed for Britain.