CFCs and SF6 as groundwater age indicators

The residence time of groundwater in an aquifer can range from just a few years to tens of thousands of years, sometimes more. Dating groundwaters therefore requires a range of chemical and isotopic techniques to cover the time range of relevance.

Chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) are man-made substances introduced to the environment since the early 1930s and late 1970s respectively and are particularly useful for dating groundwaters less than 100 years old.

BGS has been investigating the occurrence of CFCs and SF6 in groundwaters as a tool for age dating and interpreting groundwater flow mechanisms. Analysis is carried out in our GeoEnvironmental Laboratories using gas chromatography with a cryogenic purge-and-trap system.

Man-made environmental tracers

Figure 1: Records of tracer atmospheric mixing ratios can be used via Henry's Law to calculate the variations in time of concentrations in groundwater

Some man-made compounds are widely distributed in the near-surface environment and variations in their quantities means they can be used as tracers to determine pathways and timescales of environmental processes. They also serve to identify anthropogenic contamination.

One of the principal uses of such environmental tracers is to determine the ages of soil waters and groundwaters. Data for tracers provide independent determinations of groundwater recharge rates and flow velocities, often with greater accuracy than with more traditional hydraulic methods if the hydraulic properties of aquifers are poorly characterised or spatially variable.

The value of CFCs and SF6

During the 20th century, CFCs were used widely in refrigeration and foam production, as solvents and as propellants in aerosols. SF6 is used in the electrical industry and for glazing. The use of these compounds as indicators of groundwater age is possible because:

  • concentrations have been building up in the atmosphere at a known rate since their introduction
  • they are well-mixed in the atmosphere
  • like all gases, according to Henry's Law, they have a characteristic solubility in water (at a given temperature)
  • concentrations in air and 'young' water are sufficiently high to measure

Concentrations of these tracers dissolved in rainfall provide a unique signature of the atmosphere at a given time and thereby date the rainfall, or recharge, event. Estimating groundwater ages can be done by comparison with calculated historical groundwater concentrations, assuming equilibrium with observed atmospheric mixing ratios (Figure 1).

Models of groundwater flow

Water abstracted from a borehole or flowing from a spring is typically a mixture of waters from all the flow lines reaching the discharge point. The age frequency distribution and mean age of the mixture affect the interpretation of the groundwater residence time and the fluxes of solutes.

Four hypothetical mixing models can be used to describe some of the variation seen in groundwater mixtures: piston flow (PFM), exponential piston flow (EPM), exponential mixing (EMM) and binary mixing (BMM).

PFM: water reaching the open section of a borehole or discharging from a spring may have had a near uniform flow path and can be approximated with a piston-flow model, analogous to water flowing through a pipe from recharge to discharge without mixing in transit.

EPM: an aquifer receives distributed recharge in an up-gradient unconfined area, then continues beneath a down-gradient confined area.

EMM: discharge from an unconfined aquifer receives uniform areal recharge.

BMM: simple mixing occurs between two waters of separate origin; binary mixing of young water with old (pre-tracer) water is one of the simplest models to consider and is perhaps the most important in many, though not all, fractured-rock environments. Simple dilution occurs because the old fraction is assumed to be free of the tracer and the age of the young fraction can be calculated from the ratio of the two.

Application of models

Plots of one tracer against another can be used to distinguish hypothetical groundwater flow and mixing processes (Figure 2).

Analysis of CFCs and SF6 in groundwater samples has proven a significant aid in the interpretation of, for example, flow and recharge mechanisms in the southern English Chalk aquifer (Figure 3). Compositions of CFC-12 and SF6 reflect a combination of complex mixing and piston flow, depending on location along the flow path and aquifer confinement.

Figure 2. Using a combination of tracers with different input functions (curve shapes) can help to understand the groundwater flow regime
Figure 3. Understanding the groundwater flow regime of the Chalk aquifer in Portsmouth, Hampshire through the combined use of CFC-12 and SF6

Further reading and case studies

Stuart M E, Maurice L, Heaton T H E, Sapiano M, Micallef Sultana M, Gooddy D C and Chilton P J. 2010. Groundwater residence time and movement in the Maltese Islands – a geochemical approach. Applied Geochemistry 25, 5, 609—620.

Darling, W G, Gooddy, D C, Riches, J and Wallis, I. 2010. Using environmental tracers to assess the extent of river-groundwater interaction in a quarried area of the English Chalk. Applied Geochemistry, 25, 923—932.

Morris, B L, Darling, W G, Cronin, A A, Rueedi, J, Whitehead, E J and Gooddy, D C. 2006. Assessing the impact of modern recharge on a sandstone aquifer beneath a suburb of Doncaster, UK. Hydrogeology Journal 14, 979—997.

Morris, B L, Darling, W G, Gooddy, D C, Litvak, R G, Neumann, I, Nemaltseva, E J and Podubnaia, I. 2006 . Assessing the extent of induced leakage to an urban aquifer using environmental tracers: an example from Kyrgyzstan, Central Asia. Hydrogeology Journal 14; 1-2, 225—243.

Gooddy, D C, Darling, W G, Abesser, C and Lapworth, D J. 2006. Using chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) to characterise groundwater movement and residence time in a lowland chalk catchment. Journal of Hydrology 330, 44—52.

Darling, W G, Morris, B, Stuart, M E and Gooddy, D C. 2005. Groundwater age indicators from public supplies tapping the Chalk aquifer of southern England. Water Environment Journal 19, 30—40.

Gooddy, D C and Darling, W G. 2005. The age of the Rain. Planet Earth. Autumn 2005, p32.

Morris, B, Stuart, M E, Darling, W G and Gooddy, D C. 2005. The use of groundwater age indicators in risk assessment to aid water supply operational planning. Water Environment Journal 19, 41—48.

Contact

Please contact Dr Daren Gooddy for further information