The use and development of some groundwater tracing techniques for wellhead protection : studies from the Corallian Limestone of Yorkshire
The research was driven by questions pertaining to the vulnerability and protection of public water supply wells in fissured and karstic aquifers. The requirement of the industrial and research communities for highly detectable and non-toxic ground water tracers was addressed through the development of Sulphur Hexafluoride (SF6) for this purpose. This dissolved gas tracer exhibits similar physico-chemical properties to the chlorofluorocarbons (CFCs), and these hydrologic tracers were examined together in their respective capacities as applied (i.e. injected) and environmental (already present) groundwater tracers. Public water supply wells in the Corallian limestone of Yorkshire, England, were used to test the application of SF6- A hydrogeological conceptualisation of the system prior to use of the experimental technique was built from the results of geophysical borehole logging, single-borehole dilution tests and forced and non-forced gradient (dye) tracer tests. The novel tracer SF6 was injected into a karst swallet draining the River Derwent and was tracked for 44 days to a dilution 2 x 10 times smaller than the injected concentration, and still 100 times above background. The SF6 results enabled specific elements of the conceptual model to be stated theoretically and compared directly with data for the purposes of discriminating between different processes potentially operative within the aquifer. These comparisons included an analysis of breakthrough curve (BTC) tailing as a function of a geohydrological dipole, hydrodynamic dispersion as a function of tracer transport along multiple pathways (fissures and karstic conduits), and the effects of double-porosity diffusion. Paradoxical observations between rates of tracer transport and apparent groundwater ages obtained by applying simple piston-flow models to measured CFC concentrations indicated additional processes within the aquifer. The Corallian CFC survey was supported by a detailed examination of the largest local source of CFC contamination in the area, an operational landfill site situated above the aquifer.