Testing tritium-helium groundwater dating in the Chalk aquifer of the Berkshire Downs, UK

The tritium-helium (³H/³He) dating method has been applied to the Chalk (fractured microporous limestone) aquifer in the UK for the first time. An evaluation of the results from diffusion cell versus pumped tube sampling showed generally good agreement between the two techniques. Measurements of noble gas (Ne, Ar, Kr and Xe) concentrations revealed typically low amounts of excess air in the aquifer, with little variation around a mean of 1.3 ccSTP/kg suggesting the predominance of steady recharge via the microporosity rather than via the fracture network. Chalk boreholes are generally unlined, with discrete inflows from a few fracture-related flow horizons. Despite this, attempts to detect age layering in the water column by suspension of diffusion samplers or by slow-pumping were unsuccessful. However, when short-screen piezometers were used, better evidence for an age-depth relationship was obtained. Assuming a piston-flow model of water movement, a vertical flow rate of ∼3.3 m/yr was indicated. However, a more complex picture of movement was obtained by comparing total ³H activity (including the ³He decay equivalent) against SF₆ concentration, which suggested the existence of various modes of mixing. This would be consistent with the high degree of fracturing that exists in the Chalk. (author)