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

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