Converging flow tracer tests in fissured limestone

As part of an investigation into the dual porosity behaviour of fractured limestone aquifers, a sequence of converging flow tracer tests was devised and conducted at an extensively investigated experimental site in a major aquifer in the UK. The tests were designed specifically to produce detailed, high-resolution information about the tails of the breakthrough curves typically observed in this kind of aquifer and test. A set of mutually compatible, low detection limit tracers was identified through laboratory investigations. Two tests were carried out over distances of 20 m and 40 m along each of the two radii towards an abstraction borehole where tracer concentration was monitored. Simple dual porosity models were calibrated using the data from one test on each radius. Blind validations of these models were undertaken by attempting to predict the outcome of the second test on each radius, producing one success and one significant failure.