Abstract
Multiple tracer tests have been undertaken in the Chalk aquifer of Hertfordshire over distances in excess of 3 km. Both fluorescent and bacteriophage tracers were used. Unlike most previous tests, which have involved introducing tracer into features such as swallow holes and observing its discharge at springs, the present study involved inserting tracers at an arbitrary point in the aquifer and detecting them at wells (both pumped and static). Flow speeds of about 2 km/day were indicated, but recoveries were very low, suggesting highly dispersive flow within fissures but with much of the tracer either being adsorbed or travelling too slowly through the aquifer to be detected within the two-month period of each experiment. The field results were interpreted on the basis of a Monte Carlo simulation model of flow and dispersion in fissures, based on classical well hydraulics and both Fickian and non-Fickian dispersion. Travel times and dilution factors showed general agreement with the fissure-flow and matrix-diffusion aspects of the model, but field tracer recoveries were too low for detailed comparison. The tests were undertaken as part of a study to determine whether or not, and, if so, in what time, a substance accidentally entering soakaway drains at the M25/M1 interchange would reach any source of public water supply.
Published Version
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