Experiments in tracing underground waters in limestones

Two experiments were conducted in the Carboniferous Limestone aquifer of the Mendip Hills, Somerset, to compare the relative merits of differing groundwater tracers. The tracers employed were lithium acetate, polyethylene powder, Lycopodium spores and the fluorescent dye Pyranine Conc.; all were introduced into the aquifer at stream sinks. The first two tracers were totally unsuccessful. In the two experiments the rates of travel of the fluorescent dye and Lycopodium spores were very similar. Only relative concentrations can be measured from the recovery of spores as opposed to absoute concentration for dye which were determined using a field continuous flow fluorometer. The quantitative dye concentration data can be used with simultaneous measurements of discharge at the spring to deduce the flow geometry of the system; the input, tributary, distributary and maximum discharges in the system and the volume of the underground conduits. Data are presented which indicate that exchange takes place between the flooded conduit, and the surrounding fissured bedrock in a manner analogous to bank storage in rivers flowing on alluvium.In conclusion fluoresent tracers are valuable for the detailed investigation of a single sink to rising system; Lycopodium spores, which can be used to trace several sinks simultaneously, are a valuable technique in exploratory tracing.