Radon as tracer for the determination of mean residence times of groundwater in decontamination reactors

Soil and groundwater at industrial sites are commonly contaminated by chemicals having been handled, The groundwater of former gasoline production site in Germany is heavily contaminated by aromatic hydrocarbons (mostly benzene) and actually treated in bioreactors anaerobic conditions. The decontamination unit consists of eight cylindrical reactors. In each case four of them are arranged in a subsequent flow regime. One line is filled with sand and the other one with pumice granules. For the determination of reaction kinetics it is crucial to know the mean residence time of the groundwater in these reactors. Most of the commonly tracers used (dyes and salts) did not give reliable results because of the interaction with the mineral matrix in the reactors. In our work radon (222Rn) solved in the groundwater is used as tracer. Because of the anaerobic conditions in the reactor nitrogen is the carrier gas for the radon spiking of the groundwater. The flow rate of groundwater through the reactors is 1 l/h. During 8 h the radon spiked groundwater with a radon activity concentration of about 2 kBq/l was injected into the natural groundwater which has a much lower radon concentration. At the outlet of the reactor the radon concentration of the discharged water is measured online. The increasing radon concentration at the exit of the reactor indicates the shortest residence time of the water. The time dependent radon concentration trend provides detailed information about the flow behaviour and residence times of water in the reactor.