Radon, CO₂ and CH₄ as environmental tracers in groundwater/surface water interaction studies - comparative theoretical evaluation of the gas specific water/air phase transfer kinetics

The applicability of radon as environmental tracer in groundwater/surface water interaction studies has been documented in a considerable number of publications. In some of these reports it has also been suggested to validate the radon based results by using CO₂ and CH₄ as supplementary tracers. The on-site measurement of the 3 gaseous parameters relies on their extraction from the water followed by the measurement of their concentration by means of mobile gas-in-air detectors. Since most related practical applications require the recording of time series, a continuous extraction of the gases from (e.g.) a permanently pumped water stream is necessary. A precondition for the sound combined interpretation of the resulting time series is that the individual temporal responses of the extracted gas-in-air concentrations to instantaneously changing gas-in-water concentrations are either identical or in reproducible relation to each other. The aim of our theoretical study was the comparison of the extraction behavior of the 3 gaseous solutes with focus on the individual temporal responses to changing gas-in-water concentrations considering in particular the gas specific water/air phase transfer kinetics. We show that the overall mass transfer coefficients of radon, CO₂ and CH₄ result in a virtually similar temporal response to aqueous concentration changes, thus confirming the straightforward combined measurement/utilization of the dissolved gases as environmental tracers in groundwater/surface water interaction studies. (authors)