Estimating Groundwater Inflows to Small Streams Using 222Rn and Argon

English Abstract:

Radon is a radiogenic isotope derived from the disintegration of ²³⁸U. Dramatic loss of ²²²Rn is produced when groundwater is discharged into rivers, where it rapidly degasses to the atmosphere. By measuring ²²²Rn in the aquifer and in the river, it is possible to simulate groundwater-river exchanges if the radon degassing rate (a function of river width, depth and turbulence). Argon is a noble gas which is present in rivers in equilibrium with the atmospheric concentration. Performing argon tracing in the river could provide an estimate of its degassing rate to be used as a proxy for the ²²²Rn degassing rate. The objective of this work was to quantify exchanges between groundwater and small rivers using the noble gases argon and radon as tracers of groundwater discharge processes. ²²²Rn activities in groundwater and river water was analyzed during the 2013 and 2014 summers in two watersheds from the Centre-du-Québec and Vaudreuil-Soulanges regions of southern Quebec (Canada), i.e. the upper Nicolet River and the Raquette River respectively. Sampling in 2013 was dedicated to collect 130 groundwater samples. In 2014, the sampling focused on measuring ²²²Rn in the two rivers. Groundwater ²²²Rn activities show a relation with the local geology. Groundwater from the metasedimentary rocks of the Appalachians and in Grenvillian age magmatic rocks of Mount Rigaud show ²²²Rn activities between 40 and 400 Bq/L while groundwater from the St. Lawrence Platform shows ²²²Rn activities from 3 to 30 Bq/L. In the river, concentrations in ²²²Rn are between 0.1 and 4 Bq/L. Localized groundwater inflows to the two rivers are shown by peaks of ²²²Rn activities but exchanged flowrates need to be confirmed with a field-measured estimate of degassing rates Tests are currently under way to perform a laboratory-scale continuous argon injection that will help identify the required conditions for field-scale injections to be performed next summer