Radon isotopes as hydrogeological tracers in a decommissioned uranium mining area (Brazil)

This study explored the combined use of radon isotopes (²²²Rn and ²²⁰Rn) as natural hydrogeological tracers in a decommissioned uranium-mining area in Brazil. The main objectives were to evaluate whether radon isotopes could identify groundwater inflows potentially associated with acid mine drainage (AMD) along a stream adjacent to a waste-rock pile, and to assess the feasibility of estimating groundwater flow in the area, a key parameter in hydrogeological studies that is still often difficult to obtain. Two field campaigns were conducted in different seasons under contrasting hydrological conditions. Groundwater samples exhibited ²²²Rn activity concentrations on the order of 10⁵-10⁶ Bq m^-3, markedly higher than those measured in surface water. This contrast, approximately three orders of magnitude on average, confirms the sensitivity of ²²²Rn as a tracer of groundwater inputs. Localized ²²²Rn enrichments in the stream indicated two potential groundwater inflow zones, while in situ ²²⁰Rn measurements provided qualitative to semiquantitative support for localized near-source inputs. Groundwater flow, expressed as Darcy velocity, was estimated exclusively from ²²²Rn using the single-well radon-deficit method. The dual-isotope approach effectively differentiated diffuse versus point-source contributions and highlighted zones potentially associated with AMD generation. While groundwater inflows were clearly detectable during the dry season, data interpretation during the rainy season was constrained by dilution effects. The combined use of ²²²Rn and ²²⁰Rn improved the hydrogeological conceptual model and proved to be a powerful tool for identifying groundwater-surface water interactions in the mining area. The findings also reinforce the need for monitoring campaigns that span seasonal periods at a given study site.