The seasonal fluctuations and accumulation of iodine-129 in relation to the hydrogeochemistry of the Wolf Creek Research Basin, a discontinuous permafrost watershed

The long lived radioisotope ¹²⁹I is a uranium fission product, and an environmental contaminant of the nuclear age. Consequently, it can trace anthropogenic releases of ¹²⁹I in watersheds, and has been identified as a potential means to distinguish water sources in discharge (Nimz, 1998). The purpose of this work was to identify the sources and mass input of ¹²⁹I and trace the transport, partitioning and mass balance of ¹²⁹I over time in a remote watershed. We monitored ¹²⁹I and other geochemical and isotope tracers (e.g. δ¹⁴C_DIC, δ¹³C_DIC, δ²H, δ¹⁸O, etc.) in precipitation and discharge from the Wolf Creek Research Basin (WCRB), a discontinuous permafrost watershed in the Yukon Territory, Canada, and evaluated the use of ¹²⁹I as a water end-member tracer. Radiocarbon and geochemical tracers of weathering show that discharge is comprised of (i) groundwater baseflow that has recharged under open system conditions, (ii) spring freshet meltwater that has derived solutes through closed-system interaction with saturated soils, and (iii) active layer drainage. The abundance of ¹²⁹I and the ¹²⁹I/¹²⁷I ratio correlated with geochemical tracers suggests varying contributions of these three water end-members to discharge. The ¹²⁹I concentration was highest at the onset of freshet, reaching 17.4 × 10⁶ atoms/L, and likely reflects the lack of interaction between meltwater and organic matter at that time. This peak in ¹²⁹I was followed by a decline over the summer to its lowest value. Mass balance calculations of the ¹²⁹I budget show that the input to the watershed via precipitation is nearly one order of magnitude higher than the output suggesting that such arctic watersheds accumulate nearly 90% of the annual input, primarily in soil organic matter. Temporal variations in discharge ¹²⁹I concentrations correlated with changes in discharge water sources suggesting that ¹²⁹I is a promising hydrologic tracer, particularly when used in concert with other stable and radioisotopes.