Plutonium Isotopes: An Effective Tool for Fluvial Sediment Sourcing in Urbanized Catchments

Abstract

Much of the Earth's land surface is experiencing anthropogenically driven land use change and erosion at unprecedented rates (Owens, 2020;Russell et al., 2017;Shi et al., 2021). The rapid conversion of natural and rural landscapes into urban systems has diminished agricultural productivity, increased sediment pollution, and has counteracted urban sustainability (Chang et al., 2020;. The global urban population is expected to double by 2050 (Bettencourt et al., 2018), which will be met with increasing urban development as well as agricultural activity to satisfy population demands.Managing erosion and tracing sediment back to its origin is complicated by uncertainties in the connectivity of pathways, flow-dependence of source-zone activation, and sensitivity of tracers to successfully identify sources (Husic et al., 2020;Michalek et al., 2021;Owens, 2020). The most widely employed radioisotope, Cesium-137 ( 137 Cs, t 1/2 = 30.2 years), was deposited on surficial soil in large quantities during nuclear weapons testing in the 1960s (Ritchie & McHenry, 1990) . However, in the intervening decades, 137 Cs has reduced to just ∼25% of its original levels, rendering its application more challenging as time progresses limiting its utility as a tracer with conventional techniques (Alewell et al., 2017;Collins et al., 2020) although recent analytical advancements are allowing sensitive detection of 137 Cs (Povinec, 2018). A recent study of radionuclides in sediment tracing calls on the community to identify new tracers for soil redistribution as depleted tracers lose some viability (Alewell et al., 2017). Plutonium ( 239+240 Pu) has garnered significant attention as an alternative tracer of erosion as it was globally deposited in the same manner as 137 Cs, but consists of two major long-lived isotopes ( 239 Pu, t 1/2 = 24,110 years;