Spatial and temporal variability of $^7$Be and $^{210}$Pb wet deposition during four successive monsoon storms in a catchment of northern Laos

Abstract : Fallout radionuclides $^7$Be and $^{210}$Pb have been identified as potentially relevant temporal tracers for studying soil particles dynamics (surface vs. subsurface sources contribution; remobilization of in-channel sediment) during erosive events in river catchments. An increasing number of studies compared $^7$Be: $^{210}$Pb activity ratio in rainwater and sediment to estimate percentages of freshly eroded particles. However, the lack of data regarding the spatial and temporal variability of radionuclide wet deposition during individual storms has been identified as one of the main gaps in these estimates. In order to determine these key parameters, rainwater samples were collected at three stations during four storms that occurred at the beginning of the monsoon (June 2013) in the Houay Xon mountainous catchment in northern Laos. Rainwater $^7$Be and $^{210}$Pb activities measured using very low background hyperpure Germanium detectors ranged from 0.05 to 1.72 Bq L$^{−1}$ and 0.02 to 0.26 Bq L$^{−1}$, respectively. Water $\delta$18O were determined on the same samples. Total rainfall amount of the four sampled storms ranged from 4.8 to 26.4 mm (51 mm in total) at the time-fractionated collection point. Corresponding cumulative $^7$Be and $^{210}$Pb wet depositions during the sampling period were 17.6 and 2.9 Bq m$^{-2}$, respectively. The $^7$Be: $^{210}$Pb activity ratio varied (1) in space from 6 to 9 for daily deposition and (2) in time from 3 to 12 for samples successively collected. Intra-event evolution of rainwater $^7$Be and $^{210}$Pb activities as well as $\delta$18O highlighted the progressive depletion of local infra-cloud atmosphere radionuclide stock with time (washout), which remains consistent with a Raleigh-type distillation process for water vapour. Intra-storm ratio increasing with time showed the increasing contribution of rainout scavenging. Implications of such variability for soil particle labelling and erosion studies are briefly discussed and recommendations are formulated for the collection of rainwater signature in studies based on the $^7$Be: $^{210}$Pb ratio method, especially in tropical areas under high erosive pressure.