Beryllium-7 wet deposition variation with storm height, synoptic classification, and tree canopy state in the mid-Atlantic USA

Abstract

Short-lived fallout isotopes, such as beryllium-7 (7Be), are increasingly used as erosion and sediment tracers in watersheds. Beryllium-7 is produced in the atmosphere and delivered to Earth's surface primarily in precipitation. However, relatively little has been published about the variation in 7Be wet deposition caused by storm type and vegetation cover. Our analysis of precipitation, throughfall, and sediments in two forested, headwater catchments in the mid-Atlantic USA indicates significant variation in isotope deposition with storm type and storm height. Individual summer convective thunderstorms were associated with 7Be activity concentrations up to 5.0 Bq L−1 in precipitation and 4.7 Bq L−1 in throughfall while single-event wet depositional fluxes reached 168 Bq m−2 in precipitation and 103 Bq m−2 in throughfall. Storms originating from the continental USA were associated with lower 7Be activity concentrations and single-event wet depositional fluxes for precipitation (0.7 – 1.2 Bq L−1 and 15.8 – 65.0 Bq m−2) and throughfall (0.1 – 0.3 Bq L−1 and 13.5 – 98.9 Bq m−2). Tropical systems had relatively low activity concentrations, 0.2 – 0.5 Bq L−1 in precipitation and 0.2 – 1.0 Bq L−1 in throughfall, but relatively high single-event depositional fluxes due to large rainfall volumes, 32.8 – 67.6 Bq m−2 in precipitation and 25.7 – 134 Bq m−2 in throughfall. The largest sources of 7Be depositional variation were attributed to storm characteristics including precipitation amount and maximum storm height. 7Be activity associated with fluvial suspended sediments also exhibited the highest concentration and variability in summer (175 – 1450 Bq kg−1). We conclude the dominant source of variation on event-level 7Be deposition is storm type. Our results illustrate the complex relationships between 7Be deposition in precipitation and throughfall and demonstrate event-scale relationships between the 7Be in precipitation and on suspended sediment.