Direct Channel Precipitation and Storm Characteristics Influence Short‐Term Fallout Radionuclide Assessment of Sediment Source

• Published in: Water resources research 2018-07, Vol.54 (7), p.4579-4594
• Main Authors: Karwan, D. L., Pizzuto, J. E., Aalto, R., Marquard, J., Harpold, A., Skalak, K., Benthem, A., Levia, D. F., Siegert, C. M., Aufdenkampe, A. K.
• Format: Article
• Language: English
• Subjects: Beryllium; Beryllium 7; Catchment area; Catchments; channel interception; Chemical precipitation; Deposition; Discharge; Erosion; extreme event; Fallout; Flood peak; Hurricanes; Land use; Lead; Lead‐210; Overland flow; Precipitation; Radioisotopes; Radionuclide kinetics; Ratios; Sediment; Sediments; Soil erosion; Storms; Summer; Summer storms; Summer thunderstorms; suspended sediment; Suspended sediments; Throughfall; Thunderstorms; Watersheds; Wet deposition
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2017WR021684

Fallout radionuclides (FRNs) and their ratios, such as Beryllium‐7 (7Be) and excess Lead‐210 (210Pbxs), have been used to determine suspended sediment source and age in catchments. These models are based on numerous assumptions, for example, that channel deposition of FRNs from precipitation is negligible in comparison to their delivery to the channel from land surface erosion during individual storm events. We test this assumption using a mass balance approach during eight storms from summer 2011 to fall 2012 in a mid‐Atlantic United States piedmont region watershed with mixed land use. Event peak discharge and storm type corresponded to the importance of direct channel FRN deposition from precipitation. During relatively low discharge summer thunderstorms, with minimal overland flow, less than 1% of 7Be and 210Pbxs flux deposited on the watershed exits the watershed associated with suspended sediment. The majority but not all deposited on the stream channel exits the watershed associated with suspended sediment (60% of 7Be and 80% of 210Pbxs). Here precipitation and throughfall onto the wetted channel area can be responsible for any FRN newly associated with suspended sediment, as opposed to landscape surface erosion. Furthermore, FRNs can be stored with sediments in the channel between events. Events with higher discharges, including hurricanes, show the opposite pattern—FRN flux associated with suspended sediment exported from the reach is greater than channel FRN wet deposition, suggesting net erosion from the watershed landscape and/or stored material during these types of storms. Key Points Radionuclides used in short‐term sediment age and source evaluation can enter streams by channel interception and landscape erosion Channel interception can provide all of the 7Be and 210Pbxs delivered to the channel during low‐discharge thunderstorms Efforts to age or source sediment using fallout radionuclides must consider stream channel interception under low runoff conditions