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

Abstract

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.

Concentration–discharge relationships describe solute and sediment mobilization, reaction, and transport at event and longer timescales

“…Thus, although in‐ and near‐channel sources predominate at lower flows, at event flows above approximately 1 m 3 s −1 , we posit that the weaker positive slope of the C‐Q relationship for TSS is due to the activation and connection of potentially less erosive particulate sources throughout the watershed, which become increasingly hydrologically connected to the channel during higher flows (Rose et al, ). The general findings of this work are corroborated with fallout radionuclide studies showing lower discharge summer storms export suspended solids recently stored within the channel network, whereas extreme events and rain on partially frozen ground associated with higher run‐off export a larger variety of particulate sources (Karwan et al, ).…”

A Process‐Based Framework to Characterize and Classify Runoff Events: The Event Typology of Germany

“…This might help explain previously detected disparate trends in unstratified floods. The same approach could be also applied for disentangling the variability of solutes and particulates exports from catchments during different runoff event conditions (as recently suggested by Karwan et al (2018) and Knapp et al (2020)), thus providing additional insights on variability of water quality metrics observed in streams.…”

Seasonal and Longitudinal Variations in Suspended Load Connectivity Between River Channels and Their Margins

“…Thus, most of the groundwater entering the stream is derived from relatively shallow flow paths that are more oxygenated than deeper groundwater flow pathways. The radionuclide activity ratios of the suspended load in Mink Brook are all consistent with exposure ages of greater than 1 year and thus not consistent with newly precipitated particles seen by Karwan et al (2018).…”