Investigating the Use of Fallout Radionuclides Beryllium-7 and Excess Lead-210 as Short-Term Stream Sediment Chronometers in the Agricultural Upper Midwest, USA

Abstract

Sediment pollution from agricultural fields is a concern for surface water quality in the Upper Midwest even with best management practices in place to prevent field erosion. Legacy stores of previously eroded field sediment may build up in streams over time and lead to persistent water quality degradation. It is thus important to understand sediment settling and resuspension behavior in streams. In the first chapter of my thesis, I use the fallout radionuclides beryllium-7 and excess lead-210 to estimate the relative time since precipitation exposure, or “age”, of suspended sediment at various points in a stream system in eastern Wisconsin during storms. The purpose of this work was to characterize patterns of sediment transport in Plum Creek. From the first chapter it was clear that the sediment aging technique I used is heavily influenced by the variation in radionuclide deposition by precipitation. Depositional fluxes and concentrations of beryllium-7 and excess lead-210 in rain and snow are assumed to be reasonably well-correlated in many sediment aging studies that use these nuclides. However, growing evidence suggests the depositional patterns of these FRNs vary with numerous environmental conditions. I expanded on these findings by using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model to trace the back trajectories for storm series for which the beryllium-7 and lead-210 activities of precipitation were measured. I used this analysis to fill knowledge gaps that affect the sediment aging technique I employed in my first chapter.