Tracing the Depositional Fluxes of Po-210 and Pb-210 As a Tool for Sediment Resuspension Study in a Shallow Water System in Southeast Michigan

Abstract:

From the measurements of ²¹⁰Po and ²¹⁰Pb in atmospheric dry and bulk depositional flux and in aerosols in southeast Michigan, we observed the following: i) the ²¹⁰Po/²¹⁰Pb activity ratio in bulk precipitation was lower than that in aerosols and this was attributed to the possible presence of volatile ²¹⁰Po in the atmosphere; and ii) it was proposed that only a small fraction of aerosols actively participate in the scavenging of particle-reactive radionuclides based on lack of correlation between the activity of ²¹⁰Pb and aerosol mass concentration. We extended this study to investigate the particle-cycling in a shallow, dynamic freshwater system in southeast Michigan. The ²¹⁰Po/²¹⁰Pb activity ratios in bulk deposition is mostly <0.1 while in benthic sediments, this ratio is usually ~1.0. This activity ratio in finer resuspended sedimentary particulate matter is altered from the scavenging of Po and Pb derived from the atmospheric deposition, and thus, the ²¹⁰Po/²¹⁰Pb activity ratio in suspended particulate matter can be utilized as a tracer for particle cycling. We measured the concentrations ²¹⁰Po and ²¹⁰Pb in a suite of surficial benthic sediments and particulate matter collected in sediment traps from five different locations in the Clinton River that discharges in to Lake St. Clair in southeast Michigan to quantify the sediment resuspension rates and to determine the particle residence time. The mean ²¹⁰Po/²¹⁰Pb activity ratio of suspended trap and surficial bottom sediments were 0.72 and 0.75, respectively, indicating that the sediment trap particles were mostly derived from resuspended bottom sediments. Particle residence time varied from 0.3 to 4 days for ²¹⁰Pb and 0.9 to 13.4 days for ²¹⁰Po. Sediment resuspension rate calculated via a single box model approach yielded a mean resuspension rate of 0.5 g cm^-2 yr^-1 using ²¹⁰Pb and 0.2 g cm^-2 yr^-1 using ²¹⁰Po. A comparison of the ²¹⁰Po and ²¹⁰Pb fluxes in the sediment trap to that in the direct atmospheric deposition indicates that only about 1% of the ²¹⁰Pb and about 0.1% of the ²¹⁰Po were derived from direct atmospheric deposition while most of the trapped ²¹⁰Pb and ²¹⁰Po were delivered by the resuspension of bottom sediments.