Investigations of the partitioning and residence times of Po-210 and Pb-210 in a riverine system in Southeast Michigan, USA

摘要

Some of the daughter products in the ~(222)Rn-decay series, such as ~(210)Po and ~(210)Pb, have been widely used as tracers and chronometers in aqueous systems. We measured the concentrations of ~(210)Pb and ~(210)Po in the dissolved (≤0.5 μm), bulk (unfiltered) and paniculate phases (≥1 μm) collected in the Clinton River in the Lake St. Clair watershed in Southeast Michigan in order to investigate their partitioning between paniculate and dissolved phases. Activity measurements of the dissolved and particulate phases revealed that an average of 38% (range: 12-59%) and 33% (range: 12-66%) of the total ~(210)Pb and ~(210)Po, respectively, in the water column was found in the paniculate phase. The activity of dissolved and total ~(210)Pb was higher than that of ~(210)Po because of the higher atmospheric depositional fluxes of ~(210)Pb compared to ~(210)Po. Although the calculated K_d values of ~(210)Pb and ~(210)Po were similar, there was an inverse relationship between the K_d and suspended paniculate matter concentration, indicating the presence of a panicle concentration effect and we attribute this observation to the presence of significant amounts of colloidal ~(210)Po and ~(210)Pb in the dissolved phase. The fractionation factors for Po and Pb were found to be less than 1 in most cases. The first-order box model calculation-based residence times with respect to scavenging varied from 2 to 25 days for ~(210)Pb and 19-78 days for ~(210)Po, indicating higher particle-reactivity of ~(210)Pb compared to ~(210)Po.