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

Some of the daughter products in the Rn-222-decay series, such as Po-210 and Pb-210, 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 mu m), bulk (unfiltered) and particulate phases (>= 1 mu m) collected in the Clinton River in the Lake St. Clair watershed in Southeast Michigan in order to investigate their partitioning between particulate 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 Pb-210 and Po-210, respectively, in the water column was found in the particulate phase. The activity of dissolved and total Pb-210 was higher than that of Po-210 because of the higher atmospheric depositional fluxes of Pb-210 compared to Po-210. Although the calculated K-d values of Pb-210 and Po-210 were similar, there was an inverse relationship between the K-d and suspended particulate matter concentration, indicating the presence of a particle concentration effect and we attribute this observation to the presence of significant amounts of colloidal Po-210 and Pb-210 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 Pb-210 and 19-78 days for Po-210, indicating higher particle-reactivity of Pb-210 compared to Po-210. (C) 2014 Elsevier Ltd. All rights reserved.