Mobility of 137Cs in freshwater lakes: A mass balance and diffusion study of Lake St. Clair, Southeast Michigan, USA

Cesium-137 is one of the most widely utilized anthropogenic radionuclides, both as a tracer and chronometer in the environment. Its application as an effective chronometer requires a thorough understanding of its geochemical behavior in aqueous systems. In this study, we collected and analyzed a suite of time-series water samples over a period of 8 months (April November 2015), for particulate and dissolved Cs-137 activities from a watershed in southeast Michigan, USA, using Cu2Fe (CN)(6)-coated cartridges developed to pre-concentrate dissolved Cs-137. We also conducted a series of laboratory experiments with the natural freshwater sediment samples and water with different chemical composition. There were seasonal variations of both particulate and dissolved Cs-137 activities in the watersheds of Lake St. Clair. The distribution coefficients of Cs-137 determined over a period of 8 months varied between 0.14 x 10(5) and 6.1 x 10(5) mL g(-1) (mean: 2.9 x 10(5) mL g(-1)). The annual input and export flux of total Cs-137 activity via rivers into and out of Lake St. Clair were calculated to be 3.6 x 10(10) Bq and 1.6 x 10(10) Bq, respectively. The amount of 137Cs derived by diffusion from interstitial pore water to the water column was estimated to be 0.30 x 10(10) Bq (8.3% of the total input flux) which is similar to the percentage of Cs-137 desorption (13%-20%) from sediment placed in oxic soft water system over a period of 30-106 days. For the same concentrations of NH4+, Mn2+, K+, Mg2+, Ca2+, Na+ and Sr2+ in a distilled water, our lab work showed that the sediment-sorbed Cs-137 is displaced by ions in the order NH4+ > Mn2+ > K+ > Mg2+ approximate to Ca2+ > Na+ > Sr2+. In laboratory studies, Cs-137 sorbed onto sediment was found to be less mobile in oxic soft water (K-d: 2.0 x 10(3) mL g(-1)) and more mobile in anoxic soft water (K-d: 0.2 x 10(3) mL g(-1)). In a hard water system, however, there is no significant difference in K-d values for both oxic and anoxic conditions. The sequence of K-d values is: oxic soft water > oxic hard water > anoxic hard water > anoxic soft water. The desorption experiments with Cs-137-sorbed sediments also confirmed that Cs-137 is much more mobile in soft water than hard water. This mobility of Cs-137 under oxic hard water system makes sedimentation rate estimation by Cs-137 (based on the time the introduction of Cs-137) problematic in some soft water lakes, and estuarine and coastal waters. (C) 2017 Elsevier Ltd. All rights reserved.