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 ¹³⁷Cs activities from a watershed in southeast Michigan, USA, using Cu₂Fe(CN)₆-coated cartridges developed to pre-concentrate dissolved ¹³⁷Cs. 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 ¹³⁷Cs activities in the watersheds of Lake St. Clair. The distribution coefficients of ¹³⁷Cs determined over a period of 8 months varied between 0.14 × 10⁵ and 6.1 × 10⁵ mL g⁻¹ (mean: 2.9 × 10⁵ mL g⁻¹). The annual input and export flux of total ¹³⁷Cs activity via rivers into and out of Lake St. Clair were calculated to be 3.6 × 10¹⁰ Bq and 1.6 × 10¹⁰ Bq, respectively. The amount of ¹³⁷Cs derived by diffusion from interstitial pore water to the water column was estimated to be 0.30 × 10¹⁰ Bq (8.3% of the total input flux) which is similar to the percentage of ¹³⁷Cs 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 ¹³⁷Cs is displaced by ions in the order NH₄⁺ > Mn²⁺ > K⁺ > Mg²⁺ ≈ Ca²⁺ > Na⁺ > Sr²⁺. In laboratory studies, ¹³⁷Cs sorbed onto sediment was found to be less mobile in oxic soft water (K_d: 2.0 × 10³ mL g⁻¹) and more mobile in anoxic soft water (K_d: 0.2 × 10³ 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 ¹³⁷Cs-sorbed sediments also confirmed that ¹³⁷Cs is much more mobile in soft water than hard water. This mobility of ¹³⁷Cs under oxic hard water system makes sedimentation rate estimation by ¹³⁷Cs (based on the time the introduction of ¹³⁷Cs) problematic in some soft water lakes, and estuarine and coastal waters.