Trace element remobilization following the resuspension of sediments under controlled redox conditions: City Park Lake, Baton Rouge, LA

Abstract

The sediments of City Park Lake (Baton Rouge, Louisiana, USA) are characterized by Pb concentrations that exceed the corresponding National Oceanic and Atmospheric Administration probable effect level by more than a factor of two and concentrations of As, Cd, Cu, Ni and Zn that exceed the corresponding threshold effect levels. The sediments of the lake are likely to be dredged because of concern over shoaling of the lake. There is concern that exposing these sediments to an oxidizing environment could mobilize toxic metal(loid)s that are effectively sequestered under reducing conditions. To address this concern, mixtures of sediments collected from seven locations in City Park Lake were incubated in microcosms under controlled oxidizing and reducing conditions. The slurries were sampled at time intervals over a period of nearly 750 h, and the dissolved Fe, Mn and trace metal concentrations determined. Eh and pH were negatively correlated, with Eh declining by 600 mV per unit increase in pH. Of the toxic metals, only the concentration of Cu was positively correlated with Eh, apparently because of the sequestration of Cu as a very insoluble sulfide precipitate under low Eh conditions. Precipitation of the other metals as sulfides was evidently insignificant because of the low S content of the sediments. Concentrations of the remaining potentially toxic metals were negatively correlated with Eh, apparently because of their sequestration as oxides and/or adsorption to Fe oxides or oxy-hydroxides. Concentrations of Ca, Mg, and Sr were positively correlated with Eh, presumably because of their sequestration in carbonate precipitates under low Eh/high pH conditions.