Modeling diagenesis of lead in sediments of a Canadian Shield lake

Triplicate porewater lead concentration profiles were determined on six occasions in a Canadian Shield lake. Total Pb concentrations were also measured in a dated core obtained at the same site. This information, as well as an extensive dataset comprising ancillary geochemical measurements on porewaters and sediment and the population densities of benthic animals, is used in a one-dimensional transport-reaction diagenetic model to investigate the transport and mobilization of Pb in these sediments. Application of the model consistently indicates the presence of a zone of Pb production to the porewaters that lies above a zone of Pb consumption. The profiles of various porewater constituents and thermodynamic calculations indicate that Pb is mobilized in the zone of production by the reductive dissolution of iron oxyhydroxides, whereas it is removed in the zone of consumption by precipitation as a solid sulfide. Rate constants are estimated for reductive iron dissolution (k(d)(Fe(III)) = 2.0 +/- 0.5 x 10(-1) cm(3) Mol(-1) s(-1)), Pb adsorption on iron oxyhydroxides (k(ads)(Pb) = 98 +/- 55 cm(3) Mol(-1) s(-1)), and Pb precipitation (k(ppt)(Pb) = 8 x 10(-20) Mol cm(-3) s(-1) to 16 +/- 13 x 10(-22) mol cm(-3) s(-1), depending on the solubility product assumed for the precipitation of PbS). According to model calculations, diagenetic processes, such as remobilization, molecular diffusion, bioturbation, and bioirrigation have a negligible influence on the solid phase Pb profile. In agreement with this finding, the present-day fluxes of dissolved Pb by diffusion (J(D)(Pb) = -6.5 x 10(-11) mol cm(-2) yr(-1)), bioturbation (J(B)(Pb) = -1.1 x 10(-13) Mol cm(-2) yr(-1)), and bioirrigation (J(I)(Pb) 1.5 x 10(-1)1 Mol cm(-2) yr(-1)) are small compared to the flux of Pb deposited with settling particles (J(S)(Pb) 5.3 x 10(-9) Mol cm(-2) yr(-1)). Copyright (C) 2004 Elsevier Ltd