The sorption of trace metals (Zn, Ni, and Cu) by surficial sediments of four acid (pH < 5.6) and three circumneutral (pH 6.4-6.5) lakes were compared via distribution coefficients (K(D)). These were calculated from measurements of porewater metal concentrations in the uppermost layer of lake sediments and total sediment-sorped metal, as determined by the sum of the amounts of metal recovered from the easily reducible (ER) (exchangeable and bound to oxides of Mn), reducible (R) (bound to oxides of Fe), alkaline extracted (ORG) (organically bound) fractions of sediment. Porewater Cd concentrations were undetectable in all lakes. A comparison of the relative amounts of metal recovered from the three sediment components was used to illustrate differences in the partitioning of the trace metals among the seven lakes. Calculated K(D)s for Zn and Ni showed reduced pH dependence with decreasing lake pH (least-squares linear regression, P less-than-or-equal-to 0.01). No such pH dependence was noted for Cu. The comparison of the relative amounts of metal recovered from each of the three sediment components revealed that the partitioning of the trace metals Zn, Cd, and Ni differed among the seven lakes. Less metal was recovered in the ER fraction, and more was recovered in the R and ORG fraction of sediments sampled from the four acid vs. the three circumneutral lakes. In circumneutral lakes, more of the metal was recovered in the ER sediment fraction. Based on the three sampling locations in each study lake, we suggest that differences in the concentration of metals in the ER vs. R and ORG sediment component in surficial sediments of recently acidified vs. circumneutral lakes may be the result of two concurrent processes: (1) a pH-dependent decrease in the sedimentation of ER metal, and (2) a concurrent pH-induced precipitation of Fe-humic complexes involving the coprecipitation of trace metals with strongly bound Fe-humic complexes.
