Publication

High concentrations of Pb are found in surface soils of even remote ecosystems as a result of atmospheric deposition. This study conducted at the Hubbard Brook Experimental Forest provides key information on Pb speciation in soil solutions measured by ultrafiltration and ion-exchange techniques. About 50% of total filtrate Pb (passing a 0.45-mu m filter) was found to be in the colloidal form (> 3000 MW) beneath O-a horizons. Colloidal Pb concentrations in the B-s horizon were less than 10% of O-a concentrations. Of total truly dissolved Pb (passing a 3000 MW ultrafiltration membrane), less than 10% was organically complexed. Although a positive correlation existed between filtrate Pb and filtrate organic carbon (DOG), this was caused partially by Pb association with organic colloids and partially by a noncausal covariation of DOC and dissolved Pb as they were both retained during transport down the soil profile. A computed distribution of inorganic Pb species showed that under acidic conditions, which prevailed in soil solutions of HBEF (pH range 4.0-4.7), free dissolved ionic Pb2+ dominated and Pb complexes with Cl-, F-, SO42-, OH-, and HCO3- ligands were negligible. High acidity (low pH) resulted in Pb2+ desorption from soil solids. On the other hand, low pH reduced Pb complexation with both organic and inorganic dissolved ligands, thus decreasing the chance for Pb transport. It is hypothesized that the Pb released from the surface organic layer was rescavenged in the mineral soil mainly through two relatively independent processes: (1) adsorption of ionic Pb2+; (2) adsorption of organic colloids together with associated colloidal Pb. Because both colloidal and dissolved forms were effectively removed during transport down the soil profile, mobilized Pb from the surface organic layer was entirely retained in lower horizons before reaching streams.