Sorption of zinc by Lake Michigan sediments: Implications for zinc water quality criteria standards

Abstract

A series of sorption and leaching tests were performed to investigate the uptake and release of zinc from Lake Michigan sediments. In general it was found that these sediments had a moderate affinity for zinc in the neutral to slightly alkaline pH range. If the pH of the sediment slurry was decreased to 6 or less, zinc release was found. Increasing the pH to 7 or greater did not result in the zinc being readsorbed. The sorption of zinc by the sediments was rapid, usually complete in 1–2 h. These results are in accord with the results of the other studies conducted with other waters and sediments. Field observations in Lake Michigan have shown that high concentrations of zinc in tributary waters rapidly decrease within a few kilometers of the tributary mouth to an essentially constant lake-wide concentration. This concentration ranged from 2 to 4 times the typical tributary concentrations. These results may be explained by the uptake of zinc by the suspended sediment in the river and nearshore waters, with settling to the quiescent sediment-water interfaces of the deeper areas of Lake Michigan, often far removed from the tributary input. The rapid, essentially irreversible sorption of zinc by aquatic sediments raises important questions concerning the appropriateness of using the US EPA water quality criterion for zinc as a state water quality standard. The US EPA criterion is based on the total zinc concentration. In many aquatic systems most of the zinc would be adsorbed by suspended sediment, and therefore unavailable to aquatic organisms. It is recommended that the US EPA criteria be used to indicate potential zinc water quality problems where “excessive” concentrations of zinc are investigated on a site-specific basis, using a hazard assessment approach designed to evaluate the actual impact of the zinc in impairing beneficial uses of the water.