Arsenic sedimentation along the slope of a lake basin

Lake Lansing, Michigan was treated with sodium arsenite for macrophyte control in 1957. Seven 1.5-m sediment cores were taken along a line through the littoral zone to the deepest portion of the lake and analyzed for arsenic. In each core, arsenic concentrations going from the surface downward increased to some maximum and then receded to background. Depth of peak concentrations followed two patterns: three littoral cores showed peak arsenic at 0.13 m from the sediment surface, and four cores from progressively deeper portions of the lake showed a regular decrease in peak depth from 0.32 m to 0.17 m. Sediment accumulation rates were calculated along this transect. Particle-size sorting of sediments along this slope was also measured. This work suggested that sediments originated from wetland vegetation at the edge of the lake. Turbulent movement of water in the shallows caused suspension and down-slope movement of small particles. Fewer particles of wetland origin were available for sedimentation beyond the region of highest fallout (3.75 m), thus accounting for progressively lower sedimentation rates in deeper portions of the basin. A model was developed to predict the time it would take for surface sediments to reach pretreatment concentrations of arsenic levels; for Lake Lansing it would be more than 100 years.