ARSENIC TRANSPORT BETWEEN WATER AND SEDIMENTS

Arsenic discharged into the Moira River has accumulated in the sediments of Moira Lake during the past century. The chronology of arsenic concentrations in the sediments, established using Pb-210 dating, has a subsurface concentration maximum (> 1000-mu-g g-1) that reflects higher inputs to the lake 15 to 45 years ago. The distribution coefficient (K(d)) of arsenic in the surficial sediments was low (4000-6000 L kg-1) and decreased below the sediment water interface. Higher concentrations of exchangeable As also were extracted deeper in the sediments. As a result, arsenic is mobile in the sediment column and the flux of arsenic via diffusion and particle resuspension from the sediments into the water is greater than current external loading from the Moira River. Less than 20% of the external input of arsenic is buried in the lake sediments. Using these flux measurements and a one dimensional model of arsenic transport in the sediment column, we constructed the history of arsenic exchange between water and sediments throughout the past century. The simulations predict that arsenic input into the water from the sediments has been > 20% of external loading for the past 25 years and will continue to be important in the future as diffusion and resuspension regenerate arsenic from the mixed layer of the sediments into the overlying water.