ROLE OF SEDIMENT POREWATER IN THE CYCLING OF ARSENIC IN A MINE-POLLUTED LAKE

The surficial sediments of Moira Lake averaged 545 mug.g-1 of arsenic, with maximum values close to 1 mg.g-I at depths of 23-27 cm below the sediment water interface. The sediment porewater is the linking agent between the sediment solid phase and the overlying lake water. The de pth distributions of total As in interstitial waters at all stations a re characterized by subsurface maxima, with concentrations four to six times greater than in the lake waters. In all the porewater profiles examined. inorganic As [As(III)+As(V)] comprised most of the As. The A s(III) (arsenite) percentage ranged from 66-83%. Methylated-As compoun ds represented <2% of the total As. The strong correlation of As with Fe and Mn together suggests that both Fe and Mn are involved in the As mobility. The elevated As concentrations in bottom sediments represen t a continuous internal mining of As towards the interface all year ar ound. The concentration gradient of total dissolved As indicates that an upward diffusion of As towards the water column with the estimated annual fluxes being 0.8-3.8 mug.cm-2.y-1.