APPLICATION OF A MASS-BALANCE MODEL TO ASSESS IN-PLACE ARSENIC POLLUTION

A simple, steady-state mass balance model based on aquivalence as an equilibrium criterion is used to estimate chemical behavior in a lake and, together with the linear additivity principle (LAP), to quantify chemical contributions, especially that of in-place pollution. Since the model is linear, it is argued that chemical contributions from particular sources that vary spatially or temporally can be quantified and their effect isolated from total chemical dynamics. This is demonstrated by considering three loading scenarios for Moira Lake in eastern Ontario that has received elevated arsenic (As) inputs since 1866. Steady-state estimates of precontamination and high contamination conditions are generated by linearly scaling model results obtained with a unit inflow of 0.001 mg/L (unit profile). Results from field studies and the model indicate that although most As is dissolved, a basin-specific fraction of loadings undergo particle-mediated sediment-water exchange. Most loadings are exported, but the lake is a net sink for As. The present postcontamination scenario is comprised of a steady-state component due to present inputs plus an unsteady-state component attributable to in-place pollution. The latter,treated assuming pseudo-steady-state conditions, contributes As to the lake through sediment-water exchange processes, dominated by resuspension of contaminated sediments. Model results suggest that present remediation efforts aimed at reducing As concentrations in Moira Lake are accompanied by substantial As remobilization from contaminated sediments with the lake becoming a net As source, an important consideration for future management decisions.