Clark Island Sediment Remediation - Effects of a granular cap on metal fluxes

Abstract

The Clark Island remediation site is located in Francis Lake, St. Lawrence River. Capping is the selected remediation option considered for a ten ha area along the northern shores of the island where the sediments are contaminated with iron oxides and other metals. In the present project, a three-months experiment was performed on twelve mesoscale sediment-cap-water systems to assess metal fluxes within the sediments and across the sediment-water interface. Methods included in situ extraction of metals on passive samplers (DGT’s) both in the sediment and in the water flowing across the sediment surface. Uncovered sediments were found to release Zn, Se, Mn, Cu, As, Ni, Cd and Pb, whereas no release was observed for Cr and Hg. Passive samplers (DGT-probes) revealed reduction of ferric oxides in carbon enriched sediments and substantial recycling of associated metals within the sediment-cap systems. Some evidence was found on leakage of Ni through the cap of carbon enriched sediments, but in general the cap remained highly efficient throughout the three-month period, reducing all metal fluxes (Ni included) by 89% or more. A two-layered cap was recommended using a finegrained “diffusion barrier” below a coarser top layer. The primary function of the top cap is to prevent erosion of the lower cap and to enhance downwards penetration of oxygen from the river water.