Effectiveness of very thin soil layers in chemical release from bed sediment

The chemical containment effectiveness of both the natural recovery and the "particle broadcasting" processes for remediating contaminated bed-sedimen ts depend upon very, thin soil and/or sand layers millimeters in depth. Con ventional capping for in situ chemical containment of bed-sediment or dredg ed material typically involves thick layers of 30 to 90 centimeters in dept h. Few, studies have been conducted with thin layers of candidate natural m aterials. A steady-state benzoic acid dissolution test apparatus and proced ure, devised to realistically simulate bed-sediment chemodynamic conditions , was used to measure chemical flux through thin layers (I to 8 nim) of soi l, sand, and ideal porous media. The thin layers were found to be very effe ctive. Flux reductions ranged from 81 to 96%, with fine sand being slightly better than top soil. Design algorithms developed for the thick layers use d in conventional capping design will underpredict the flux through very, t hin layers. Advective flow, induced by, surface roughness is proposed to ex plain the higher average measured-to-predicted flux ratio of 1.67.