Abstract

 Sediments contaminated with organic compounds generate gas bubbles that transport solid and liquid contaminants up to the overlying water. Such a situation has been observed at a Superfund site with PAH-bearing sediments on a tributary of Lake Superior, which borders the US and Canada.
  The objectives of this assessment were to (1) measure related fluxes, (2) understand the transport mechanisms, and (3) determine how to prevent the transport. The authors devised a flux meter to measure the rate of discharge of groundwater, gas and PAH-bearing material from the sediment. The flux meter was first proven at the bench scale, then field deployed. Pressure transducers were installed in the water and sediments at each field flux meter location to document hydraulic interactions. Additional bench-scale flux meters were constructed to evaluate capping effects on the fluxes from consolidation, groundwater advection and bubble transport. Related studies were conducted at WL Delft where a long-term model was developed.
The following results were found. Only warm sediment generated gas. Groundwater advection retards the formation of gases. Gas ebullition is related to changes in pressure. Gas bubbles migrate by forming channels in fine-grained sediment. Rising bubbles erode NAPL and sediment from the channel walls. NAPL creates a shine on the surface as the bubbles burst. Neither NAPL nor contaminants migrate into a sandy cap under the conditions tested. Bubble transport can be controlled or eliminated by: (1) insulating the sediment to temperatures below the bubble-generating threshold, thereby eliminating biogenic gas production, (2) densifying and strengthening the sediment via consolidation, making channel formation more difficult, and (3) interrupting channelization. All three of these controls are provided by proper capping material.