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 on a tributary of Lake Superior, which borders the United States and Canada.
  The research: (1) measured gas generation rates as a function of sediment temperature in the gas production facility at WL|Delft Hydraulics, (2) measured multi-phase flow of pore water, NAPL and gas with seepage induced consolidation tests to simulate consolidation beneath a subaqueous in-situ cap and a subaqueous dredged material disposal cell cap, (3) calibrated and validated modeling by comparing a hind-cast to measured conditions, and (4) modeled potential future effects of remediation based on the finite strain consolidation program DELCON. This model includes the combined effect of consolidation, gas bubble formation and transport, and multiphase pore fluid flow.
  We found the following. Gas was generated only from sediment containing high TOC. Gas generation rates in the sediment were biogenic and sensitive to temperature. Sediments are cooler at depth. Capping reduces gas generation by insulating the sediment to temperatures below the threshold of biogenic gas production. The NAPL was highly viscous and to move it, the gradient must be high and sustained. Those conditions were not created during capping, so NAPL won't move when capped. Consolidation parameters could be generalized to other soil mixtures based on the clay fraction. Gas bubbles migrate after cracking the fine-grained matrix and forming channels. Consolidation increases the resistance to channel formation by densifying and strengthening the sediment. No such channelization can occur in sandy material such as that used in the modeled caps. Hind-cast gas fluxes correspond well with measurements in the field.