Hydrogen sparging of aquifers contaminated with chlorinated solvents has shown promise as a method to enhance microbial dechlorination in situ. A concern, however, is the ability to distribute hydrogen effectively throughout the contaminated interval such that complete dechlorination can occur. As is true for aerobic biosparging application, the horizontal and vertical extent of residual gas saturation formation during hydrogen sparging is limited to only a small, conical region around and above the screened interval because of the low density and viscosity of hydrogen compared to water.
The objective of this project was to assess the potential for hydrogen-generated foams to more effectively contact contaminated aquifers with electron donor and support rapid reductive dechlorination processes as compared to conventional hydrogen sparging. Studies were conducted to investigate foam application in or near DNAPL source zones where contaminated aquifer volumes to be contacted were not prohibitory.
This technology incorporates air or hydrogen to generate foam from a co-injected surfactant solution that is intended to effectively displace and degrade residual DNAPL. One role of the foam is to promote lateral gas distribution in order to contact the heavier-than-water DNAPL at the aquifer base and form a more effective downgradient barrier to contaminant migration. Dilute surfactant solution and hydrogen can continue to be injected intermittently to degrade any remaining chlorinated hydrocarbons.
Various surfactants were investigated to determine both their ability to generate foam and their possible inhibiting effect on dechlorination. Hydrogen is not useful in the immediate DNAPL vicinity, and the overall desired level of dechlorination did not occur.
Batch studies performed with mixed dechlorinating cultures to determine which dechlorinating organisms were being affected by surfactants yielded organism-specific, mixed results. Combined column and modeling efforts indicated that gas could be distributed more uniformly when foam was generated than when gas was injected without surfactant.
Foam-enhanced biosparging might prove useful as a low-level chlorinated contaminant barrier; however, use of hydrogen foam in a source zone where there is considerable liquid contaminant would be of limited value because biodegradation is inhibited both by dissolved contaminant concentrations near saturation and by surfactant concentrations well below those expected to be injected.