| Abstract: | Perchlorate and nitrate in vadose‐zone soil can be continuing sources of groundwater contamination and technologies for remediation of these sources are limited. Gaseous electron donor injection technology (GEDIT) is an in situ approach that was demonstrated at a site in California. A mixture of hydrogen, carbon dioxide, liquefied petroleum gas (LPG), and nitrogen was injected into the vadose zone over a period of 5 months followed by 3 months of LPG alone. Perchlorate and nitrate plus nitrite nitrogen concentrations were reduced by over 90% and both were capable of being reduced to nondetectable concentrations. Hydrogen was required for perchlorate destruction, although concentrations as low as 0.5% were effective. Hydrogen was not required for nitrate destruction. Contaminant destruction was observed in both fine‐grained and coarse‐grained soils ranging from clay to gravel. Hydrogen was an effective electron donor because of its low molecular weight and high diffusivity which likely promoted its penetration into low‐permeability formations. Contaminant destruction was also observed in moisture contents ranging from 6.8% to 36% demonstrating that GEDIT was effective in low‐ and high‐moisture soils. The radius of influence (ROI) for perchlorate destruction was approximately 3.0 to 4.6 m while that for nitrate was greater than 17 m. The ROI was limited by use of a single injection location. Use of a grid injection system is anticipated to greatly increase efficiency. This research represents the first demonstration of GEDIT for treatment of contaminants in the vadose zone. |
