As of 2005, the Department of Defense (DoD) has identified nearly 6,000 sites at its facilities that require groundwater remediation and has invested $20 billion for site cleanup over a 10-year period. At many of these sites, unsaturated chlorinated volatile organic compound (CVOC) source zones located above the water table are producing and sustaining groundwater plumes. Many of these unsaturated sources are currently being treated with soil vapor extraction (SVE) technologies. Long-term SVE projects can be very costly, as the treatment process for the recovered vapors is expensive.
The overall objective of this project was to demonstrate hydrogen-based treatment (H2T) as a remediation technology for the unsaturated zone, either as the initial remediation technology applied at a site or as a polishing technology that will allow DoD site managers to shut down an existing expensive, low performance SVE system, but where monitored natural attenuation may not be sufficient to control the groundwater plume that is sourced by the residual contaminants in the unsaturated zone. With such a technology, the cost for remediating these groundwater plumes can be greatly reduced, and a much more sustainable remedy can be implemented. This demonstration answers key questions about the performance, implementability, and cost of the technology.
In the H2T system, a mixture of nitrogen, hydrogen, propane, and carbon dioxide gases are injected into an unsaturated treatment zone through a series of widely spaced injection points to degrade chlorinated organic compounds. Nitrogen serves as a non-explosive carrier gas to flush oxygen from the soil gas, enhancing conditions for the anaerobic degradation of chlorinated solvents. Propane is used as an inexpensive electron donor for scavenging oxygen (i.e., naturally occurring aerobic bacteria will use the propane to remove oxygen). Hydrogen is used as the electron donor for dechlorinating bacteria to stimulate biodegradation of chlorinated organic compounds, forming innocuous daughter products such as ethane or ethene. Nitrogen and hydrogen can be purchased and delivered to the site (which are refilled or changed out regularly by the gas provider as part of the gas delivery contract) or generated on site depending on the size of the H2T application (i.e., total flowrate and treatment time). The stoichiometry of the dechlorination reaction indicates that for every 1 mg of hydrogen utilized by dechlorinating bacteria, 21 mg of tetrachloroethene (PCE) can be completely converted to ethene.
In the unsaturated zone, the H2T process relies on a gas injection skid consisting of piping, gages, safety equipment, a process control system, and gas supply vessels that could connect to a piping manifold and injection wells at the site. At some sites, one advantageous configuration could be the conversion of a low-performance SVE system to H2T, where the existing SVE blower and treatment system is decommissioned and replaced by the H2T injection skid connected to the existing manifold and injection wells.
Over the 6-month test, a total of 830,000 standard cubic feet of gas was injected into a fine-grained vadose zone at a former missile silo site in Nebraska with the following average composition: 10% hydrogen, 79% nitrogen, 10% propane, and 1% carbon dioxide. The hydrogen gas was designed to stimulate biodegradation of the chlorinated solvent contaminants that persisted in this zone even after 3 years of SVE. Because of inconclusive sampling results during the test, the total gas flow rate and hydrogen composition were doubled for the last month of the injection phase (2.5 scfm to 5.0 scfm and 10% to 20%, respectively). A subsequent increase in hydrogen and propane concentrations and decrease in oxygen concentrations were observed.
Mass in Treatment Zone
The molar mass of chlorinated compounds was unchanged (7.1 moles before vs. 7.1 moles after). Therefore, while the system was successful at converting TCE, a cis-DCE stall condition appeared to be present at the site. Key conclusions from the test:
The demonstrated H2T system was more successful than the existing SVE system at removing TCE from the fine-grained soils at this test site, but it was not successful at removing a significant fraction of the cis-DCE. To help drive a full-scale H2T treatment zone to deeply anaerobic conditions, some type of barriers over the top and around the sides of the treatment zone (even something as simple as adding water to reduce the gas permeability of the soils) might help break out of a cis-DCE stall condition.
Key H2T implementation issues are summarized below.
The unit cost for a full-scale H2T system (assumed to be about 50,000 cubic yards) is projected to be $49 per cubic yard. This would compare to the following costs per cubic yards: $37 for a new-build soil vapor extraction system; $20 to keep an existing SVE system in operation for another two years; and $97 for excavation. Sensitivity analyses were performed to evaluate the effect of gas flowrate and ROI on the unit cost of H2T implementation. It was concluded that while the cost of H2T was greater than SVE system operation, the decision to switch to H2T operation over an SVE system should be made based on the overall performance and not only on the cost assessments.