In-Situ Abiotic/Biotic Remediation
At certain sites, the combination of abiotic/biotic treatment can be very effective in a reasonable timeframe. Emulsions containing zero-valent iron (ZVI) and a carbon source such as vegetable oil or lactose, mixed with water, are injected into the subsurface to reduce chlorinated ethenes (e.g., PCE, TCE) in groundwater. The abiotic process occurs as the chlorinated ethenes come into contact with ZVI. The biotic process relies on native bacteria that are present in the subsurface and uses organic carbon injected into the subsurface to stimulate biodegradation. The addition of an enhanced carbon-rich food source is much like fertilizing a crop. These amendments can also be mixed with surfactants and amended with bacteria (see descriptions of Enhanced Bioremediation and Surfactant Enhanced Recovery).
Other compounds are similarly used to degrade energetics from munitions fabrication and use, as well as some metals and pesticides.
Emulsions can be delivered to the subsurface by several injection methods, including hydraulic fracturing, pneumatic fracturing, and direct injection. It also can be directly mixed into the soil using deep-soil mixing equipment or placed into an open excavation.
Methane is produced as a fermentation product of various organic compounds. The presence of methane indicates that highly-reducing conditions have been established in the aquifer. Additionally, methanogenic conditions are considered necessary for the conversion of vinyl chloride to nontoxic ethene.
Limitations and Concerns
As is always the case when injecting a substrate into the subsurface, it works when it (the abiotic compound or the bacteria) come into contact with the contaminant.
The technology is better suited for more permeable subsurface areas. Rebound may occur if significant contaminant mass is present in low-permeability strata (e.g., fine clays) at the site.
Multiple rounds of injection may be needed, depending on the contaminant concentrations, the adsorbed mass of contaminants, and the effectiveness of the delivery method.
The radius of influence of each injection point must be carefully measured before full-scale application.
Mineral by-products such as arsenic and manganese can be mobilized. The potential for the mobilization of these metals requires monitoring.
Depending on the permeability of the subsurface, injection may be highly pressurized, resulting in symptoms similar to fracturing, as well as the surfacing of liquids. Each site must be carefully screened and tested before full implementation.
Abiotic/biotic techniques have been successfully used to remediate soils and groundwater contaminated chlorinated ethenes, such as PCE and TCE. This technique has also been used for energetics such as TNT, RDX, and perchlorate, as well as was some pesticides and heavy metals (e.g. hexavalent chromium).
Technology Development Status
Commercial mixtures of organic and inorganic materials are available for most applications of this technology.
http://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminants-on-Ranges/Protecting-Groundwater-Resources/ER-1376/ER-1376/(language)/eng-US (PDF at end of this fact sheet is very large). Also see http://www.ncbi.nlm.nih.gov/pubmed/12026076
Other Resources and Demonstrations
See http://www.epa.gov/tio/download/remed/engappinsitbio.pdf for engineering approaches to in-situ bioremediation.
See https://portal.navfac.navy.mil/portal/page/portal/navfac/navfac_ww_pp/navfac_nfesc_pp/environmental/erb/documents-z/zvi-pres.pdf for a description of the pilot test of ZVI at Hunters Point Naval Shipyard.
See http://www.clu-in.org/download/contaminantfocus/dnapl/Treatment_Technologies/600r08148.pdf for a monitoring method to assess degradation
See http://serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-1421/ER-1421/(language)/eng-US for results of a study using abiotic/biotic techniques for reducing NMDA. N-nitrosodimethylamine (NDMA) is an emerging contaminant of concern. It can be formed through propellant production, chlorination, and the treatment of wastewater. It is a potent carcinogen whose action level in drinking water is 0.7 parts per trillion. NDMA moves rapidly in groundwater because it does not sorb strongly, and it is slowly degraded in both aerobic and anaerobic environments. See also http://www.pnl.gov/main/publications/external/technical_reports/PNNL-18547.pdf
For an innovative study using pine bark to reduce hexavalent chromium (chromium VI) to chromium III, see http://aebrc.org/korean/files/enhanced_abiotic_reduction_of_cr_vi__in_a_soil_slurry_system_by_.pdf