Chlorinated solvents are present in groundwater at an overwhelming number of Department of Defense (DoD), Department of Energy (DOE), and related contractor sites. A significant number of these sites have volatile organic compounds (VOCs) present as free-phase dense nonaqueous phase liquids (DNAPL) that may act as a long-term source of VOCs to groundwater. Due to the slow dissolution of solvents from residual or pooled DNAPL source areas, conventional treatments such as pump-and-treat serve solely as containment technologies and require long operational periods (i.e., decades or longer) to satisfy the need for protection of human health and the environment, incurring high operation and maintenance (O&M) costs over that period.
Significant attention has been devoted in the past few years to research and field applications of source treatment technologies, as they have the potential to lower the overall cost and time required for remediation of contaminated aquifers. Recently, a small-scale field pilot test of emulsified zero-valent iron (EZVI) was conducted under the National Aeronautics and Space Administration (NASA) Small Business Technology Transfer (STTR) program to assess the ability of this technology to treat a trichloroethene (TCE) DNAPL source zone. The pilot test showed promising results as a method for significantly reducing both mass and flux from DNAPL source zones. However, additional field demonstration research was required to improve the EZVI delivery approach, clarify the relative degradation contributions of the zero-valent iron (ZVI) versus biodegradation promoted by the emulsifying agents, and validate the technology for widespread use for DNAPL source zone treatment at DoD and related private sector sites. NASA holds the patent for this technology, and as a U.S. Government technology, no fees for the use of EZVI will be levied on any federal facility.
The objectives of the field demonstration were to evaluate the ability of the two most promising injection technologies to evenly distribute the EZVI in a controlled manner and to evaluate the ability of EZVI to significantly reduce the mass flux of dissolved-phase VOCs from a DNAPL source zone and to reduce the DNAPL mass in the source. Additional objectives were to provide reliable technical data relevant to field-scale EZVI trials, including documenting the benefits of the technology in terms of expected reduction in the duration and cost of remediation of DNAPL sites, to develop a Guidance Manual to assist DoD managers and practitioners with appropriate selection and implementation of the EZVI technology, and to provide information to the Marine Corps Recruit Depot (MCRD) Partnering Team for use in the Feasibility Study for Site 45.
Laboratory and field research has demonstrated that zero-valent metals will reductively dehalogenate dissolved chlorinated solvents, such as tetrachloroethene (PCE) and TCE, to ethene. EZVI can be used to enhance the destruction of chlorinated DNAPL in source zones by creating intimate contact between the DNAPL and the ZVI particles.
The EZVI is composed of food-grade surfactant, biodegradable oil, water, and ZVI particles, which form emulsion particles. Each emulsion particle or droplet contains ZVI particles in water surrounded by an oil-liquid membrane. Since the exterior oil membrane of the emulsion droplet has hydrophobic properties similar to that of DNAPL, the droplets are miscible with DNAPL. It is believed that as the oil emulsion droplets combine with DNAPL TCE, for example, the TCE is sequestered in the oil and then dissolves into the aqueous droplet containing ZVI that was within the oil emulsion droplet. It is also believed that the final degradation by-products from the dechlorination reaction are driven by the increase in concentration inside the aqueous emulsion droplet to diffusion into the nonaqueous phase (oil and TCE), then out into the surrounding aqueous phase. While the ZVI in the aqueous emulsion droplet remains reactive, the chlorinated compounds are continually degraded within the aqueous emulsion droplets, thus maintaining a concentration gradient across the oil membrane and establishing a driving force for additional TCE migration into the aqueous emulsion droplet where additional degradation can occur.
Through this demonstration, significant reductions in the estimated mass of PCE DNAPL (~93% reduction) and estimated total mass of target VOCs (~86% reduction) in the Pneumatic Injection test plot following EZVI injection were observed. Significant reductions in the mass flux of the parent compounds PCE (~85% reduction) and TCE (~86% reduction) and of the degradation product cis-1,2-dichloroethene (cDCE) (~71% reduction) along with significant increases in the mass flux of the degradation products vinyl chloride (VC) and ethene in the Pneumatic Injection test plot following EZVI injection also were observed. Degradation of PCE and its daughter products within the Pneumatic Injection test plot was further supported by the compound-specific carbon-13 and chlorine-37 isotope results obtained by the U.S. Environmental Protection Agency. In addition, DNAPL was pumped from some wells where it was previously absent, indicating that some of the DNAPL was mobile. An increase in concentrations of daughter products (VC and ethene), however, indicated that mass was not only displaced, but also degraded.
In the cost assessment, more than 62% cost savings compared to pump-and-treat was observed, and the cost savings for the EZVI injection alternative can be increased if microscale ZVI is used in place of nanoscale ZVI. The cost for in situ chemical oxidation falls between the EZVI injection alternatives, where microscale ZVI and nanoscale ZVI are used.
At full scale, an underground injection control permit will be required in most jurisdictions for the injection of EZVI and the extraction and re-injection of contaminated groundwater, if co-injection of groundwater with the EZVI is being conducted. There is also a potential that the use of nanoscale ZVI (rather than microscale ZVI) will be a concern to the public and to regulators. Daylighting of EZVI may occur if a vertical pathway connects the injection interval with the surface. Vertical pathways should be plugged with bentonite prior to EZVI injections to prevent this.