Formatted Title

Colloidal Zero-Valent Iron Injection and Low-Temperature Thermal for Enhanced Biotic/Abiotic Degradation of a TCE DNAPL Source

Background/Objectives

Full-scale bioremediation of a trichloroethene (TCE) source zone and plume began in 2005 beneath an active manufacturing building near Portland, Oregon, using food-grade emulsified vegetable oil (EVO). Remediation targets a 4,000-square-foot source zone (TCE concentrations greater than 10,000 micrograms per liter [µg/L]) beneath a former vapor degreaser and a 12-acre downgradient plume. Dense non-aqueous phase liquid (DNAPL) was observed during drilling and subsequent groundwater monitoring; the maximum baseline TCE concentration in groundwater (1,170,000 µg/L) exceeded the TCE solubility limit. Through 2014, five source zone injections of high-concentration EVO emulsion were successful at treating the downgradient plume and reducing TCE mass in the source; however, TCE concentrations remained high in the source. EVO injections in 2014 and 2018 also included ferrous sulfate or colloidal zero-valent iron (cZVI) to stimulate abiotic degradation concurrent and complementary to biodegradation. Beginning in 2022, enhancements of source zone treatment included pH buffering, additional cZVI injection, and low-temperature thermal treatment. As described by the Arrhenius equation, biotic and abiotic degradation rates approximately double for every 10 degrees Celsius (°C) temperature increase.

Approach/Activities

Biotic and abiotic reactions can reduce TCE to non-toxic end products. The biodegradation of TCE is enhanced by injection of fermentable substrates. The microbial process of reductive dechlorination utilizes volatile fatty acids and hydrogen produced by fermentation to transform TCE to break down products cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) and to end products ethene and ethane (E+E). Ferrous sulfate and zero-valent iron stimulate abiotic reduction of TCE and cDCE primarily through β-elimination, producing chloroacetylene and acetylene, which are further reduced to E+E. Detection of acetylene in groundwater samples is a clear indicator of abiotic TCE reduction; however, acetylene is highly reactive and short lived and is rarely detected even where conditions are optimal for β-elimination to occur.

In February 2022, the low-temperature thermal treatment system began operations to heat the source zone to approximately 30–35°C, which is optimal for biological reductive dechlorination. In August 2022, approximately 27,000 gallons of slow- and fast-release buffer and nutrients were injected at 10 source injection wells located on 10- to 20-foot centers. cZVI (3,500 pounds) was also injected at seven source zone wells with the highest TCE concentrations, and where dissolution and desorption continued to be dominant over biodegradation.

Results/Lessons Learned

At the end of 2022, the combination of thermal, pH buffer, and cZVI enhancements resulted in TCE molar sum for source wells at an all-time low and E+E at an all-time high. The TCE concentrations at the seven cZVI-injected wells decreased by 18 to 98 percent. Acetylene detections, definitive proof of enhanced β-elimination, were as high as 300 µg/L in 2018 and 2022 at source wells. Acetylene at the cZVI-injected wells increased by seven to 36 times compared to prior detections. From baseline through 2022, the size of the source zone (TCE concentrations greater than 10,000 µg/L) has decreased by 85 percent. Effective treatment continues downgradient of the source, with E+E detections dominant on a molar basis over chlorinated ethenes at nearly all wells beyond the source.