The development of chemically-reducing conditions in the subsurface is an important strategy for in situ restoration of groundwater impacted by chlorinated solvents. This can be accomplished by emplacement of reducing materials (e.g., zero valent iron, edible oil) or active pumping of reductants (e.g., sulfate, nano-minerals, labile carbon source). For actively pumped restoration activities, there is significant uncertainty regarding how long the enhanced reduction will persist once the site has been returned to more-passive conditions (i.e., active-to-passive transition). If, during active treatment, a reservoir of reactive reduced species is produced (e.g., precipitation of reactive minerals), then those minerals may continue to reduce chemicals well after active treatment has ended. However, if the reactive species are short-lived, then the ongoing effect on groundwater restoration may be limited.

The objective of this project was to examine the formation of meta-stable reactive species under realistic groundwater scenarios, including the addition of precursor materials that have been reported to enhance abiotic degradation (e.g., magnetite, sulfate, ferrous iron, lactate). By conducting a matrix of experiments varying carbon source (lactate), carbonate concentration, magnetite mineral addition, as well as aqueous ferrous iron and sulfate addition, it was hoped that the factors controlling abiotic degradation could be isolated and quantified.

The matrix of experiments was conducted in large sand columns, which were initially allowed to run for several months to establish steady-state conditions. After steady-state conditions were established, a cocktail of chlorinated compounds was continuously injected for a period of approximately two weeks to develop a steady-state concentration profile within the column. At the end of that period, the concentrations of each of the chlorinated compounds was analyzed in water samples from along the length of the column. Column flow was then stopped to simulate the end of active pumping, and samples were periodically collected over a period of three months.

Trichloroethene (TCE) showed a consistent degradation rate for nearly all conditions tested, with the exception of the column where no carbon source (lactate) was added. Carbon tetrachloride (CT) degradation was observed at more-or-less consistent rates in four of six columns. The two columns showing lower degradation rates were the column without lactate and the column without carbonate addition. The latter had a low pH (~3.5), which was eventually increased to 7.2 using PIPES buffer. When the pH was increased in the column without lactate addition, the CT rate increased by an order of magnitude, but was still an order of magnitude less than the four columns with both lactate and carbonate. For both TCE and CT, an important result is that biologically-active conditions were critical for creating an environment in which abiotic degradation could occur.

This project was undertaken to develop a data set for use in a concurrent larger-scale project (ER20-1357). The data set was also used to evaluate a decision tree approach to identify which subsurface conditions resulted in enhanced and sustained degradation of chemicals. (Project Completion – 2023)