DTSC-approved pilot demonstrates in-place treatment options for TCE at HMSA

The Department of Toxic Substances Control (DTSC) has recently approved a pilot in-place groundwater treatment at the HMSA site, presenters said, and project staff demonstrated how injected amendments could be used to speed breakdown of trichloroethylene (TCE).

The presenter said the demonstration used a not-to-scale model to show how wells and amendments interact underground and to compress processes that generally take months or years into a short demonstration. “DTSC has recently approved a pilot treatment for in situ or in place treatment,” the Presenter said, describing the demonstration site and the model used to visualize flow and plumes.

Why it matters: TCE is a chlorinated solvent that regulators and site managers commonly treat with long-term strategies. At the demonstration, presenters contrasted pump-and-treat — where contaminated groundwater is extracted, tested offsite and treated — with in-place (in situ) treatment that injects reagents to create chemical and biological conditions that break down contaminants near the source.

The Presenter described pump-and-treat as appropriate when concentrations are high, saying, “when you have higher concentrations of TCE, pump and treat is a remedy that makes sense.” The demonstration used colored dye to simulate a large plume and show how extracted water becomes less concentrated over time under pumping.

Ryan, identified in the transcript by first name, described the specific amendments proposed for the in situ approach. “We’re proposing to add here is an emulsified vegetable oil. So, it’s a food grade carbon substrate that stimulates the biological degradation,” Ryan said, adding the team would also use “a form of zero valent iron, which also reduces TCE.” He said combining biological and chemical reductants provides two degradation mechanisms and “greatly reduces the chance of accumulating like vinyl chloride” while promoting conversion toward ethene.

The presenters demonstrated both an injection well and a monitoring well and stressed the pilot nature of the test. The Presenter noted the team is assessing the amendment’s sphere of influence in different test areas and cited example target radii discussed in the demonstration: “one of them’s 15 ft, one of them’s 20 ft.” The team emphasized they do not presuppose this will be the final solution and that full-scale design would be based on pilot monitoring data.

Next steps and monitoring: presenters said amendments disperse from an injection well and the reactions unfold over weeks to months; the pilot will include periodic monitoring to measure daughter-product concentrations and the extent of influence. The Presenter said extracted water from pump-and-treat operations has been taken offsite for testing and treatment in past efforts at the former sodium disposal facility (FSDF), and that different parts of the site and other parties are evaluating similar technologies.

The demonstration materials and comments made clear the pilot is diagnostic: the team will use measured results from the DTSC-approved pilot to design any larger-scale in situ treatment rather than assume a specific amendment or footprint is sufficient.

The pilot demonstration did not provide a timetable for a full-scale remedy; presenters said monitoring and analysis of pilot results will inform subsequent decisions.