At a glance
Location: Washington
Geology: Fractured bedrock
Contaminant(s): Hexavalent chromium (Cr+6)
ETEC Solution(s): CarBstrate, ISD system
Unique challenge(s): Difficult Contaminate, Water body
Site Summary
A former agricultural facility in Washington faced a significant challenge. A leaking underground storage tank had released hexavalent chromium into fractured bedrock and overburden, creating conditions that were both chemically harsh and geologically complex. Groundwater in the source area was strongly aerobic and oxidized, a combination that typically inhibits successful bioremediation.
Before stepping into the field, ETEC conducted a bench-scale evaluation using site groundwater. The results were promising: even at elevated Cr+6 concentrations, microbial activity could be stimulated using a nutrient-amended substrate. This proof-of-concept positioned bioremediation as a viable path forward.
To bring the approach to life, ETEC designed, fabricated, and installed a trailer-mounted 10 gpm In Situ Delivery (ISD) system. This system would become the backbone of the pilot-scale demonstration, delivering amendments continuously and precisely into the subsurface while allowing ETEC personnel to maintain full control over treatment conditions.
Site conditions and remedial goal
The target zone encompassed roughly 80 by 120 feet with a 30-foot saturated thickness. Eleven extraction wells fed groundwater to the ISD system, where it was automatically amended with CarBstrate, and then redistributed through 20 injection wells positioned throughout the source area.
An added layer of complexity came from a nearby losing creek that seasonally communicated with the water table. This natural influx of oxygen kept the groundwater highly oxidized, maintaining Cr+6 in its mobile and toxic state. Overcoming these conditions required a strategy capable of delivering consistent hydraulic influence and sustained reducing power.
The remedial goal was clear: convert Cr+6 to stable, environmentally protective Cr+3 and achieve complete transformation across the entire treatment interval.
ETEC Solution
Following a successful bench scale test, ETEC personnel operated an ISD system, applying CarBstrate at engineered feed rates based on Cr+6 concentrations, groundwater extraction volume, terminal electron acceptor mass, and the evolving geochemistry observed during treatment. This adaptive approach ensured that the subsurface received exactly what it needed when it needed it.
As recirculation progressed, monitoring revealed that some areas were significantly more oxidized than others. Rather than slowing progress, this insight allowed ETEC to strategically increase substrate loading in those zones, rapidly shifting conditions toward the reducing environment required for chromium transformation.
Remedial outcome
The pilot exceeded expectations, demonstrating how powerful short-term groundwater recirculation can be when paired with a highly soluble electron donor and a data-driven operating strategy.
In total, the system recirculated approximately 70,000 gallons of groundwater over the six-month treatment period, continuously delivering amendments and reshaping the aquifer's chemistry in real time.
- More than 90 percent reduction of Cr+6 across the treatment area.
- At MW-7, the site's most impacted well, Cr+6 concentrations as high as 58 mg/L were reduced and replaced by 207 mg/L of Cr+3 by the end of recirculation.
- Oxidized zones that initially resisted reduction ultimately achieved complete Cr+6 transformation with targeted increases in substrate dosing.
- Once formed, Cr+3 remained stable with no rebound to Cr+6.
- Additional slug injections over the following year reinforced anaerobic conditions and ensured long-term stability.
This project highlights the strength of ETEC’s ISD technology paired with CarBstrate: even in a highly oxidized aquifer, aggressive recirculation and precise amendment delivery can achieve complete transformation of Cr+6 within months. The result is a cleaner, safer aquifer and a proven framework for future chromium remediation efforts.
