To Treat A Comingled TCE And Hexavalent Chrome Plume
Background/Objectives
Site groundwater was impacted by a TCE plume co-mingled with a Hexavalent Chromium
(Cr[VI]) plume. High Cr(VI) concentrations inhibit the biodegradation of TCE, complicating
remedies for mixed plumes. The selected remedy employed sequential applications of
In-situ Chemical Reduction (ISCR) with calcium polysulfide (CPS) to initially reduce Cr(VI)
concentrations followed by enhanced anaerobic bioaugmentation (EAB) and ISCR to
degrade TCE.
Approach/Activities
Calcium precipitation is common during CPS injections resulting in negative impacts to soil
permeability, injection equipment, well screens, and radius of influence. Our work focused on
bench-scale development of pH-adjusted CPS solution and field implementation of the CPS
solution to reduce unacceptable secondary impacts to soil permeability and groundwater
geochemistry. Subsequent EAB was combined with ferrous lactate ISCR to polish off residual
Cr(VI) concentrations.
Results/Lessons Learned
After the CPS injection, Cr(VI) concentrations within the injection area ranged from
non-detected to about 1 mg/L. Cr(VI) concentration rebound was not observed
3 years after injection. Three years after the ISCR/EAB injection, TCE and Cr(VI)
plumes continue to shrink and rebound has not occurred. Geochemistry continues
to remain in the pH neutral anoxic range conducive to biotic and abiotic degradation.