Formatted Title
Application of a Combined Technology for the Treatment of an Aerobic Aquifer Impacted with Carbon Tetrachloride and Chloroform
Background/Objectives
The site is in a highly industrialized area of northern Italy, where groundwater is contaminated with carbon tetrachloride (CT) (>10 mg/L), chloroform (CF) (>10 mg/L), hexavalent chromium and, to a lesser extent, tetrachloroethene and trichloroethene (<1 ppb). The EHC® Liquid technology deploys in situ chemical reduction (ISCR) mechanisms for treatment of impacted groundwater. It is comprised of two ingredients which are easily combined and diluted for injection: i) ELS™ Microemulsion; a controlled-release food-grade carbon in the form of lecithin, and ii) EHC® Liquid Reagent Mix; an organo-iron compound. The addition of organic carbon in a saturated zone is widely known to promote conventional enzymatic reductive dechlorination (ERD) reactions. As bacteria ferment the ELS™ Microemulsion component, they release a variety of volatile fatty acids (VFAs) such as lactic, propionic and butyric, which diffuse from the site of fermentation into the groundwater plume, and serve as electron donors for other bacteria, including dehalogenators. Lecithin itself is primarily composed of phospholipids, with both hydrophilic and hydrophobic properties in the molecular structure. Further, phospholipids support ERD by providing essential nutrients (carbon, nitrogen, phosphorus) to bacteria. Synergistically, the soluble organo-iron component contains a source of a ferrous iron (Fe[II]) that can combine with other elements to biogeochemically form a variety of reactive iron minerals (e.g. magnetite, pyrite). These minerals are capable of ISCR of contaminants as they oxidize further to the ferric (Fe[III]) state via one electron transfer. The Fe[III] can then be “recycled” back to Fe[II], as long as other electrons from supplied carbon and indigenous carbon are available.
Approach/Activities
In the intervention areas and the respective downstream sectors, standard pump-and-treat wells were located, and designed to accelerate the removal of various contaminants. However, the presence of active pumps inside, or in the immediate vicinity, of the EHC® Liquid injection zones could have compromised ISCR/ERD effectiveness. This is a function of an increase in groundwater flow rate and potential removal of the injected emulsion and EHC® Liquid mix. For this reason, a strategy was planned to optimize the onsite wells by reducing the groundwater extraction rates, thus protecting effectiveness of the ISCR/ERD treatment. Through use of mathematical modelling, optimal extraction rates were defined which would keep the natural seepage velocity to less than 300 m/year in the ERD treatment area. Application of the EHC® Liquid remedial reagents was performed via direct injection through fixed Manchette tubes distributed in the source area and hot spots.
Results/Lessons Learned
Less than 12 months after injection of EHC® Liquid into the main source area and hot spots, concentrations of CT and CF contaminants were rapidly reduced up to 95% compared to pre-treatment levels. Requisite remedial target values were reached in all main monitoring piezometers in the areas. The major observations of field parameters in the ISCR and ERD treatment areas included: i) an increase of manganese and Fe[II] in solution as anaerobic cometabolites, ii) a decrease of competing electron acceptors dissolved oxygen and sulfate, stability of pH in the neutral range, and iii) establishment of negative Redox around -150 mV. Lastly, Cr[VI] concentrations have been observed constantly below detectable values with no rebounds.