Combining Chemical Oxidation and Enhanced Reductive Dechlorination for DNAPL Source Area Treatment at a Danish Megasite: Pilot Test Evaluation

Background. Kærgård Plantage is the largest and most complex contaminated site in Denmark. From 1956 to 1973, a pharmaceutical manufacturing facility, with regulatory approval, discharged 280,000 m3 of wastewater containing approximately 60,000 tons of various salts, sulfa drugs (antibiotics), barbiturates, lithium, mercury, cyanides, aniline/pyridine and organic solvents (PCE, DCM, benzene, toluene, alcohols) to six pits situated on a plantation approximately 400 meters from the beach. Given the complex and highly concentrated mixture of contaminants, implementation of multiple or sequenced remediation technologies may be required to achieve cleanup goals.

Activities. Bench tests with soil and groundwater from the site were performed to evaluate the feasibility of treating residual chlorinated solvent dense nonaqueous phase liquid (DNAPL) and other contaminants present in the saturated zone beneath two of the waste pits (Pits 1 and 2). Technologies included in the bench test were chemical oxidation using modified Fenton’s reagent, MFR (hydrogen peroxide + chelated Fe2+), chemical oxidation using activated sodium persulfate (ASP) and biological enhanced reductive dechlorination (ERD) via biostimulation/bioaugmentation.

Based on the results obtained in the laboratory, MFR and ERD were selected for pilot testing in the field. Three pilot tests were initiated in June 2010: Pilot Test 1 - ISCO using MFR (Pit 1, 3-6 m bgs.) in an area of Pit 1 impacted with high DNAPL and a complex mixture of sulfonamides, barbiturates, and ketones; Pilot Test 2 - ISCO using MFR followed by ERD (Pit 1, 7-9 m bgs.) in a less contaminated area of Pit 1; and Pilot Test 3 - ERD (Pit 2, 6-11 m bgs.) in an area of Pit 2 impacted with low amounts of DNAPL and moderate dissolved phase chloroethene concentrations, absent significant concentrations of sulfonamides, bariburates and ketones. The test plots for Pilot Tests 1 and 2 are collocated in Pit 1 and consist of an ~8 m x 8 m test cell with six monitoring wells screened at two depths and five injection wells screened at three depths. Pilot Test 1 involves four successive MFR injections over a six month period in a treatment interval 2 m below ground surface (bgs). Pilot Test 2 involves two MFR injections at a depth of 3 m bgs, followed by one to two batch injections of emulsified soybean oil and Dehalococcoides (KB-1). The design for Pilot Test 3 (ERD in Pit 2) consists of a closed-loop groundwater recirculation cell (25 m long) in which electron donor (lactate and ethanol) is delivered to the treatment zone via forced gradient flow field. Performance is monitored in a series of wells located between the injection and extraction wells.

Results. Despite very high contaminant concentrations, soil samples collected after the 2nd MFR injection event in Pit 1 showed a significant reduction in contaminant concentrations. Water analyses showed that the concentrations of organic contaminants still are very high and that metals were mobilized. Lead and copper were found in the highest concentrations in the test cell and downgradient. Results indicated a radius of influence between 1.5 and 2.5 m. Results from ERD pilot test showed that injection of electron donor stimulated partial dechlorination of PCE to cis-1,2-dichloroethene, vinyl chloride and ethene within its first three months of operation. However, the rate of dechlorination was slower than expected based on bench test results, and pH appeared to decrease due to HCl generation. pH buffering with NaHCO3 and bioaugmentation with KB-1 were implemented to accelerate dechlorination, but results are not yet available. All the pilot test will be completed in June 2011and will be presented at the conference.