Comparison of Laboratory Experiments of Chemical, Biological, and Thermal Methods for Treatment of Chlorinated Solvent DNAPL at Kærgård Plantage in Denmark

The Kærgård Plantage megasite on the western coast of Denmark represents one of the most difficult remediation challenges in Scandinavia. Disposal of an estimated 280,000 m³ of pharmaceutical wastes at the site from 1956 to 1973 resulted in the development of a complex mixture of contaminants in soil and groundwater, including sulfonamides, barbiturates, aniline, pyridine, chlorinated solvents (chloroethenes), fuel hydrocarbons, mercury, cyanide, lithium and many other compounds.  Wastes were disposed in six pits that continue to leach contaminants to groundwater. Contaminants in groundwater are estimated to discharge into the ocean at a rate of 20 m³/year, and public health concerns have prompted the closing of a 1.5 kilometer section of beach at the site. In 2008, the waste pits were excavated down to the water table, and the project is currently focused on evaluation of alternative in situ technologies for remediation of the residual wastes below the water table - first in the laboratory and then in pilot tests. Given the complex and highly concentrated mixture of contaminants, implementation of multiple or sequenced remediation technologies may be required to achieve cleanup goals.  An important design objective is to prevent or minimize mobilization of mercury and cyanide.     

A series of bench tests have been 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.  Technologies included in the bench test are chemical oxidation using MFR: modified Fenton's reagent (hydrogen peroxide + chelated Fe²⁺), chemical oxidation using persulfate and  ASP: activated sodium persulfate (with NaOH, hydrogen peroxide and  Fe²⁺), biological enhanced reductive dechlorination (ERD) via biostimulation/bioaugmentation, and steam enhanced extraction. Treatment have been assessed through monitoring changes in contaminant concentrations, select degradation intermediates, and groundwater toxicity (Microtox® bioassay) consequent to treatment. The tests have been performed with soil and groundwater from the site containing different levels of chlorinated solvent DNAPL.

Chemical oxidation using both persulfate activated with hydrogen peroxide and modified Fenton's reagent was effective towards the contaminants at Kærgård Plantage and no significant difference in mobilization of metals was observed for MFR and ASP. Reaction rates, heat and gas production was controlled and acceptable in all experiments. In the ERD tests with low concentrations of PCE (C₀ ~ 25 mg/L PCE), addition of electron donors simulated complete dechlorination to ethene within 170 days of donor addition.  Dechlorination was more rapid in bioaugmented microcosms (63 days); however, the indigenous bacteria were capable of complete dechlorination without bioaugmentation. In the high DNAPL test and high strength mixture tests, however, significant dechlorination was not observed, as ERD processes in these tests appeared to be inhibited. The steam enhanced extraction tests have not been performed yet, but the results will be ready in the summer 2010. The results from all the laboratory experiments will be used to decide which methods should be tested in the following pilot tests at the site.