Denitrification in a BTEX Contaminated Aquifer Containing Reduced Sulfur
Abstract
At a former gasworks plant in Duesseldorf (Germany) a massive soil and groundwater contamination with BTEX (up to 100 mg/l) and to a minor extent with PAH (up to 10 mg/l) were detected. Mainly due to sulfate and iron-(III) reduction, a natural biodegradation has occurred and restricted the length of the contaminant plume in the direction of groundwater flow to only 600 m. The active remediation strategy at this site includes nitrate-enhanced in-situ bioremediation of the remaining contaminants in the plume. Nitrate was infiltrated in the contaminated aquifer during a field test to study the efficacy of enhanced natural attenuation. Degradation of hydrocarbons under denitrifying has been proved by numerous laboratory and field studies. However, at this site the competing reaction of nitrate with hydrocarbons and reduced sulfur components has to be considered. The oxidation of pyrite by nitrate in pristine aquifers is well known. The Duesseldorf aquifer contains FeS, pyrite and Fe-calcite precipitated during over 50 years of natural attenuation. The hydrogeochemical transport model PHREEQC-2 is used to simulate the distribution of chemical species and reaction rates along the flow path between the infiltration well and two multilevel wells . The complicated suite of reactions caused by the reduction of nitrate is evaluated by the comparison of modeled and measured data. At the Duesseldorf site the concomitant presence of nitrate, Fe(II) and BTEX/PAHs showed that the reactions did not evolve to thermodynamic equilibrium and were controlled by kinetics. The very good fit of observed and model calculations illustrates that the inorganic chemical reactions during the field test are generally well understood. The kinetic reactions could be modelled with rate equations from the literature based on oxygen, and which were extended with nitrate. Denitrification rates with BTEX compounds and with FeS were found to be comparable, but the oxidation of Fe(II) and FeS occurred prior to the reaction of the organic pollutants. Thus, reduction products which accumulated earlier during the natural attenuation of BTEX were the first to react when nitrate was added as electron acceptor for enhanced bioremediation of the pollution. For BTEX degradation, a rate law based on Monod kinetics for natural organic matter was adapted to fit the observed decrease of nitrate. Sulfides may be common at other petroleum polluted sites, and enhanced remediation by addition of oxidants may be retarded by the unintended and possibly unexpected oxidation of these solids. The Duesseldorf case is therefore of interest for indicating the complex geochemical reactions which occur when other electron donors are present in the aquifer besides the aromatic compounds targeted for the remediation.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2001
- Bibcode:
- 2001AGUSM...H32B07E
- Keywords:
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- 1831 Groundwater quality;
- 1832 Groundwater transport