Enhanced Natural Attenuation for the in-situ biodegradation of heterocyclic hydrocarbons in groundwater

Heterocyclic hydrocarbons (NSO-HET) are ingredients of tar oil, commonly found downgradient of former gasworks sites. Most NSO-HET are highly mobile due to their high water solubility and low biodegradation rates under anaerobic conditions.

Based on the extension and contaminant distribution of the plume downgradient of a former gasworks in Southern Germany, the most applicable technology for enhancing the natural degradation of PAH, BTEX and NSO-HET was selected and tested under controlled conditions in a large physical model (artificial aquifer in the Large Flume of VEGAS).

Based on a technical review, numerical simulations and experimental results from 2-D (flume) experiments the technology of a Groundwater Circulation system was selected. The Groundwater Circulation Wells technology (GCW) can be applied to achieve a maximum mixing of the electron acceptor solution with the groundwater. A spherical groundwater circulation is induced by means of ex- and infiltration ports in vertical wells. Infiltration and exfiltration ports are located in hydraulically separated filter sections.

A GCW was installed in the Large Flume to mix either oxygen from the ambient air or hydrogen peroxide homogeneously in the groundwater contaminated with the relevant NSO-HET from the field site. During the investigations, an almost complete degradation was determined for both types of electron acceptors. Hydrogen peroxide (50 mg/L) is disproportioned within less than two days into water and oxygen causing an oxygen concentration of more than 20 mg/L. The aerobic degradation of the contaminants is significantly increased, even upgradient of the GCW within the inner circulation of the wells system.

Oxygen infiltration was found to be less efficient, limited by the lower concentrations of oxygen (< 5 mg/L) but it causes lower costs for the operation than hydrogen peroxide. To reduce costs for the long-term operation of this ENA measure, it is advised to create aerobic conditions using hydrogen peroxide release and then switch to aeration in order to maintain the biological degradation. The results of the Large Flume experiment indicated a periodical infiltration of air / hydrogen peroxide.

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Koschitzky, Hans-Peter

Haslwimmer, Thomas
Trötschler, Oliver

VEGAS

From: 2002-10-01

To: 2005-03-31