Quantification of Natural Attenuation of N-S-O Heteroaromatic Compounds in Groundwater at Field Scale

N-S-O heteroaromatic compounds (HET) can typically be found in groundwater at tar oil contaminated sites. Despite the fact that most of these substances are toxic and potentially carcinogenic, they are not yet routinely monitored. Goal of this contribution is to present results from the investigation of the in-situ natural attenuation potential of HET in groundwater at field-scale. The research has been performed at the `Testfeld-Sed' (TFS) experimental site, a former gasworks site in southern Germany. At the TFS site, the ongoing quantitative determination of the natural attenuation potential of HET is performed using an integral investigation approach at multiple control planes (integral pumping tests, based on the inversion of concentration time series measured during pumping), which are positioned at different distances downgradient of the source zone. Natural attenuation processes result in a reduction of the total contaminant mass flow with increasing transport distance. Degradation rates can be obtained from (multi-)process-based reactive transport modeling which considers all processes contributing to mass flow reduction. Two control planes (CPs) at a distance of about 150 m were used for the integral measurements at the TFS site. The concentration time series already indicate that contaminants like BTEX and PAH almost vanished on their way from the upstream to the downstream CP, while HET were still present. The determined mass flow rates further indicate that some of the HET (especially Methylbenzofurane and Dimethylbenzofurane) are not only very mobile, but also highly persistent. This confirms the dominating role of HET as major contaminants at the TFS site. However, the observed decrease in the total Dimethylbenzofurane mass flow rates already indicates good chances for enhanced natural attenuation (ENA) measures (e.g. injection of H2O2 as additional electron acceptor to stimulate biodegradation) which are planned as next steps. Acknowledgement: The authors greatly acknowledge the financial support of this study within the KORA-framework of the German Ministry of Education and Science (BMBF), grant 02WN0361.