Modeling the long-term and transient evolution of biogeochemical and isotopic signatures in coal tar-contaminated aquifers

Abstract

Reactive transport modeling is a critical element in assessing the potential of natural attenuation of groundwater pollutants. In the present study, we developed a comprehensive quantitative model that incorporates the key processes affecting the long-term fate of complex organic compound mixtures released from coal tar-type dense nonaqueous phase liquid sources. The model framework addresses the simulation of the long-term dynamics of source zone depletion, the fate of the released compounds during reactive transport in the groundwater, the evolution of the aquifer's biogeochemical response, in particular its redox conditions, and the redox-dependent carbon isotope fractionation of selected organic compounds. The modeling framework was applied for the interpretation of observed biogeochemical and isotopic data from a well-characterized coal tar-contaminated site in northern Germany. The simulations highlight the diversity of fates of the individual compounds, which result from their widely varying physicochemical characteristics, and also how complex interactions develop over the lifetime of the contamination. The highly transient release of contaminants from the coal tar as pool and as heterogeneously distributed blobs in the source zone triggers continuously changing biogeochemical conditions and isotope signatures. The modeling results illustrate how difficult and uncertain the assessment of contaminant fate can be if the collected data cover only a small time window relative to the transport time scale. This emphasizes the need for a holistic understanding of the governing processes that control the effectiveness of monitored natural attenuation before it is implemented as a passive remediation strategy at nonaqueous phase liquid-contaminated sites. Copyright 2011 by the American Geophysical Union.