Abstract
The monitored natural attenuation (MNA) of groundwater contamination is in many cases considered to be a cost-effective and
appropriate strategy to manage the remediation of contaminated aquifers. A prerequisite for the public acceptance of such NAremediation
approaches is that the efficiency of the presumed NA-processes should be proven. This proof must be based on an
extensive hydrogeological and in-situ characterization of the ongoing aquifer processes. Additional investigations of aquifer
samples in the laboratory (microcosms) are often questioned because they are not representative.
The aim of this research was therefore to evaluate and compare the microcosm-monitoring of degradation activity directly to
results obtained from field investigations at the same site, a former coking plant in Castrop-Rauxel in Germany. The in-situ
monitoring of the contaminant degradation at this site revealed a clear decrease in the concentration of the major contaminants,
benzene and naphthalene along the flow direction of the plume. Furthermore, comparative isotopic analysis of the sulfur pools
indicated that sulfate-reducing processes could play a predominant role in this degradation process. However, attempts to prove
the microbial degradation of the organic contaminants directly by analysis of the isotopic fractionation of the contaminant carbon
failed, especially in the case of benzene and naphthalene. The assumption that a very active microbial population was actually
involved in the natural attenuation process of the prevalent contaminants at the site could be shown more convincingly with
laboratory microcosm experiments. The two monitoring approaches therefore complemented each other very well and should
therefore both be implemented in standard natural attenuation monitoring programs.