Investigation and quantification of the stationarity of PAH plumes in groundwater
At contaminated sites, natural attenuation processes impede the spread of contaminants in the aquifer. Under favourable conditions, these processes may even lead to the stagnation or shrinkage of the contaminant plume. In order to consider these processes in the treatment of residual wastes detailed exploration of site characteristics is required. Gathering this informationis often very costly and time consuming.
In the framework of this project investigation and monitoring methods for different parameters were devised, which can be used for the integrated evaluation of the natural attenuation potential of former gas work sites.
A multilevel multitracer experiment using three dyed tracers (Uranine,Sulforhodamine B und Sodium Naphthionate) was accomplished to determine the hydrolgeological conditions (longitudinal dispersivity, velocity, and vertical dispersion effects) at a PAH-contaminated model site. To reduce material and personnel costs, a new field fluorometer was developed, that allowed a simultaneous and selective measurement of the concentrations of each of the three dyed tracers. In addition to the automatic field measurementsconducted at the test site, the fluorescence intensities were measured for selected samples in the lab by a conventional filter fluorometer.
Also, a new method was designed to characterise the availability ofthe sediment-bound PAH. Based on an extraction sequence the less available PAHs wereseparated from the more available PAHs. The extraction, which was carried out on a gravelysand material from the test site, clearly demonstrated that despite of the aged creosotecontamination, a substantial percentage of the PAH mass downstream the old waste deposits was readily available. Furthermore there was no evidence that considerable amounts of the PAH were bound to inner surfaces which are inaccessible to common extraction procedures.
The finding that PAHs were highly available could be corroborated through the application of a newly conceived infinite-sink-extraction technology. Additionally, the transferability of the extraction sequence as well as the infinite-sink-extraction could be ensured by its application to a PAH-contaminated soil with a high organic content.
The validity of the PAH availability was furthermore confirmed by the results of comprehensive soil and groundwater investigations which were carried out during the course of the dissertation. Unlike the elongated PAH contaminant plume in the groundwater, appreciable PAH concentrations in the soil were measurable only in an area immediately bordering the old waste deposits. In regions in which the groundwater concentrations of PAH were measuredat a few hundred μg/l, the sediment PAH concentration was near the analytical detection limit. A relevant contribution of a PAH transport by colloids in groundwater could be ruled out based on the results of the LIBD and ESEM measurements. The PAH partitioning between solids and groundwater which was determined analytically, contradicted the theoretical approachcommonly found in the literature which was based on a correlation between Kow and Koc.
Along the plume notable concentration reductions of the studied contaminants (PAH, BTXE, and CHC) suggest the action of biological degradation processes. On the basis of the degradation series (PCE → TRI → DCE → VC), a concentration pattern typical for dechlorination processes could be observed in the case of the CHC's.
Two additional independent parameters, the Fe(III)/Fe(II) ratio and the aquifer temperature, were investigated within the course of the project to qualitatively verify the existence of microbiological degradation processes at the test site. To determine the Fe(III)/Fe(II) ratio in the sediments a wet chemical extraction process was used. With the change of this ratio alongthe plume regions of stronger iron reduction could be identified and conclusions about the availability of iron could be drawn. The existence of microbiological degradation processes was also confirmed by the temperature measurements with built-in temperature sensors. As expected a good correlation between the temperature and the contaminant concentration could be observed. So a horizontal and a vertical temperature profile could be measured downstream of the contamination source, which indicates increased microbiological activity.
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Bundesministerium für Bildung und Forschung (BMBF)
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