NUMERICAL SIMULATION OF THE MOBILIZATION OF NEARBY CONTAMINANTS CAUSED BY REGULARLY-SCHEDULED CONVENTIONAL GROUNDWATER SAMPLING
At the study site in Elsterweda, Germany, the quaternary aquifer was contaminated with 470 t of unleaded gasoline due to a train accident in 1997. After the removal of 257 t of contaminants, the active remediation measurements were ceased in 2006 and Monitored Natural Attenuation (MNA) was implemented as remediation measure. From that point in time, a contaminant plume developed. An observation well at the Elsterwerda site, located adjacent to the plume, detected increasing concentrations of pollutants over time, even though a natural migration to this well had not been anticipated. Numerical simulations were run to determine whether this migration was induced primarily by the groundwater sampling process itself.
The groundwater modeling software Feflow 6.1 was used to perform the simulations. The progression of the Elsterwerda contaminant plume was simulated over a period of five years using transient, mass transport models. A scenario without groundwater sampling as well as one with groundwater sampling at the observation well–150 l of water pumped four times a year–were considered. At specific points between the plume and the observation well, the concentration developments of the dissolved contaminants were plotted over time and compared to one another. Additionally, the sensitivity of the model was tested through varying hydraulic and mass transport parameters, such as permeability, sorption, and degradation.
The simulations showed that groundwater sampling has the potential to draw contaminants into an area which they would not have migrated to otherwise. As a result, contaminant concentration levels at the observation well increased to the point that they exceeded threshold levels. Furthermore, regularly-scheduled conventional groundwater sampling can have a significant long-term impact on the progression of the contaminant plume.