Regional long-term model for the simulation of groundwater flow and transport for the region Untere Mulde/Fuhne, Germany

For the model area of the Untere Mulde/Fuhne, a detailed regional numerical groundwater flow model was established with a modelled time span from 1840 to 2005. Lignite mining in this area started in 1840 north of the town of Bitterfeld and continued up to 1990 over the west and south to the east. This resulted in a spatially diffuse spreading of contamination from the related chemical industry. The aim of the model was to develop strategies that explain the recent contamination better than local and short term models. Additionally, a stepwise method for the calibration of long-term models was developed although such a procedure may only be partly applicable to other sites due to site-specific requirements and data availability. The basis for model development was the outline of varying groundwater flow directions and gradients induced by open pit lignite mining that led to a strong time-dependent spreading of contaminants. Several unique geological settings such as Quaternary glacial channels had to be implemented in the model as well as the technical infrastructure. The results of the model are presented as pathlines and distribution plots of a conservative tracer. Using this model, a first possibility is given for the interpretation of the recently found distribution of contaminants. The Quaternary channels, for example, led to preferred flow paths and contamination of the Tertiary aquifer. The development of an adjusted and stepwise method with a calibration of time-independent parameters by fitting to short time measurements, a deductive prolongation of time-dependent but stationary time series and a correlation of non-stationary boundary conditions with time series data beyond the modelled area was proven justified.