Modelling Groundwater Travel Time Distributions in the Hainich CZE
2 Thüringer Landesamt für Umwelt, Bergbau und Naturschutz, Referat 83, Hydrogeologie, Bodenkunde
3 Universität Potsdam, Mathematisch-Naturwissenschaftliche Fakultät, Institut für Erd- und Umweltwissenschaften
P 11.2 in Verweilzeiterkundung in Grundwasserleitern mittels Tracern und Grundwasseraltern
The Earth’s Critical Zone (CZ) is the biodiverse layer interfacing the atmosphere and the geosphere and providing the habitat for the majority of terrestrial biota, including humans. Environmental change caused by intense agricultural usage, pollution, increasing water demand for irrigational purposes and climate change impose critical stress on this zone. The influencing surface factors can be quantified and measured on small scales but the propagation of these signal through the upper soil layer towards the deeper subsurface is poorly understood. The Hainich Critical Zone Exploratory (CZE) is located in the north-west of Thuringia in central Germany and serves as a large-scale natural lab to observe surface and subsurface processes. Several test sites are equipped with diverse sensors to record the bio-, eco-, geo- and hydrological parameters.
In order to understand the hydrogeological system, it’s major pathways as well as the flow rates and travel times in coherence with the surface input signal, we have established a numerical groundwater flow model of the Hainich CZE following the transect of the observation wells. This built up the basis for a detailed particle tracking analysis to estimate the distribution of travel times of groundwater in the subsurface which can help to estimate the biogeochemical activity and thus the degradation capability of microbes to chemical substances.
We released a total amount of 31.500 particles at 63 different time steps, each with a static vector field from a model forced by temporal fluctuating recharge as well as constant head boundary conditions. The particles were appointed at each well in the CZE starting at the depth of the screen and following the advective flux reversely. In order to account for dispersion and diffusion the random walk method was chosen. The travel times for every time step and for each well were plotted, averaged and fitted with a gamma distribution. We found out that the sensitivity of the obtained results strongly depends on the magnitude and orientation of the advective flow field, thus resulting in a high variability throughout the investigated time steps. This is caused by temporal fluctuations of inflow (recharge, lateral inflow) as well as spatially heterogeneous hydraulic conductivities. Water reaching the shallow screens of the wells entered the soil in an area locally surrounding the wells (1 - 2 km) and travelled for less than a year to approximately 20 years. The water at the deeper screens spend several decades to 150 years in the subsurface until it reached the well and stemmed from regional areas covering even larger parts than whole modelling domain.