3. Studies
4.2 Subsurface flow paths and flow systems on different scales
Local flow systems are still of major importance concerning the substance input, since upper aquifers are most vulnerable to anthropogenic impacts. Whether an area contributes to surface water
eutrophication or not, depends on the hydraulic connectivity to the surface water as well as on the substance load within a specific area. Both are mainly determined by the sediment distribution and characteristics. One method to identify possible sediment heterogeneities delivers Lehr et al. (2015, study I). They were able to identify parts of the aquifer with different connectivity to the surface water on a scale of 101 m. The investigation period covered four years and included flood events of several months as well as a restoration measure of the river; nevertheless the general linkage between groundwater and surface water remained the same for the whole period. Hence, spatial heterogeneity of the connectivity is larger than the temporal one. Hence, the subsurface flow nearby surface waters can be seen as temporally constant at least for a given timescale. Furthermore, the study confirms the findings of Schornberg et al. (2010), who showed that heterogeneity of the exchange between
groundwater and surface water bases on the sediment distribution of the aquifer and not the sediments in the river. The River Spree represents this case. Even if hydraulic connectivity of the meander has been improved by removing the mud layer, the overall pattern of the connectivity remained.
A similar pattern of higher spatial and lower temporal heterogeneity was also found by Lewandowski and Nützmann (2010) for the nutrient concentrations within the same aquifer and with the same spatial resolution as in the study of Lehr et al. (2015, study I). Since there was no clear evidence on the reason of these findings, Pöschke et al. (2015 b, study II) performed small scale geochemical investigation (10-1 – 101 m) on the upper aquifer and found an even larger spatial heterogeneity. Implying that the horizontal flow component is small in comparison to the vertical one, otherwise equilibrium would establish. Further, the results of the Multi-Level Sampler suggest that there is a vertical layering of the groundwater. The upper 1.5 m of the aquifer is impacted by water level fluctuations, which induces different redox conditions throughout the year and therefore variable and high nutrient concentrations in the upper part of the aquifer. Below concentrations are generally low and temporal constant
meaning that there is another groundwater origin. It is still unanswered if and how these vertical layers are interacting. Does the interaction take place already on the flow towards the surface water or is it restricted to the area, where the water enters the surface water? Also if the part of the aquifer with the high nutrient concentrations is responsible for the eutrophication of surface waters, is still unknown. However, there are evidences for a lake from Crowe and Schwartz (1981) who discovered, that the water quality of the surface water only changes significantly, when the main part of the aquifer is subjected to a change in concentration. This would not be the case at the aquifer of the River Spree, since the upper 1.5 m of the aquifer represents about 7.5 % of the total aquifer volume.
The numerical modeling of groundwater discharge towards Lake Stechlin was able to illustrate that different vertical flow systems need to be considered when determining the groundwater flow towards a lake in lowland head areas with complex aquifer systems. In general, it can be assumed, that flow systems are more or less constant over a given time, since the temporal variations are small in aquifer systems. Skøien and Blöschl (2003) explain that a landscape tends to remove the temporal variability which is described by a meteorological force and adds spatial variability. The latter one is given by the sediment distribution (Lischeid, 2008). Nevertheless, Winter (1983) discovered a temporal change in the vertical extension of the flow systems due to differences in groundwater recharge. This can also be assumed for the catchment of Lake Stechlin, since there are long term fluctuations of the water levels of P03 and P37 which are not reflected in the lake. Additionally, Holzbecher (2001) describes variable horizontal extension of the groundwater catchment of Lake Stechlin, which also relates to the long term weather conditions. In conclusion, a long term temporal variability of the subsurface catchment of Lake Stechlin can be expected. This must be considered when research questions are addressing transient system behavior, e.g climate change studies.
For the Lowland of North Eastern Germany it could be conclude, that as long as there is no significant change in the water levels of the surface waters, the spatial extension and distribution of the
detection of the flow paths should focus on the description of the sediment distribution to determine areas with higher transport capacities for mass, substances and energy. Hence, investigations on smaller spatial scales are necessary for these areas. In contrast, the regional flow system can be seen as the lever for the whole hydrogeological system. These areas govern the groundwater discharge over larger scales. Due to the spatial extension of the systems, the reaction on temporal changes is generally slow. Therefore, long term observations are required to capture changes. However, the temporal scales depend on reaction time, which in turn depends on the sediment. Nevertheless, a general
characterization of geologic settings is sufficient.