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N5 Effects of bank filtration on lake ecosystems
Doctoral student: Mikael Gillefalk 
Supervisors: PD Dr. Sabine Hilt , Prof. Dr. Wolf M. Mooij , Prof. Dr. Mark Gessner , Prof. Dr. Reinhard Hinkelmann , Dr. Michael Hupfer 
Bank filtration describes the process of surface water infiltrating into the groundwater from lakes or rivers. In large parts of the world, especially in Central Europe, this phenomena is induced by installing groundwater wells close to surface water bodies, lowering the groundwater surface. The passage through the soil functions as a first, cost-efficient cleaning step for drinking water production. Earlier research dealing with bank filtration has focused on the abstraction capacity and the cleaning efficiency. With the research project N5, we want to change perspective and instead focus on ecosystems.
The overall aim of project N5 is to investigate the effects of bank filtration on lake ecosystems, thereby opening a new research field. Our goal is to carry out fundamental research, studying particular parameters, bringing the state of the art forward but also not to forget the broader picture and be able to provide a knowledge base upon which well-informed decisions about lake management can be made. These two aims go hand in hand.
The research is conducted using a wide variety of methods, including field measurements, sampling, lab work and experiments. Also, the ecosystem model PCLake (https://en.wikipedia.org/wiki/PCLake ) is used to simulate the effects of bank filtration on shallow lakes.
- Colourful periphyton filters
- © M. Gillefalk
Early results show that bank filtration might be the reason for observed differences in sediment characteristics in Lake Müggelsee. The modelling work has shown that lake types are crucial when trying to predict the relative effect of bank filtration. Many parameters come into play, two of them are groundwater nutrient concentrations and lake depth.
To give some understanding to what the difference in groundwater nutrient concentration might mean, imagine two different lakes. Both have the same depth, size, environmental setting and they both are groundwater-fed with the same amount of water. One thing differs, however, and that is that in one case, the groundwater is very rich in nutrients causing eutrophication and making the lake turbid, in the other, the groundwater nutrient concentration is low and this lake is clear. If bank filtration would start in both cases, the effect of it would be different. The eutrophic lake will get rid of a big nutrient inflow and might be given the chance to recover. The other lake, however, will be less resilient to higher nutrient loading (from surface water) than before. The both lakes ended up in the same situation, the response to this new situation was different, and especially their relative change compared to each initial condition was very different.
- Bifurcation plot – low groundwater nutrient concentration
- © M. Gillefalk
- Bifurcation plot – high groundwater nutrient concentration
- © M. Gillefalk
The modelling work has shown similar results with regard to lake depth, where shallower lakes suffer more from bank filtration than do deeper lakes.
- Other UWI projects: N6 , N7 , T6  (IAHR conference paper)
- Common topics group: Interfaces in urban surface waters , Surface water – groundwater interactions  and Modelling 
collaborations: NIOO 
Initial project plan