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Urban Water InterfacesT4 Impact of residual wastewater constituents

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T4 Impact of remaining wastewater constituents on interfaces in surface waters

Doctoral student: Robert Ladwig

Supervisors: Dr. Michael Hupfer, Prof. Dr. Reinhard Hinkelmann, Prof. Dr. Sven-Uwe Geißen

Background

Wastewater treatment plants serve to minimize the pollution of aquatic systems aiming for an efficient elimination of nutrients and harmful substances. However, treated wastewaters often contain dissolved substances that are either not eliminated during the process or are even added during the treatment process. Discharging these residual substances into urban lakes will strongly interact with their interfaces. Ecological systems that are either limited by phosphorus, nitrogen or both can experience dramatic changes when receiving increased loadings of these nutrients. Especially in the context of global warming the need for a dual management of nutrients will increase due to longer vegetation periods in summer, stronger stability of the water column, ideal conditions for the development of toxic algae blooms and an increased depletion of oxygen in deeper water layers. Thus, the success and efficiency of the management of urban lakes are crucial for the understanding of the processes of all parts of the managed system, as well as their interactions.

Aims

The aim of this project is to investigate the water management of urban lakes focusing on their technical interfaces. The heavily modified urban lakes in Berlin, Germany, have experienced diverse urban stressors like morphological degradation or eutrophication and, in some cases, are nowadays supported by a complex setup of management measures which acts as a “life-support system” for the lake. As field site I decided for Lake Tegel in the Northwest of Berlin (Figure 1), which is nowadays affected by discharges of treated effluents (originating from a wastewater treatment plant and a phosphorus-elimination plant), mixing events with nutrient-rich river water, bank filtration, hypolimnic aeration, a lake pipeline, diffuse inflows and recreational activities. Main focal points of interest are the impact of global warming as well as alterations of the management setup on the lake and how the inflow of treated effluents can affect the sediment-water interface, for instance the retention or release of phosphorus.

Figure 1: Lake Tegel, Berlin
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Methods

For the investigation of past management measures at Lake Tegel several sediment cores were taken from 2015-2016. Using statistical methods (k-means clustering, principal component analysis and self-organizing maps) the sediment composition of Lake Tegel was compared within different sites of the lake and with sediment cores of other lakes. Further, water samples are taken on specific dates from 2017-2018 for analysis of total phosphorus, soluble reactive phosphorus, nitrate, ammonium, nitrite and dominant phytoplankton species. Concurrent measurements of physicochemical parameters like temperature, flow velocity and electrical conductivity are also taken. Beginning in summer of 2017 temperature and electrical conductivity loggers will be deployed at Lake Tegel to get data with a high temporal resolution.

To assess the impact of changes in management measures on the ecological system coupled modelling tools that are also simulating water quality are used, for instance General-Lake-Model coupled to Aquatic-Ecodynamics-Model-Library for vertical 1D-simulations and TELEMAC-MASCARET coupled to DELWAQ for 2D-simulations. These models are calibrated with past field data from previous studies and are used to quantify future changes in the lake system.

Preliminary results

The impact of the management measures was investigated by using statistical methods and could be visualized in the lake sediment composition changing from high abundance of heavy metals and reducing redox conditions to less impacted sediments in recent layers (see Figure 2). The abundance of heavy metals in recent lake sediments of Lake Tegel is similar to a lake with low urban impact and is lower than in a close urban lake suggesting that the management measures were successful in the reduction of heavy metals.

Figure 2: Clustered vertical profiles of sediment cores at Lake Tegel. A: Profiles represent respective cluster established by k-means. B: Box-and-whisker-plots of elements, scores and proxies, colors represent the respective clusters
Lupe

Using a vertical 1D-hydrodynamic model coupled to an ecological module (GLM-AED2) was proven to be sufficient in replicating observed data of temperature, phosphate, oxygen and nitrate. The results show that global warming will shift Lake Tegel from a dimictic state to a monomictic one with an increase of the temperature differences between surface and bottom layer in summer and a decrease in winter (see Figure 3). This more stable summer stratification will cause an enhanced depletion of oxygen and the establishment of phytoplankton, which eventually can cause the formation of toxic algae blooms that benefit from an increase in water temperature and buoyancy. Further, one can use the model to distinguish the effects of different management setups (either a regularly functioning elimination plant or a weakened one as seen in Figure 3).

Figure 3: Simulated changes of future summer and winter temperatures (2008-2100) for regular and weakened setup of the elimination plant (in terms of daily averaged or reduced discharges)
Lupe

In the final step, a 2D-model (TELEMAC-MASCARET) is set up to incorporate the complex hydrodynamics of the lake including wind fetch, river mixing and turbulences caused by islands (see Figure 4). This final model will be coupled to a water quality model (DELWAQ) to explore how different management measures (e.g. changing the inflow dynamics, improving upstream nitrate elimination techniques) could affect the lake ecosystem.

Figure 4: Transient 2D-simulation showing calculated velocities at Lake Tegel in summer 2008
Lupe

Collaborations

Further links

IGB: http://www.igb-berlin.de/profile/robert-ladwig

Researchgate: https://www.researchgate.net/profile/Robert_Ladwig

Github: https://github.com/robertladwig

Twitter: @hydrobert

 

Initial project plan

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