Stormwater impact guidelines for dissolved oxygen (DO) were applied to the Berlin River Spree, which (a) receives the effluents of more than 100 combined sewer discharge points and (b) is subject to significant anthropogenic background pollution. Discrimination of DO depressions, which are the direct result of combined sewer overflows (CSO) from DO depressions which are not related to CSO was achieved by combining stormwater impact guidelines with the analysis of data for: (i) rain events before critical DO depressions, (ii) water temperature (T) and conductivity as indicators for CSO impact in the river and (iii) T and DO before critical DO depressions to assess the effect of background pollution. Results indicate that the River Spree is in a critical state regarding DO for two main reasons: (a) upstream of the stretch with CSO discharge points because of background pollution and (b) downstream of the stretch because of CSO. Highly critical situations with DO < 2 mg L-1 only occurred under CSO influence. Nevertheless, the analysis underlines the importance of measures to reduce both CSO and background pollution in urban rivers.

Eine optimierte Abwasserbehandlung führte seit den 1990er Jahren zu stark abnehmenden, kontinuierlich aus Punktquellen in die Vorfluter eingeleiteten Nährstofffrachten (HEINZMANN, 1998, SENSTADT, 2001), wodurch sich die Wasserqualität der aufnehmenden Gewässer Berlins merklich verbesserte. Episodische Belastungen durch Mischwasserentlastungen stellen jedoch weiterhin eine bedeutende Ursache einer herabgesetzten Wasser- und Sedimentqualität und eine der wichtigsten Managementaufgaben für die Berliner Stadtspree und der Kanäle dar (vgl. LESZINSKI ET AL., 2006, RIECHEL 2009). Hinsichtlich des von der EU-WRRL geforderten guten ökologischen und chemischen Zustandes der Binnengewässer bzw. des guten ökologischen Potenzials für stark veränderte und künstliche Gewässer, stellt die Lebensraumfunktion für die aquatischen Lebensgemeinschaften der Berliner Gewässer das wesentliche gewässerinterne Schutzziel dar. Neben dem erheblichem ökologischen Gefährdungspotenzial, das insbesondere von extremen Ereignissen der Mischwasserentlastung ausgeht, reduzieren vorrangig hydromorphologische Defizite (Stauhaltung, Uferbefestigung, Sohleintiefung, etc.) die Lebensraumqualität für die aquatischen Lebensgemeinschaften. Aufgrund der Schifffahrtsnutzung der Berliner Spree und der Kanäle stellen Wellenschlag und Sunk- und Schwalleffekte während Schiffspassagen eine zusätzliche, bedeutende Belastung dar (vgl. LESZINSKI ET AL., 2006). Wie in der Studie „Immissionsorientierte Bewertung von Mischwasserentlastungen in Tieflandflüssen“ (LESZINSKI ET AL., 2007) dargelegt, liegen die in Laboruntersuchungen ermittelten Ansprüche bzw. Toleranzen hinsichtlich der Wasserqualität für die Fischarten und Arten wirbelloser Bodenorganismen der Berliner Spree und der Kanäle in einem vergleichbaren Bereich (JACOB ET AL., 1984, LAMMERSEN, 1997). Die Herleitung von Gütestandards hinsichtlich der Wasserqualität für die Fischfauna schließt somit den Schutz der Lebensgemeinschaft der wirbellosen Bodenorganismen mit ein. Ebenso besteht bei beiden Organismengruppen ein grundsätzlicher, vergleichbarer funktioneller Zusammenhang zwischen der Ausprägung der Lebensgemeinschaft und der hydromorphologischen und strukturellen Lebensraumausstattung des Gewässers (z.B. SHELDON, 1968, KARR & SCHLOSSER, 1978, MINSHALL, 1984; MINSHALL & ROBINSON, 1998, TANIGUCHI & TOKESHI, 2004). So korrelieren Artenzahl und Diversität beider Organismengruppen höchst signifikant negativ mit dem Ausbaugrad der Ufer. Als Resultat der verschiedenen Belastungen findet sich in der Berliner Stadtspree eine extreme Dominanz von wenigen sehr anspruchslosen, toleranten Arten. Folglich sind Verbesserungen des ökologischen Zustandes und des Besiedlungspotenzials für wirbellose Bodenorganismen und Fische neben der Reduzierung der negativen Auswirkungen der Mischwasserentlastung, vorrangig durch Aufwertung der Uferstrukturen zu erreichen. Strukturelle Aufwertungen der Ufer müssen zusätzlich die hydrodynamische Belastung durch den schiffsinduzierten Wellenschlag berücksichtigen, um einerseits das Besiedlungspotenzial zu erhöhen, andererseits die Ufer vor Erosion zu schützen. Die vorliegende Studie gibt Hinweise auf die Möglichkeiten und Grenzen einer Revitalisierung der Berliner Stadtspree und der Kanäle am Beispiel der Fischfauna, indem sie die wesentlichen Belastungen und deren Auswirkungen skizziert. Potenzielle Maßnahmen zur Aufwertung der Uferstruktur sollten aufgrund der oben angesprochenen sehr ähnlichen Wirkmechanismen zwischenUmweltausprägungen und Zusammensetzung der Lebensgemeinschaften beiden, wirbellosen Bodenorganismen und Fischen, zu Gute kommen. Zur Beurteilung möglicher struktureller Maßnahmen wird zunächst davon ausgegangen, dass die negativen Auswirkungen der Mischwasserentlastung derart minimiert werden können, dass sie keine akute Beeinträchtigung der Wasserqualität und der aquatischen Lebensgemeinschaften mehr verursacht. Des Weiteren soll beurteilt werden, ob durch solche Maßnahmen ein Lebensraum für Fischarten geschaffen werden kann, die höhere Ansprüche an die Sauerstoffbedingungen im Gewässer haben als die aktuelle Lebensgemeinschaft.

Wet weather discharges from urban catchments are widely recognised as a major cause of unsatisfactory receiving water quality. Among stormwater discharges the impact from combined sewer overflows (CSO) plays a prominent role. The dynamic character of the discharge events lead to particular stress on the water bodies. Legal requirements for CSO follow the precautionary principle and usually set emission standards. Within the Urban Waste Water Treatment Directive 91/271/EEC of May 1991 it is written that “member states shall decide on measures to limit pollution from storm water overflows”. The directive does not give standards but solely proposes that “such measures could be based on dilution rates or capacity in relation to dry weather flow, or could specify a certain acceptable number of overflows per year”. The European Water Framework Directive 2000/60/EC of October 2000 goes beyond and asks for a combined approach to river basin management. On the source side, it requires that all existing technology-driven source-based controls must be implemented as a first step. On the effects side, it provides a new overall objective of good status for all waters, and requires that where the measures taken on the source side are not sufficient to achieve these objectives, additional ones are required. To assess the impact of CSO on the Berlin receiving water the research projects MONITOR and SAM-CSO are carried out in cooperation between Kompetenzzentrum Wasser Berlin, the Berliner Wasserbetriebe and the Senate Department of Environment Berlin. The objective of the projects is to identify and make available receiving water parameters (immission parameters) for the decision making process concerning the optimisation of the urban drainage system. Further on, a method for the evaluation of measures of combined water treatment on the basis of these immission criteria will be defined. The evaluation shall be based on both, available measurement data from the sewer system and the receiving water and simulations with an integrated model for the coupled drainage-river-system. The paper will present the methodology of the project. Special focus is on the description of the processes within the Berlin water bodies (stagnant lowland rivers) and the compilation of relevant physical-chemical and ecological parameters for the assessment of CSO.

The present study “Literature review on impact-based guidelines for stormwater treatment” provides an overview of international guidelines, which evaluate acute impacts of combined sewer overflows (CSO) on receiving surface water bodies. The overview should serve as a basis for the assessment of measured and simulated CSO impacts on Berlin surface waters within the projects “Monitor-1” and “SAM-CSO”, which are currently carried out at the Berlin Centre of Competence for Water. In contrast to the classical approach of sewer emission thresholds, impact-based guidelines focus on possible effects of CSO in the receiving surface water. Impact-based guidelines aim at deriving locally adapted measures to minimize CSO impacts to surface waters. Thanks to this local approach, potential protection measures can be planned dependent on the state of a specific river, reservoir or lake. The following study focuses on acute CSOimpacts, which were identified as relevant for the biocenosis of the River Spree in Berlin within the KWB project ISM: (i) Increased levels of unionised ammonium (NH3) through ammonium input. (ii) Low levels of dissolved oxygen (DO) through the input of degradable organic components, which lead to DO consumption. Guidelines from Germany, Austria, Switzerland, United Kingdom, France and USA are considered along with the approach by Lammersen, which assembles a number of scientific publications. The Austrian guideline (ÖWAV-RB 19) stops at distinguishing whether further investigations are necessary. In the US “CSO control policy” further analysis is delegated mostly to local institutions. The French “Arrêté du 22 juin 2007” also asks to take into consideration the local situation of the receiving water but does not give any limit values. The remaining four approaches provide a detailed evaluation scheme for critical NH3 and DO conditions, using duration-frequency-relationships. These relationships assume that pollution events of a specific duration may only occur in defined recurrence intervals (e.g. Figure 4.1). The Swiss guideline (STORM) is not suitable for dammed lowland river systems such as the Berlin River Spree, since it focuses on fast flowing rivers with salmonid fish populations. As a result there remain three approaches, which are interesting for the Berlin situation: the UPM guideline from the UK, the BWK-M7 guideline from Germany and the Lammersen-approach, which summarizes various scientific results. Apart from the dependency of critical concentrations on event duration and recurrence frequency, influence of temperature, pH and concurrent NH3-concentrations or DO-minima are considered by UPM and the Lammersen-approach. The relationships used by the three approaches for NH3 and DO are similar (see Figures 4.1, 4.3 and 4.4). Nevertheless, their comparability is limited, as the approaches generalize various local situations and cannot be derived strictly scientifically. As a first step we therefore recommend applying the three approaches to existing data from the River Spree and count the respective numbers of critical events. Based on the results it is possible to assess to which extent each approach is applicable for the situation in Berlin. As a second step experts need to evaluate the resulting critical events to distinguish suboptimal from lethal situations. For instance, the Lammersen-approach judges both (i) a two-day period with DO < 5 mg L-1 and (ii) a 30-minutes event with DO < 1.5 mg L-1 as critical. However in the Berlin River Spree (i) occurs basically continuously throughout the summer season and is tolerated by local fish species, whereas (ii) would probably lead to a major fish kill. As a consequence the prevention of (ii) should be given first priority. Based on the experience gained from the assessment of river monitoring data, simulation results can be evaluated in a third step. All the considered guidelines propose numerical simulation of sewer and receiving surface water systems. However only simple model approaches are discussed in detail, while specialized literature is suggested for complex cases. If numerical simulations are used for the planning of concrete measures, model uncertainties must be indicated to avoid feigning accuracy of results that cannot be provided. The Swiss STORM guideline suggests using Monte-Carlo simulations to calculate probabilities of the recurrence of critical events for possible management measures. We suggest a similar approach for the Berlin situation. Thus, decision makers could weigh cost against probability of success for proposed measures.

Urban water courses are considerably degraded in terms of their hydrology, riparian and channel morphology, substrate heterogeneity and habitat features as well as water and sediment quality. In addition, the combined sewer overflows and the ecotoxicological impacts of its components lead to a change of the physical-chemical and microbial mass balance affecting the biocenoses of higher trophic levels. Combined sewer overflows are therefore an additional stress to the ecological status of the urban course of the River Spree and of its channels, which is damaged already by both preload and background load of the aquatic environment. With regard to the assessment of the ecological water status, the European Water Framework Directives gives priority to the aquatic biocenoses in their capacity as ecological quality parameters. Against this background, an immission-oriented approach for the assessment of combined sewer overflows has to describe also their impacts on the biocenoses of the macrozoobenthos, the fish fauna, the macrophytes and the phytoplancton. These biocenoses are protected against the harmful impacts resulting from CSO only if the modification of their physical and chemical environment is avoided or reduced to an ecologically tolerable level respectively. In case that unfavourable impacts cannot be completely eliminated, the degree of impairment and the number of damaging CSO discharge events, which appear to be acceptable, should be defined. The present study is based on the bibliographic study „ Impact of urban use on the mass balance and the biocoenosis of lowland rivers under special consideration of combined sewer overflows” and deals with the assessment of CSO impacts on the ecological situation of the urban Spree and the channels (Cyprinid water bodies). In general, the immissionoriented assessment of CSO impact on the biocenoses (macrozoobenthos, fish fauna) requires the observation of the intensity, duration and frequency of occurrence of the individual events based on the assumption that, due to the background pollution, top priority is currently given to the acute CSO impacts. Requirements for the protection of aquatic biocenoses are developed with regard to the target parameters oxygen and ammonium/ammoniac and ecological tolerances of the biocenotic subjects of protection, which are strongest influenced by CSO. Initially, it is discussed to what extent the already existing results from laboratory investigations can be transferred to field situations. Next to the commonly accepted threshold values for oxygen concentrations during continuous persistent loads, particular requirements for the oxygen balance in case of peak loads are formulated.

The objective of the studies performed in the scope of the Integrated Sewage Management (ISM) project on combined sewer overflows in Berlin, Germany was to develop methods that would make it possible to assess wastewater management measures performed under the city’s water management permit as well as more sophisticated strategies (e.g., global real time control) through the application of water body-related criteria. For this purpose, a preliminary study was first performed to characterize the underlying water body-specific processes and hydraulic, physical, chemical and ecological parameters relevant to the status of Berlin’s surface waters (LESZINSKI et al., 2007a). The second step involved the development of a method for water quality-oriented assessment of wastewater management measures (LESZINSKI ET AL., 2007b). In addition to the already recognized thresholds for dissolved oxygen concentration during continuous, long-term water load conditions, particular focus was placed on formulating requirements for oxygen demand under peak load conditions. Ammonia toxicity due to sewage input, another important stress factor for aquatic ecosystems, was also analyzed and threshold values for both chronic and acute peak ammonia loads were defined. The results of the third phase of this research are described in this report. Two numerical simulation models (for urban drainage networks and surface waters) were combined and the feasibility of the developed method was evaluated based on the case of a combined sewer overflow event documented by the surface water monitoring. The simulations were performed using InfoWorksTM CS hydrological/hydrodynamic urban drainage network modeling software (ISM model) and the GERRIS/HYDRAX/Qsim unsteady ecosystem modeling system. The latter model was developed by the Federal Institute of Hydrology in Koblenz and is used by the Senate Department of Health, Environment and Consumer Protection (SenGesUmV). The present report describes the theoretical principles of the utilized models, the base of data available for analysis of the selected event, and the assumptions made in cases of missing input data for hydraulic modeling as well as for the water quality simulations. The one-dimensional hydraulic modeling results for the branched surface water system of the reach Berlin-Charlottenburg demonstrated that the hydraulic conditions can be simulated with satisfactory accuracy using the current data. In the case of water temperature, it was also possible to achieve a high degree of agreement between the measured and computed values in spite of the lack of highresolution temporal input data from the tributaries (Landwehr Canal, Panke River, BerlinSpandau Ship Canal). However, this was not the case for dissolved oxygen concentration, the main parameter used for evaluation of combined water treatment. The DOC simulations computed using input data based on a monthly sampling interval did not show satisfactory agreement with the online measurements in the water system. Dry-weather biological processes, which were associated with high-level, short-term oxygen enrichment or consumption, could not be depicted in the simulations. After completion of the water quality simulations, the effect of variation of individual input parameters was assessed. This analysis showed that no significant improvement of agreement with the measured values could be achieved by adjusting the assumptions for individual parameters (chlorophyll-a and BSB5). In the case of ammonia, the second most important parameter, the available sampling data from the tributaries in the investigated water system were collected only once a month, if at all. Therefore, it cannot be expected that the temporal distribution of this parameter was correctly reflected by the model. The number of validation measurements taken within the water system was also insufficient. Summarizing the results of the study of the linked urban drainage/surface water quality model, which was tested for the first time, it can be concluded that InfoWorks CS and GERRIS/HYRDRAX/Qsim provide problem-oriented simulation tools for reaching the objective of ISM study of assessing various scenarios for reduction of impacts from combined sewer overflows. By contrast, the available data are deficient and do not allow to adjust and calibrate the models to meet the specific needs of this task, particularly in light of the fact that short-term effects of combined sewer overflows are to be analyzed.

Urban water courses are considerably degraded in terms of their hydrology, riparian and channel morphology, substrate heterogeneity and habitat features as well as water and sediment quality. In addition, the combined sewer overflows and the ecotoxicological impacts of its components lead to a change of the physical-chemical and microbial mass balance affecting the biocenoses of higher trophic levels. Combined sewer overflows are therefore an additional stress to the ecological status of the urban course of the River Spree and of its channels, which is damaged already by both preload and background load of the aquatic environment. With regard to the assessment of the ecological water status, the European Water Framework Directives gives priority to the aquatic biocenoses in their capacity as ecological quality parameters. Against this background, an immission-oriented approach for the assessment of combined sewer overflows has to describe also their impacts on the biocenoses of the macrozoobenthos, the fish fauna, the macrophytes and the phytoplancton. Initially, the most important factors, mechanisms and processes determining the mass balance of a water course are described. Particular attention is given to the mass balance of eutrophic lowland streams and rivers and of river-lake–systems. In this context, the abiotic mass balance is discussed together with the biotic use of resources. After introducing the basic processes of the mass balance, the impacts of the anthropogenic use on these processes are subsequently described with regard to Berlin’s specific water resources environment. The result is a compilation of the hydraulic, physical-chemical and ecological parameters relevant to Berlin’s water resources serving for water quality assessment purposes. Starting from the ecological processes disturbed by the anthropogenic use, the potential effects of the combined sewer overflow are examined. The parameters selection is concentrated on the essential processes connected to combined sewer overflow issues. Based on the large number of stress factors and their interactive impact system, those influences of the combined sewer discharge are worked out which have to be categorised as particularly jeopardising and which are important target values for the future water quality simulation. Due to the high background load, the highest priority has to be given to the acute load caused by nutrients and carbon load peaks resulting from combined sewer discharges, since they overcharge the self-cleaning potential of the urban course of the River Spree and its channels. Even if the organic substances and the chemical contaminants discharged lead to chronic loads, the main objective is to avoid to the greatest possible extent the temporary but extremely hypoxic conditions, since combined sewer overflows cause fish die-offs when the water resources situation is already critical. Primarily, the water quality modelling has to be concentrated on the realistic mapping of the highly dissolved concentration charts of the target parameters oxygen and ammonia, since the degree of the biocenoses’ damage is rather determined through discharge duration, discharge intensity and frequency than through the medium rates of pollutant loads.