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Combined sewage is a mixture of domestic sewage and stormwater runoff. Combined sewage is typically conveyed away from urban areas through underground sewer systems and treated at wastewater treatment plants (WWTP). However, during medium and large storms the underground sewer systems are overloaded and, to protect the city assets from uncontrolled flooding, several emergency outlets are built along the network. These outlets, denoted as combined sewer overflows, ensure that excess water is discharged into nearby streams and other surface waters during periods of overloading. Similar structures are located at WWTPs to divert any flows exceeding the maximum treatment capacity of the plant (the so-called bypass flows). On an annual basis only a minor fraction of the total combined sewage volume is discharged through these structures, but during very extreme storms the magnitude of wet discharges can result in a manifold flow increase to the overall river flow compared to dry weather conditions. Together with emissions from separate stormwater systems, these emissions are denoted as wet weather discharges.Wet weather discharges were studied intensively both nationally and internationally in the period from 1975–2000. It was recognized that especially emissions from combined sewage during wet weather heavily impacted small creeks and lakes (Eriksson et al., 2007a; Kjølholt et al., 2001). Large measurement campaigns were initiated to quantify these emissions and to establish causal relationships between the emissions and impacts. Impacts were often divided into physical changes (erosion, deposition), aesthetical pollution, eutrophication, pathogenic pressure, oxygen depletion, toxic and/or xenobiotic components, and changes in the ecosystem (community dynamics) within and around the surface water.The implementation of related environmental legislation in Denmark and across the EU during the period 1987–2000 mainly focused on continuous emissions. Construction of wastewater treatment plants for industrial emissions, as well as domestic sewage has reduced the annual loadings to surface waters substantially during the last decades. The enactment of the Water Framework Directive (2000/60/CE), however, moved the focus from single discharge points to a more holistic evaluation of the ecological status of the receiving water body. Although not directly addressed, intermittent discharges play an important role in affecting the overall quality of the receiving water body. Some surface waters have clearly improved their status thanks to the implementation of these environmental legislations and the construction of treatment plants, while other surface waters still struggle to achieve the desired quality standard.In general, the following types of measures are considered to further improve the chemical and/or ecological status:• Reduction of emissions from continuous sources (e.g. waste water treatment plants, industrial emissions, agriculture).• Reduction of emissions from intermittent (e.g. wet weather) sources.• Change in land use and/or banning of specific compounds in the area.• Change in (base) flow of surface water.• Biomanipulation of the aquatic ecosystem to favour its transition to a better ecological status.• Changes in hydromorphological (physical) conditions (river aeration, re-meandering, etc).Furthermore, urban areas are affected by processes that are intensifying the pressure on receiving water bodies caused by wet-weather discharges. These processes include increasing urbanization and subsequently impermeabilization of existing urban areas, which lead to a rise in the runoff flows and volumes. Changes in rainfall patterns caused by climate change can also contribute to more frequent and/or greater wet-weather discharges. These changes might contribute to deteriorate the current status of receiving water bodies, requiring additional measures.This report revisits the importance of wet weather discharges, notably combined sewer overflows, in relation to the goal of achieving good chemical and ecological status of surface waters. The report mainly focuses on the discharges of ammonium/ammonia, organic matter, and the resulting oxygen depletion, as based on a review of the existing scientific literature, they cause the most evident negative impacts on the water bodies, and it is possible to quantify cause-effect relationships between discharges and the status of the receiving water body.• Chapter 2 defines all the elements of the integrated urban water system that are considered in the report, and outlines our understanding of how the different elements interact, which may affect the chemical and ecological status of the receiving water body as a consequence of wet weather discharges.• Chapter 3 discusses our current understanding of the pollutants discharged through wet weather discharges, providing an overview of existing measurements and comparing them to environmental quality standards for good chemical status.• Chapter 4 describes the indicators that are used to evaluate the impacts of wet weather discharges on the ecological status of the receiving water body. A specific focus is given to identifying a link between insufficient ecological status and existing stressors, including combined sewer overflows.• Chapter 5 takes a holistic perspective by exploring our understanding of the causal relationships existing between wet weather emissions and good ecological status, including stress-factors affecting the ecological status other than wet weather discharges.• Chapter 6 describes the state-of-the art in relation to the monitoring of water quality (both at discharge points and the receiving water body).• Chapter 7 provides an overview of current regulation approaches for overflow discharges and water quality criteria at the international level.• Chapter 8 illustrates the different modelling approaches that can be used as tools to interpret the integrated measurements coming from monitoring programs, and thus to support future decision-making (i.e. next generation regulations).• Chapter 9 then proposes a procedure to establish operational guidelines for the regulation of wet weather discharges from urban areas.

Increasing modifications in land use and water management have resulted in multiple stressors impacting freshwater ecosystems globally. Chemicals with the potential to impact aquatic habitats are still often evaluated individually for their adverse effects on ecosystem health. This may lead to critical underestimations of the combined impact caused by interactions occurring between stressors not typically evaluated together, e.g. xenobiotic groundwater pollutants and trace metals. Although water managers need a set of measures for determining water body status, approaches capable of clarifying the many existing discrepancies between chemical and ecological status are still urgently needed.To address this issue, we identified sources and levels of chemical stressors along a 16-km groundwater-fed stream corridor (Grindsted, Denmark), representative for a mixed land use stream system. Potential pollution sources included two contaminated sites (factory, landfill), aquaculture, wastewater/industrial discharges, and diffuse sources from agriculture and urban areas (Sonne et al., 2017). Ecological status was determined by monitoring meiobenthic and macrobenthic invertebrate communities.The stream was substantially impaired by both geogenic and anthropogenic sources of metals throughout the investigated corridor, with concentrations close to or above threshold values for copper, nickel and zinc in the stream water, hyporheic zone and streambed sediment. The groundwater plume from the factory site caused elevated concentrations of chlorinated ethenes, benzene and pharmaceuticals in both the hyporheic zone and stream, persisting for several km downstream. Impaired ecological conditions, represented by a lower abundance of meiobenthic individuals, were found in zones where the groundwater plume discharges to the stream. The effect was only pronounced in areas characterized by high xenobiotic organic concentrations and elevated dissolved iron and arsenic levels – linked to the dissolution of iron hydroxides caused by the degradation of xenobiotic compounds in the plume. The results thus provide ecological evidence for the interaction of organic and inorganic chemical stressors, which may provide a missing link enabling the reconnection of chemical and ecological findings. This study highlights the importance of stream-aquifer interfaces for ecosystem functioning in terms of biological habitat, and that multiple stressor systems need to be tackled from a holistic perspective.