The Soil and Water Assessment Tool (SWAT) has been applied to the Ic watershed, Brittany, France, to evaluate scenarios for reduction of nitrate in stream water. For the simulated period the model showed fair results with a mean index of agreement of 0.64 at the watershed outlet for discharge and nitrate loads. The management goal for the watershed is the meeting of drinking water threshold at the watershed outlet. An analysis of observed data revealed that nitrate loads would have to be reduced by at least 17% on average to reach that goal. Scenarios investigated cover fertilizer reduction and the introduction of wetland buffer zones. Decreased nitrogen inputs were realized on a) selected subbasins and b) all agricultural fields; wetlands were placed at three model subbasins. Most effective measures were a 50% fertilizer decrease on selected subbasins resulting in a range of 13 22 % reduction of nitrate loads with a high uncertainty. Consequently, none of the tested measures is likely to achieve a sufficient reduction. Combined measures such as enhanced fertilizer management and concurrent introduction of wetlands seem to be the most promising way to approach the drinking water threshold.

The Aquisafe project is a cooperation of the Indiana University Purdue University Indianapolis (IUPUI, USA), the German Federal Environment Agency (UBA, Germany) and the Berlin Centre of Competence for Water (KWB, Germany). The aim of the project is the development of a scheme for natural mitigation zones to protect surface waters from diffuse pollution in rural and semi-rural environments. In particular, key contaminants, applicable management and modelling tools and potential substance removal by constructed wetlands or riparian zones are being studied. Within these frameworks, two case studies are carried out in Brittany, the number one agricultural region in France. A hydrological model is currently being applied on the Ic catchment (92 km2) to test its capability of (i) understanding hydrological, basin-scale regimes, (ii) predicting the effect of mitigation measures and (iii) distinguishing diffusion pathways for different types of contaminants. In the second case study, a constructed wetland in Iffendic on the River Meu is monitored as an example of a natural and inexpensive mitigation option. On the way through the wetland nitrate concentrations from drainage inflows to the river decreased more than tenfold. In the ongoing monitoring, knowledge on hydrological flowpaths is improved to be able to quantify the retention potential of constructed wetlands in Brittany for nitrate and other agriculturally-based pollutants, such as pesticides.

The study “Trace pollutants in combined sewer overflows” provides an overview on the input of trace substances (organic or inorganic trace substances) to surface waters during combined sewer overflows (CSO). The study outlines substance pathways, types of substances, expected loads as well as possible impacts on the receiving water. The study shall aid the discussion and further handling of trace substances within the project Monitor-1, which is currently carried out at the Berlin Centre of Competence for Water (KWB). The study has identified more than 300 substances, which could reach the Berlin surface water bodies via CSO. Moreover, it is assumed that there is a large number of substances and metabolites, which are still unknown. Sewage-related substances in combined sewers can stem from: household products (e.g. surfactants from cleaning agents), leaching (e.g. amines from textile colours or Bisphenol A from plastic coatings), wash-off of cosmetic products (e.g. Benzophenone-3 from sunblocks) or health lotions, excretion of ingested products (e.g. the pain killer Diclofenac). Stormwater-related substances in combined sewers can stem from abrasion from car and railway traffic (e.g. cadmium from break lining abrasion), erosion of building materials (e.g. copper from eaves gutters), application (e.g. glyphosate for weed control on pavements) or atmospheric deposition (e.g. polychlorinated dibenzofurans from exhaust fumes). In the framework of this study available substance data was assembled, containing: general information like synonyms and CAS-No., chemical properties, elimination rate in wastewater treatment plant (WWTP), observed concentrations in surface waters and toxicity. A complete list of the substances as well as selected substance properties is attached in table A6 (Appendix). Some of these substances can be used as tracers to distinguish different pollution pathways to surface waters. Suitable indicators should enter surface waters mainly via one pathway, their half-life in surface waters should be sufficiently long, concentrations should show no seasonal fluctuations and they should be well-measurable. For instance, caffeine is a good indicator for inflows of untreated sewage via CSO, as it is very well degraded in WWTP. As an indicator for treated sewage the almost nondegradable anti-epileptic Carbamazepine could be used. Finally, polycyclic aromatic hydrocarbons (PAH) could be used as indicator for stormwater-related pollution. In road traffic, PAHs result from incomplete combustion processes, adsorb on atmospheric particles, deposit on the surface and are washed off by rainfall runoff. In this study the relevance of the path “CSO” for trace substance emissions from the Berlin wastewater system in comparison to the other paths (storm drainage and WWTP) was assessed. Therefore, a simple balance of the fraction of CSO in total emissions to the Berlin surface waters was carried out. The balance was based on the annual volumes of WWTP effluents, storm drainage and CSO. Due to a lack in substancespecific measurement information the balance was calculated dependent on the elimination rate of substances in WWTPs. Based on the resulting figures A, B, C, D and E the fraction of CSO in total loads to the Berlin surface waters can be assessed for each substance with known elimination rate in WWTPs. In the study we distinguished between: (i) sewage related substances / stormwater related substances, (ii) balance area „Berlin total“ (5 WWTPs with 622,000 m³/d, 97 km2 combined sewer system, 231 km2 storm drainage system) / balance area „Berlin city centre“ (60% of the WWTP Münchehofe with 39,000 m³/d, 83 km2 combined sewer system, 112 km2 storm drainage system) and (iii) total annual loads / event based loads.

The Aquisafe project assesses the effectiveness of natural mitigation zones in reducing diffuse pollution to surface waters. In one case study on a constructed wetland in agriculturally dominated Western France, nitrate concentrations from drainage inflows to a small river decreased up to tenfold on the way through an intermediary constructed wetland. However, only ~30 % of the total N-load is retained in the wetland, whereas ~70 % enters the river directly during high flow events as a result of low soil permeability. The study underlines the importance of flow paths and infiltration for nitrate removal in natural or constructed wetlands, which is often neglected in practice.

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.