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Care for the environment is not the only, but certainly a significant reason to separate storm and sewage water. A separate collection and discharge of storm-water may however result in direct discharge of pollutants from urban surfaces into receiving surface waters. The “Environmental Improvement” subproject studied the possibility that other, new environmental problems are created due to the measures taken in separation of storm-water and how to monitor this feature.
 Measures taken within NORIS focused on disconnection of stormwater via various technologies, and at complementary measures to treat stormwater quality. The pollutants in the storm water originate from precipitation itself, runoff or false connections. These media can roughly contain major pollutant sources as sediment, organic chemicals, bacteria, nitrates and phosphates, fertilizers, herbicides, oil, road salts, faeces, and metals etc.
Storm water and its complex content can change the hydrology, and ecosystem quality of a receiving water body. The characteristics of the water body and physical, chemical and biological processes taking place determine the type and size of impact. Furthermore, the intensity duration and frequency of storm water discharge most probably play a role as well.
Predicting ecological effects of storm water discharge to surface water in relation to measures taken is difficult and depends on:
- The variability in quality and quantity of storm-water. Reoccurrence of rain events causes a frequent discharge of varying complex mixtures to the surface water.
- Type of measure taken
- Intrinsic vulnerability of the ecosystem. Each type of ecosystem has its own susceptibility on addition of compounds.
Monitoring project:
 At the start of NORIS general knowledge about stormwater quality in combination with its impact on surface water quality and ecosystem was limited. Within the Wieringermeer pilot a large monitoring campaign focussed on long term (>>20 years after disconnection) and short term (<48 hours) environmental impact of stormwater discharge.
The pilot “Wieringermeer” of the NORIS project was aimed at disconnecting storm water from the sewer system and to evaluate the impact of stormwater discharge into the receiving surface water. Measures as disconnection of stormwater will result in reduced number of Combined Sewer Overflows (CSO’s), directly resulting in a reduced load of pollution to the aquatic ecosystem. However, disconnection of stormwater from the sewer system will reduce the number of CSO’s but will be replaced by discharge of stormwater directly to the surface water (in case of Wieringermeer). This storm water contains a mixture of compounds as it flows over roofs, roads and other surface before it enters the water.
General question in the monitoring project to be answered was:
"Do measures as “NO rainwater In Sewers” by disconnecton result in a better surface water quality?"
 More specifically, the general question was addressed to effects of discharging stormwater on the long term, and on the short term, and based on both chemical and ecological parameters.
Long term effects were defined by ecosystem “equilibrium” after 20+ years of receiving storm water. Parameters were ecology based (according to the WFD diatom & macrofauna diversity) and chemical based (e.g. several metals, mineral oil, PAHs, in both water and sediment). Short term effects were defined as changes within the ecosystem shortly (<24-48 hours) before and after stormwater discharge. Parameters were almost the same as described above.
Long term impact
Monitoring revealed hardly any long term effects of storm water run off on diatom and macrofauna species communities as this depended largely on intrinsic ecosystem features (salinity, sediment structure, nutrient status). However, Wieringerwerf’s diatom and macrofauna community was structured by specific intrinsic ecosystem drivers (N/P ratio) and the contribution of storm water discharge was therefore hard to detect. Only 14% of discharge compounds were shifting the community (Cu, EOX, Cd).
Short term impact
Short term effects were mainly denoted to increased dynamics in salinity, and increase of metals (total and dissolved Zinc and thus bio available). The concentrations of zinc in ponds after run off exceeded the Maximum acceptable Concentration, indicating temporal acute potential risk for the aquatic species community present.
Quality of storm water run off
Quality was determined by chemical parameters and toxicity endpoints (e.g mortality of crustaceans). The quality of the stormwater varied in time and space. Biological tests (bio assays) with stormwater, combined with sophisticated chemical assays (Toxicity Identification and Evaluation (TIE) revealed an extraordinary group of metals to cause mortality for crustaceans:
Metals specifically present in stuccoworks (e.g. cobalt, vanadium) were responsible for the observed mortality of animals in the laboratory. The metals could be attributed to a local plasterer who rinsed and emptied his buckets in the stormwater inlets. This example shows the importance of clear communication with inhabitants regarding the quality of storm water run off, and its environmental consequences.
Conclusions
Ecosystem susceptibility to effects of stormwater run off depends on local run off quality but as well on intrinsic ecosystem vulnerability and resilience to specific compounds. The intrinsic dependency requires a specific ecosystem effect assessment. An ecotype approach is thus most wanted to assess the risk of regional vulnerability towards urban water discharge, and the potential impact of stormwater run-off and disconnection. Only after such an assessment feasible measures within urban water management can be taken.
Protocol and workshops
Besides the pilot project in Wieringerwerf several workshops on “environmental impovement” were fascilitated. Discussions on future practise in the scope of WFD were helt, and an inventory on environmental questions and possible answers were some of the products of these wokrshops.
 A protocol was set up to help stakeholders in assessing the environmental impact of the measures that are taken in the pilot projects. To take the differences of each practical situation into account, three levels of certainty were proposed to evaluate environmental improvement/Impact (see figure 1); monitoring, modeling and expert judgment.
An environmental assessment, independent on which level of certainty, forces stakeholders to think about environmental issues in a systematic way, and therefore it is certainly preferable to not performing the impact assessment at all.
The most accurate information (level I) on environmental impact is obtained by performing monitoring activities before and after the measure has been taken. Regarding the impact emission of pollutants, the following monitoring parameter-set is proposed as relevant in field monitoring: Chemical oxygen demand; suspended solids, Metals (Zn); Diversity of diatoms and macro invertebrates.
Prediction of environmental impact (level II) by modelling is sometimes more relevant in certain situations. The disadvantage is that models require large amounts of reliable input data in order to obtain reliable output data, and that there is a large but usually unknown uncertainty involved with the output data.
When measurements and modelling are both not feasible, a prediction of environmental impact of measures may be obtained by expert judgement, based on as much information and experience as possible (level III). This is the least expensive but also least reliable way to assess environmental impact of measures that are taken.
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