Substance flow analysis as a tool for urban water management

Human activity results in the production of a wide range of pollutants that can enter the water cycle through stormwater or wastewater. Among others, heavy metals are still detected in high concentrations around urban areas and their impact on aquatic organisms is of major concern. In this study, we propose to use a substance flow analysis as a tool for heavy metals management in urban areas. We illustrate the approach with the case of copper in Lausanne, Switzerland. The results show that around 1,500 kg of copper enter the aquatic compartment yearly. This amount contributes to sediment enrichment, which may pose a long-term risk for benthic organisms. The major sources of copper in receiving surface water are roofs and catenaries of trolleybuses. They represent 75% of the total input of copper into the urban water system. Actions to reduce copper pollution should therefore focus on these sources. Substance flow analysis also highlights that copper enters surface water mainly during rain events, i.e., without passing through any treatment procedure. A reduction in pollution could also be achieved by improving stormwater management. In conclusion, the study showed that substance flow analysis is a very effective tool for sustainable urban water management.

Probabilistic risk assessment of urban stormwater discharge

David Andrew Barry, Nathalie Chèvre Rossi, Luca Rossi

Urban stormwater is an important source of detrimental impacts for receiving waters. Discharges from separated or combined sewer systems are recognized as a major contamination input to the environment. Moreover, these discharges may impact the morphological aspect of receiving waters, like rivers and streams. But these impacts are difficult to handle. First, precipitation is the main driving force of the problem and has stochastic features, which must be taken into account. Second, some variables and parameters of the urban and receiving water systems are difficult to measure or to estimate (e.g., riverbed characteristics, number of inhabitants in a watershed, imperviousness, etc.) and therefore cannot be determined exactly. Finally, factors involved in the cause-effect relationship between urban stormwater discharges and the receiving water ecosystem, i.e., the assessment criteria, are difficult to describe with a linear relationship. To tackle this problem, we developed a new impact-based approach in Switzerland to allow engineers to plan adequately treatment devices and technical solutions. To support this approach, a probabilistic software tool has been proposed to assess impacts of urban drainage to receiving waters based on new impact-based criteria. The software includes probabilistic modeling facilities (Monte Carlo simulation routine) and sensitivity analysis to focus the data acquisition on the most important parameters. Long-term simulations (10-year duration) are performed using historical rain data from different Swiss locations for the simulation of an urban watershed, upstream watershed, and the receiving water’s hydrology and water quality. This long period allows us to account for variability in precipitation. The computation typically runs 200-500 iterations. Most variables and parameters’ uncertainties are taken into consideration by generating random values of a given multivariate distribution. The results, after sorting, are plotted as a cumulative distribution function that represents the probability (on the Y-axis) complying with a criterion (on the X-axis). The guidelines and the software were proposed to authorities and to experts active in the field of urban wet-weather and ecosystem management. This new approach was accepted by the majority of the experts, even if the complexity of the planning procedure generated skepticism from some users. The concepts of uncertainty and variability were well understood by users. In fact, they are commonly used in urban wet-weather and water ecosystem management, even if not explicitly presented. For example, the definition of watershed imperviousness, the choice of rain series, the initial rain losses, the impacts of chemicals on aquatic species, etc., are parameters that are commonly estimated in a given range of reliabilities. Examples of applications and presentations of scenarios were the best way to illustrate the benefits of using the new approach. Nevertheless, the interpretation of results is quite difficult and a communication strategy, such as organization of workshops and courses is a need for the acceptance of the methodology.

2010

Estimation des rejets urbains de zinc et de cadmium au moyen d'une analyse de flux de substance. Application à la commune de Lausanne

David Gremaud

This work is focused on the problem of the contamination of surface water and sediments by heavy metals, more particularly by cadmium and zinc, contained in runoff water and wastewater. To bring to light the urban cycle of cadmium and zinc, we have used the methodology of the substance flow analyses (SFA) and have elaborated a model. This model consists of various sources, flows, processes and stocks. The zone of study of this work is the municipality of Lausanne, situated at the edge of the Lake Geneva in Switzerland. This city possesses a bay; the bay of Vidy, which knows an important problem of pollution of sediments. Indeed, the values of concentration in heavy metals of sediments, in particular in cadmium and zinc, are superior to the limit values TEC and PEC (cf. chapter 6). The sewer system is of combined type and of separated type. Waters of the combined system are brought to the STEP of Vidy to be treated. On the other hand, waters of the separated system are sent without treatment to surface water. We have identified several sources of cadmium and zinc as domestic wastewater, car traffic, rail traffic, roofs and sidings of buildings, the dry and wet deposition. We have quantified the sources and coefficients of transfer of our model with data of the literature and hypotheses. We have put these values in SimBox, a program developed by the EAWAG, which allows to model the flows of substance and make analyses of sensibility and variation of the parameters. We have obtained the following results for the municipality of Lausanne. The contributions in sediments are 6.996 kgCd/year and 2961.11 kgZn/year. The contributions in surface water are 2.328 kgCd/year and 987.49 kgZn/year. The major sources of cadmium in surface water and sediments are the street furniture (70%), domestic wastewater (14%), roofs (8%) and sidings (3%). The major sources of zinc in surface water and sediments are the street furniture (24%), sidings (20%), roofs (19%), the tires of vehicles (16%) and domestic wastewater (14%). We have proposed several technical solutions to reduce the contributions of metals in surface water and sediments as the infiltration of runoff water in grounds or their discharge in wastewater (combined sewer), roads cleaning, pretreatment followed by a discharge in surface water. We have talked about the last two examples more specifically; roads cleaning and pretreatment followed by a discharge in surface water. The roads cleaning allows a reduction in sediments of 2.6% for cadmium and 3% for zinc. As for the filtering socks placed in the garbage dumps of roads, the reduction is 29% for cadmium and 15% for zinc.

2009

Impact of climate change on runoff pollution in urban environments

David Andrew Barry, Sylvain Coutu, Sébastien Kramer

Runoff from urban environments is generally contaminated. These contaminants mostly originate from road traffic and building envelopes. Facade envelopes generate lead, zinc and even biocides, which are used for facade protection. Road traffic produces particles from tires and brakes. The transport of these pollutants to the environment is controlled by rainfall. The interval, duration and intensity of rainfall events are important as the dynamics of the pollutants are often modeled with non-linear buildup/washoff functions. Buildup occurs during dry weather when pollution accumulates, and is subsequently washed-off at the time of the following rainfall, contaminating surface runoff. Climate predictions include modified rainfall distributions, with changes in both number and intensity of events, even if the expected annual rainfall varies little. Consequently, pollutant concentrations in urban runoff driven by buildup/washoff processes will be affected by these changes in rainfall distributions. We investigated to what extent modifications in future rainfall distributions will impact the concentrations of pollutants present in urban surface runoff. The study used the example of Lausanne, Switzerland (temperate climate zone). Three emission scenarios (time horizon 2090), multiple combinations of RCM/GCM and modifications in rain event frequency were used to simulate future rainfall distributions with various characteristics. Simulated rainfall events were used as inputs for four pairs of buildup/washoff models, in order to compare future pollution concentrations in surface runoff. In this way, uncertainty in model structure was also investigated. Future concentrations were estimated to be between ±40% of today’s concentrations depending on the season and, importantly, on the choice of the RCM/GCM model. Overall, however, the dominant factor was the uncertainty inherent in buildup/washoff models, which dominated over the uncertainty in future rainfall distributions. Consequently, the choice of a proper buildup/washoff model, with calibrated site-specific coefficients, is a major factor in modeling future runoff concentrations from contaminated urban surfaces.

2012