Impact of urban areas' drainage system on the quality of water bodies and mitigation strategies

Diffuse urban pollution is a significant factor in compromising receiving water and groundwater standards required by the EU Water Framework Directive. Many studies (e.g., Ashley et al., 2005; McGrane, 2016) show that changes in the built environment and climatic forces contribute to the increase of combined sewer system overflows and of stormwater directly conveyed to nearby water bodies. These discharges are responsible for receiving water contamination, as a result of high concentrations of pathogens, BOD, suspended solids (SS), hydrocarbons, heavy metals and nutrients, thus being a significant source of water bodies’ pollution.

To mitigate/eliminate pathogens and BOD contamination sources, the combined drainage system is usually split into separated sanitary sewer and stormwater systems, although difficulties related to economic and technical feasibility may be relevant. Nevertheless, this solution does not solve completely pollution due to SS, hydrocarbons, heavy metals from urban areas runoff and nutrients from rural drainage.

Sustainable Drainage Systems (SuDS) deal with stormwater at source, helping infiltration and storage of water, increasing groundwater recharge, and reducing peak flood and volume in the drainage system. Moreover, filtration processes through porous media may reduce pollutants driven by first flush, usually controlled by stormwater tanks and sewer system spillways. However, clogging phenomenon limits drainage efficiency in the long-term, making sometimes porous media itself a source of contamination.

In the following, a PhD project focusing on the urban area of Treviso is illustrated. Treviso is crossed by the river Sile, one of the longest (95 km) European wellspring rivers, part of a protected area. The Sile river is polluted by discharges from both the existing combined sewer system and rural drainages.

While responsible agricultural practices will be promoted to mitigate the pollution originating from rural areas, a project aims to separate the combined system, developing a new pipe network for sanitary wastewater. When properly applied in the present drainage system devoted to the stormwater control only, SuDS solutions are able to mitigate pollution coming from wash-off and reduce flood peaks.

Discharge measurement stations will be realised on the Sile river upstream and downstream the Treviso town, to quantify drainage system outflows of the urban area during rainfall events and in dry conditions. Sampling for qualitative analysis will give a measure of the pollutants’ concentration.

SuDS solutions, e.g. porous pavements, infiltration trenches and vegetated swales, will be tested with laboratory equipment (6×2 m²) capable of considering the runoff and underground drainage in a fully controlled environment subjected to a prescribed rainfall intensity. By this way it will be possible to analyse the main physical processes and assess the SuDS solutions’ efficiency both in the short and long-term, using advanced mathematical models for the interpretation of results.

If the laboratory model will provide satisfactory results, a full-scale test will be developed on an experimental site in Treviso town. The installed qualitative and quantitative monitoring system will allow to determine the effectiveness of the solutions adopted.