An Integrated Catchment-Scale Approach to Urban River WaterQuality Using Constructed Wetlands

Urban catchments include land, groundwater, sewer, river, and other water components. Together, these elements form a complex, integrated urban water system. Managing river water quality in such systems is particularly challenging due to built (grey) infrastructure, which increases pollutant impact through impervious surfaces and increases stormwater runoff, limiting natural filtration processes. In response, many cities have begun to adopt constructed wetlands (CWs) as natural (blue-green) infrastructure to improve river water quality at the catchment scale. Despite their growing use, several challenges persist, including how to quantify the impact of CWs on river water quality, optimise the design of multiple wetlands, and apply these insights to catchment[1]wide planning. This study addresses these challenges by introducing an integrated planning and design framework for CWs aimed at improving water quality across urban catchments. Specifically, the framework focuses on (1) assessing pollutant removal by CWs, (2) designing CWs locally, and (3) integrating CWs into larger catchment plans.

To develop and test this approach, we first created a CW module within the Water Systems Integrated Modelling (WSIMOD) framework, enabling the simulation of interactions between CWs and other water components in urban catchments. We then applied this module to the Pymmes and Salmon Brook catchments in the UK to evaluate river water quality before and after constructing CWs. Next, we used the model to explore various design variables (e.g., area, size, configuration) for placing new CWs within each sub-catchment, quantifying their effectiveness in improving river water quality. Finally, we propose a guiding principle for CW planning based on these findings, illustrating how different spatial layouts affect the achievement of nitrogen and phosphorus targets within sub-catchments.