Predicting infiltration and pollutant retention in sustainable drainage systems: Experiments, modelling and design

A major problem of increasing urbanization is the rise in pollution caused by runoff, affecting water quality directly and due to combined sewer overflows. Among alternative strategies, Sustainable Drainage Systems (SuDS) such as rain gardens and other bioretention facilities is becoming more widespread. Previous research has focused primarily on hydrologic design, including the degree to which groundwater is replenished by these systems, and models have been developed to quantify the extent of that enhanced focused recharge. However, there are few tools for their design that adequately consider pollutant retention. We have developed a numerical model that simulates infiltration into different area systems with porous media of up to three layers, and can simulate movement and accumulation of metals considering macropore flow. This model is here further validated using new laboratory column results (for matrix and macropore flow), and applied to the design of a rain garden system for a planned roundabout in Kent, U.K, considering climate change scenarios. Results using past and potential future climate series show levels of lead can build up in the upper layers of the system, but only constitute a health hazard after approximately 10 years (when a management intervention such as replacing the upper few cm of soil is required). Implications and future opportunities are discussed.