Construction of water-soil-plant system for rainfall vertical connection in the concept of sponge city: A review

The green infrastructure (GI)/low impact development (LID) facilities aim to control the runoff and its associated pollutants in the context of climate change and accelerated urbanization, so as to make the urban development areas as close as possible to the natural hydrological cycle. Compared with the fragmented facilities construction and research, the large-scale LID planning require deep-going understanding of the impact of LID spatial allocation on surface–subsurface hydrologic dynamics, and the stability of performance for these facilities with the extension of operation time. Based on the analysis of eco-hydrological effects brought by urbanization, the water-soil–plant continuum (WSPC) system for rainfall runoff under the concept of sponge city is proposed. Current technical challenges about parameter optimization, continuous operation, coupling simulation for bioretention facilities are identified and recommendations to overcome them are provided. This new concept points out that source, form and species of non-point source pollutants (NSP). In particular, heavy metals, persistent organic pollutants (POPs) and micro plastics in runoff and their risks to bioretention soil media (BSM). In addition, mulch, litter and their decomposition process under the influence of weather and rainfall conditions; media modification technologies based on solid waste recycling; cumulative effects of pollutants and its ecological risks to soil and groundwater; simulation and optimization of the structural parameters according to local conditions, these four aspects are the bottlenecks of the application for such facilities. The current review critically points out future research directions about the attenuation process for infiltration function, ecological risk for pollutant accumulation, surface-soil-underground coupling simulation. Among them, the runoff-soil-groundwater vertical coupling simulation consists of the following three parts: surface runoff and confluence model, soil pollutant migration and fate model, groundwater simulation model, and their water-balance and energy-balance. This study will contribute to a greater understanding and key knowledge gaps in the optimization and long-term operation for GI/LID system.