Urban Water Cycle Responses to Extreme Precipitation: Contaminant Mass Balance, Redistribution, and Transfer to Groundwater

Urban areas are characterised by continuous emissions of particulate matter from multiple anthropogenic sources, which accumulate mainly on impervious surfaces such as roads, parking areas, and rooftops. During precipitation events, especially extreme rainfall, these accumulated contaminants can be mobilised and redistributed throughout the urban water cycle. This study examines contaminant mass balance and redistribution during extreme precipitation at the scale of the urban water cycle in the regional pilot-case aquifer. Understanding these processes is essential for evaluating the effectiveness and long-term risks of infiltration-based stormwater management.

A multi-compartment sampling strategy is being implemented within an urban catchment to capture the key reservoirs and fluxes governing contaminant transfer within the system. Precipitation is sampled to characterise atmospheric washout, while street dust serves as an integrated record of contaminant accumulation on impervious surfaces. Surface runoff from sealed areas is monitored at gully pots, followed by sampling of water percolating through infiltration and filtration layers of the stormwater retention system. Groundwater quality is statistically evaluated at selected monitoring wells to assess aquifer response. In parallel, recharge inputs from the aquifer hinterland, including river infiltration, are characterised to constrain background groundwater conditions.

Water samples are being analysed for physicochemical parameters and dissolved contaminants, while solid phases from precipitation, runoff, and dust are chemically, mineralogically, and morphologically characterised to identify dominant particulate contaminant carriers. These datasets provide the basis for developing a conceptual mass-balance model describing contaminant transfer across the air–surface–soil–groundwater continuum.

Preliminary results indicate that a substantial fraction of contaminants mobilised during rainfall events is initially retained within soils and filtration media, thus limiting direct transfer to groundwater. However, this retention capacity is finite and strongly depends on contaminant loads, soil properties, and hydrological conditions. In areas affected by spatially localised industrial hotspots, extreme precipitation and associated leaching processes may act as effective triggers for contaminant release and downward transport once storage capacities are exceeded. These findings highlight the need to determine when soils and infiltration systems serve as effective buffers and when they facilitate contaminant transfer. This distinction is critical for evaluating the long-term performance of infiltration-based urban water management strategies and assessing groundwater vulnerability under climate change–related extreme precipitation regimes.