Shallow Groundwater Discharge into Urban Drains: Identifying the Missing Link to Define Urban Typologies for Impact Assessment of Urbanization on Water and Nutrient Balances

Abstract:

Groundwater and surface water (GW-SW) interaction in drains of many sandy coastal plain areas displays an ephemeral hydrological regime, as often shifts occur in their hydraulic functioning from a losing to a gaining water conditions upon the position of the surrounding shallow water table (SWT). Urbanization in such areas and stormwater management strategies enhancing infiltration have the potential to alter the infiltration rates and the subsurface water storage dynamics with consequences for the residence time of the water and nutrient transformations prior their discharge into receiving SW drains. Identifying first order control on the above processes will assist the improvement of assessment tools for better urban development. This work presents findings on the hydrodynamics of the GW-SW water exchange in two drains of the Perth Coastal Plain area (Western Australia, Australia) impacted by a SWT developing on a layered variable texture soil: a peri-urban drain and a restored living stream drain in urban residential area. A multi-technique approach was used to investigate water mass balance and fluxes over a reach scale and involved continuous records of hydrometric data for GW-SW interactions, passive tracers for water pathway identification, pore water temperature for vertical water exchange, and differential SW discharge using an Acoustic Doppler Current Profiler. Results highlighted differences in the GW-SW interactions between both drains under stormflow and baseflow conditions. A substantial increase of GW discharge into the drain coincided with the full development of a SWT over a seasonal scale at the peri-urban drain, which suggests a more natural water infiltration and redistribution in the subsurface. In contrast, a large volume of infiltrated rain water was discharged into the living stream over a period of few weeks regardless of the development of the surrounding SWT, which suggests the influence of underground pipe system in water redistribution. The results contributed to identify key physical parameters to define urban typologies, quantify the subsurface storage discharge and residence time, and finally assess the transport and transformations of nutrients using a generalised Damköhler number. Future work will populate the framework with other study cases.