Characterising hydrodynamic controls on groundwater in a coastal urban aquifer using time and frequency domain responses at multiple spatiotemporal scales

Urban environments often have highly variable and evolving hydrogeology. Coastal cities present even greater challenges to hydraulic and thermal conceptualisation and parameter estimation due to their complex dynamics and the heterogeneity of ocean-influenced hydraulic processes. Traditional methods of investigation (e.g. pump tests, invasive sampling) are time consuming, expensive, represent a snapshot in time and are difficult to conduct in built-up areas, yet properties derived from them are crucial for constructing models and forecasting urban groundwater evolution.

Here we present a novel approach to use passive sampling of groundwater head data to understand subsurface processes and derive hydraulic and geotechnical properties in an urban-coastal setting. This is illustrated using twenty years of high frequency (hourly) time-series data from an existing groundwater monitoring network comprising 234 boreholes distributed across Cardiff, the capital city of Wales, UK. We have applied Tidal Subsurface Analysis (TSA) to Earth, Atmospheric and Oceanic signals in groundwater time-series in the frequency domain, and also generated Barometric Response Functions in the time domain. By also observing the damping and attenuation of the response to ocean tides with distance from the coast and tidal rivers, this combination of analyses has enabled us to disentangle the influence of the different tidal components and estimate spatially distributed aquifer processes and parameters.

The data cover a period pre and post construction of a barrage across the coastline, impounding the city’s rivers. We were therefore able to observe a huge decrease in the subsurface ocean tide signal propagation following this human intervention, through the coastal and tidal river boundaries. These changes reveal variations in hydraulic responses and values of hydraulic diffusivity between different lithologies, notably with made-ground deposits being much less sensitive to ocean tides than the underlying sand and gravel aquifer. By being able to map the spatial variations in hydraulic response and barometric efficiency for the first time (and therefore formation compressibility and extent of aquifer confinement) we have been able to refine interpretations (and in some cases overcome misconceptions) derived from previous inferences made solely from borehole logs. We anticipate that linking the improved hydraulic characterisation, enabled by the new methodology, will also help better characterisation of the subsurface thermal regime, and management of shallow geothermal energy resources in coastal urban aquifers.