Hydro-geomechanical characterisation of a coastal urban aquifer using multiscalar time and frequency domain groundwater-level responses

Hydraulic properties of coastal, urban aquifers vary spatially and temporally with the complex dynamics of their hydrogeology and the heterogeneity of ocean-influenced hydraulic processes. Traditional aquifer characterisation methods are expensive, time-consuming and represent a snapshot in time. Tidal subsurface analysis (TSA) can passively characterise subsurface processes and establish hydro-geomechanical properties from groundwater head time-series but is typically applied to individual wells inland. Presented here, TSA is applied to a network of 116 groundwater boreholes to spatially characterise confinement and specific storage across a coastal aquifer at city-scale in Cardiff (UK) using a 23-year high-frequency time-series dataset. The dataset comprises Earth, atmospheric and oceanic signals, with the analysis conducted in the time domain, by calculating barometric response functions (BRFs), and in the frequency domain (TSA). By examining the damping and attenuation of groundwater response to ocean tides (OT) with distance from the coast/rivers, a multi-borehole comparison of TSA with BRF shows this combination of analyses facilitates disentangling the influence of tidal signals and estimation of spatially distributed aquifer properties for non-OT-influenced boreholes. The time-series analysed covers a period pre- and post-impoundment of Cardiff’s rivers by a barrage, revealing the consequent reduction in subsurface OT signal propagation post-construction. The results indicate that a much higher degree of confined conditions exist across the aquifer than previously thought (specific storage = 2.3 × 10$^{-6}$ to 7.9 × 10$^{-5}$ m$^{-1}$), with implications for understanding aquifer recharge, and informing the best strategies for utilising groundwater and shallow geothermal resources