Abstract
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.
Highlights
As competitive use of underground urban spaces increases, better understanding of the subsurface and its processes is needed to de-risk development and ensure sustainability
Traditional methods often fail to account for the effects of barometric pressure on groundwater level measurements resulting in errors in estimating total head and groundwater flow direction (Rasmussen and Crawford 1997), which can be significant in areas of flat topography (Spane 2002) and have implications for contaminant pathway assessment, groundwater resource management, the use of sustainable urban drainage systems (SuDS) and the implementation of ground source heating systems
Pre-impoundment, the longest period of continuous data capture was for 2 months; the results indicated that whilst time-series length was important for the accuracy of output M2 and S2 amplitude and phase values, duration had little impact on the determination of dominant tides, which remained consistent across the different time-periods tested
Summary
As competitive use of underground urban spaces increases, better understanding of the subsurface and its processes is needed to de-risk development and ensure sustainability. By determining the groundwater response to Earth and atmospheric tides, it is possible to calculate BE from which a range of hydraulic properties, such as aquifer compressibility, specific storage and confinement may be derived. McMillan et al (2019) refer to this as tidal subsurface analysis (TSA), expressing its suitability for a range of applications but noting it is a promising, yet underused, tool This passive approach repurposes, and has the potential to add value to, commonly collected atmospheric and groundwater monitoring data enabling detailed rapid characterisation of subsurface conditions at a high spatial and temporal resolution
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