Borehole water level response to barometric pressure as an indicator of groundwater vulnerability

The response of borehole water levels to barometric pressure is a function of the confining layer and aquifer properties. This study aims to use this response as an aid towards quantitative assessment of groundwater vulnerability, applying the techniques to the confined/semi-confined part of the Chalk Aquifer in East Yorkshire, UK. Time series analysis techniques are applied to data collected from twelve monitoring boreholes to characterize and remove components contributing to the borehole water level signal other than barometric pressure, such as recharge and Earth tides. Barometric response functions are estimated using the cross-spectral deconvolutionaveraging technique performed with up to five overlapping frequency bands. A theoretical model was then fitted to the observed barometric response functions in order to obtain estimates of aquifer and confining layer properties. Derived ranges for pneumatic and hydraulic diffusivities of the confining layer vary over four orders of magnitudes (0.9 to 128.0 m2/day and 10.0 to 5.0×104 m2/day respectively) indicating that the aquifer is nowhere purely confined. Discrepancies between estimates of aquifer transmissivity derived from the barometric response function and pumping tests have been explored using slug tests and results suggest that aquifer model transmissivity are highly sensitive to borehole construction. A simple flow model, constructed to test the potential impact of confining layer heterogeneity on the barometric response function, shows that while high frequencies reflect the immediate vicinity of the borehole, low frequencies detect confining layer properties up to some 500 meters distant from the borehole. A ‘characteristic time scale’ is introduced as a function of derived properties of the confining layer and is used as a quantitative measure of the degree of aquifer confinement. It is concluded that barometric response functions are sensitive to confining layer properties and thus can provide a useful tool for the assessment of aquifer vulnerability