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

The response of borehole water levels to barometric pressure changes in semiconfined aquifers can be used to determine barometric response functions from which aquifer and confining layer properties can be obtained. Following earlier work on barometric response functions and aquifer confinement, we explore the barometric response function as a tool to improve the assessment of groundwater vulnerability in semiconfined aquifers, illustrated through records from two contrasting boreholes in the semiconfined Chalk Aquifer, East Yorkshire, UK. After removal of recharge and Earth tide influences on the water level signal, barometric response functions were estimated and aquifer and confining layer properties determined through an analytical model of borehole water level response to barometric pressure. A link between the thickness and vertical diffusivity of the confining layer determined from the barometric response function, and groundwater vulnerability is proposed. The amplitude spectrum for barometric pressure and instrument resolution favor determination of the barometric response function at frequencies to which confining layer diffusivities are most sensitive. Numerical modeling indicates that while the high frequency response reflects confining layer properties in the immediate vicinity of the borehole, the low frequency response reflects vertical, high diffusivity pathways though the confining layer some hundreds of meters distant. A characteristic time scale parameter, based on vertical diffusivities and thicknesses of the saturated and unsaturated confining layer, is introduced as a measure of semiconfined aquifer vulnerability. The study demonstrates that the barometric response function has potential as a tool for quantitative aquifer vulnerability assessment in semiconfined aquifers

Determination of fluid flow properties from the response of water wells to atmospheric loading.

The water level in a well that taps a partially confined aquifer is often sensitive to atmospheric loading. The magnitude and character of this response is partly governed by the well radius, the lateral hydraulic diffusivity of the aquifer, the thickness and vertical pneumatic diffusivity of the unsaturated zone, and the thickness and vertical hydraulic diffusivity of the saturated zone overlying the aquifer. These key elements can be combined into five dimensionless parameters that partly govern the phase and attenuation of the response. In many cases, the response of a well to atmospheric loading can be broken up into a high-, intermediate-, and low-frequency response. The high-frequency response is governed largely by the well radius and lateral diffusivity of the aquifer. The intermediate-frequency response is governed by the loading efficiency of the aquifer. The low-frequency response is governed by the vertical pneumatic diffusivity and thickness of the unsaturated zone and the vertical hydraulic diffusivity and thickness of the saturated material above the aquifer. Cross-spectral estimation is used to fit the response to atmospheric loading of three water wells to the theoretical curves in order to yield estimates of three of the key dimensionless parameters. These estimates then are used to make estimates or place bounds on the vertical pneumatic diffusivity of the unsaturated zone, the lateral permeability of the aquifer, and the composite vertical hydraulic diffusivity of the overlying saturated materials