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

Abstract

[1] 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.

Detecting the vulnerability of groundwater in semi-confined aquifers using barometric response functions

“…2. These comprise low, intermediate and high frequency ranges within the barometric pressure signal (Rojstaczer, 1988;Hussein et al, 2013). At low frequencies (stage A), gain increases and phase decreases with increasing frequency, and the behaviour is controlled primarily by the properties of the confining layer.…”

“…At very high frequencies, the movement of water cannot keep up with barometric pressure changes and the aquifer once more behaves as if unconfined (gain and phase approach zero). Barometric response functions can be determined from time series of open borehole water levels and barometric pressure, recorded at regular intervals of around four hours or less for time periods of around one year or more (Hussein et al, 2013). Automatic monitoring of borehole water levels at time intervals of one hour or less is becoming increasingly common so that the often dense networks of monitoring boreholes that exist in many major aquifers can potentially provide ready-made data sets suitable for this approach.…”

“…Automatic monitoring of borehole water levels at time intervals of one hour or less is becoming increasingly common so that the often dense networks of monitoring boreholes that exist in many major aquifers can potentially provide ready-made data sets suitable for this approach. Hussein et al (2013) showed through numerical simulations that the presence of a highly conductive pathway through a confining layer is reflected in the head variation induced by barometric pressure in the aquifer. Here the work of Hussein et al (2013) is furthered through a study of BRFs for aquifers with heterogeneous confining layers.…”

“…Hussein et al (2013) showed through numerical simulations that the presence of a highly conductive pathway through a confining layer is reflected in the head variation induced by barometric pressure in the aquifer. Here the work of Hussein et al (2013) is furthered through a study of BRFs for aquifers with heterogeneous confining layers. Through a suite of numerical simulations, the feasibility of using BRFs to detect the presence of possible pathways for contaminants within semi-confining layers is investigated and the potential implications for groundwater vulnerability assessment practice are discussed.…”

“…Acworth and Brain (2008) demonstrated that a thin weathered zone (<2 metres) in fractured granite can result in a response of borehole water level to barometric pressure indicating confined behaviour and used this response to investigate groundwater-surface water interactions. Hussein et al (2013) presented BRFs demonstrating a range in behaviour for an aquifer confined by heterogeneous glacial sediments and related these to groundwater vulnerability. These authors suggested a measure of vulnerability incorporating the thicknesses and diffusivities, estimated from BRFs, of the unsaturated and saturated zones within the confining layer.…”