Natural pore pressure fluctuations as an indicator of shallow aquitard continuity

Abstract

The natural pore pressure response of a shallow aquifer/aquitard system can be used to assess aquitard continuity. This is the first paper to examine aquitard continuity at the field scale based on the barometric head response function (BHRF) obtained from multiple-regression analysis. The pore pressure measurements made during winter provided more representative BHRFs for the bounding aquifers and the aquitard. Distinct BHRFs were found for the shallow unconfined aquifer, clay aquitard and confined aquifer. This implies how the clay aquitard isolates the underlying confined aquifer from the shallow unconfined aquifer. It is found that the BHRFs for intact clay formation are delayed and differ from those for the fractured zone and confined aquifer. For the fractured zone, the observed BHRF quickly reached equilibrium. The BHRFs for intact clay and fractured zone equilibrated at high BHRF values. For the confined aquifer, the BHRF was low and constant. Numerical simulations provided a hydraulic conductivity below 1 × 10−8 m/s for the observed BHRF. Fast Fourier transform showed that the pore pressure components with a period smaller than one cycle per day were fully explained by barometric pressure fluctuations. Furthermore, the response of a deep piezometer to the moisture loadings provided an opportunity to estimate that the annual snowfall, for the study area, lost 25–35% of its weight.