Considering barometric pressure in groundwater flow investigations

Water level elevation measurements in wells are commonly used as a basis to delineate groundwater flow patterns (i.e., flow direction and hydraulic gradient). Barometric pressure fluctuations, however, can have a discernible impact on well water levels. These barometric effects may lead to erroneous indications of hydraulic head within the aquifer. Total hydraulic head within the aquifer, not well water level elevation, is the hydrologic parameter for determining groundwater flow direction and hydraulic gradient conditions. For low‐gradient, unconfined aquifer sites exhibiting variable vadose zone characteristics (e.g., thickness, pneumatic diffusivity), barometric pressure fluctuations can cause temporal changes in lateral flow direction and flow velocity. Discrete water level measurements used to determine the average or long‐term groundwater flow conditions, therefore, may provide nonrepresentative results. Calculation of the barometric response characteristics for individual wells provides the basis to account for the temporal effects of barometric pressure fluctuations from monitor well measurements, so that average, long‐term groundwater flow pattern behavior can be determined.