IDENTIFYING AND REMOVING BAROMETRIC-PRESSURE EFFECTS IN CONFINED AND UNCONFINED AQUIFERS

Failing to account for barometric pressure effects in water level meas urements can introduce errors by misestimating the total head and by a dding noise to water level measurements, For determining the total hea d in an aquifer, we assert that the air pressure head at the water sur face in the well must be added to measured water levels (equivalent to using an absolute pressure transducer) even though the resulting valu es may have larger temporal and spatial variability than the original water level measurements, At the Savannah River Site in South Carolina , the average barometric pressure variation is 6 to 7 cm, with a range of over 30 cm, Failure to account for barometric pressure variability could result in misestimation of the direction and magnitude of the h ydraulic gradient at the site, We also demonstrate procedures for remo ving barometric effects, such as to reduce noise during an aquifer pum ping test, and to identify mechanisms by which barometric pressure aff ects water levels, Three mechanisms are summarized including: an insta ntaneous response for confined aquifers; a delayed response due to bor ehole storage in confined and unconfined aquifers; and a delayed respo nse in unconfined aquifers due to the passage of barometric pressure c hanges through the unsaturated zone, Using data from the Savannah Rive r Site, barometric efficiencies are estimated using linear regression and a modification of Clark's Method, Delayed responses are estimated using regression deconvolution, The type of barometric effect provides diagnostic information about whether the aquifer is confined or not, the presence of borehole storage or skin effects, and the air diffusiv ity coefficient within the unsaturated zone, We also show how removal of barometric pressure effects improves the ability to observe otherwi se unnoticeable effects.