Water-level fluctuations due to barometric pressure changes in an isolated portion of an unconfined aquifer

The potential effect of barometric pressure changes on the water-level elevations in wells tapping unconfined aquifers is often ignored or considered to be negligibly small. However, under certain circumstances, wells in unconfined aquifers can have barometric efficiencies greater than those typically observed for wells in confined aquifers. This is illustrated by results of a study performed at a federal Superfund site where a contaminated portion of the unconfined aquifer is isolated by a soil-bentonite cutoff wall and clay cap. A well located within the containment system displays water-level fluctuations which mirror barometric pressure changes. During the study, water-level elevation ranged 36.6 centimeters (cm) [1.20 feet (ft)] in response to barometric pressure changes of 2.87 centimeters mercury (cm-Hg) [1.13 inches mercury (in-Hg)]-a barometric efficiency of about 94 percent(%). A well screened similarly outside the containment system did not display any observable barometric pressure effects. Vertical transmission of barometric pressure changes to the water table inside the containment system is retarded by the clay cap, and the soil-bentonite cutoff wall retards the lateral transmission of barometric pressure changes from the unsaturated zone outside the containment system. Water-level fluctuations of the magnitude observed during this study may need to be considered when developing monitoring programs to assess the performance of remedial systems.