State of the Practice Worldwide: Interaction of Perched and Regional Aquifers at the Site Impacted by Volatile Organic Compounds

A series of field measurements were used to evaluate the mutual interaction between the perched water zone and the regional aquifer under the specific site conditions of the contaminated site. The perched water zone apparently developed after the demolition of a building used for chemicals by leaving low permeability relics of the building slab after demolition and the presence of a relatively thin clayey horizon (approx. 2 m thick). The regional aquifer consists of weathered shale bedrock beneath the perching layer. The difference between the watertables of the perched and regional aquifers is between 0.5 and 1.2 m. The perched water was found to contain elevated concentrations of volatile organic compounds during initial sampling. A study was conducted to examine the perched water more closely and to describe the (1) controls on perching behavior, (2) contaminant flux through the unsaturated zone (soil gas) and to the regional aquifer, and (3) differences in the water quality of the perched groundwater and that in the regional aquifer. The data evaluation showed that the perched aquifer system is dynamic both in vertical and horizontal flow. Nonetheless, the hydrochemical data from the two aquifers were distinct in chemical composition despite their close proximity.