Confined aquifer responses to pumping – potential impacts on plume migration

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Pressure waves are transmitted in confined aquifers with relatively little damping compared with surficial phreatic aquifers. Confined aquifers may therefore be subject to abrupt and far-reaching changes in the rate and direction of groundwater flow due to changes in well pumping. Where there is a mechanism for contaminants to reach a confined aquifer, the contaminant migration within the aquifer can be significantly influenced by flow field variability. A groundwater flow and mass transport study recently conducted for a site in Nassau County, New York had to account for rapid flow changes and complete 360-degree variability in groundwater flow direction over time. Study results were presented using animations, which clearly illustrated the dynamic nature of groundwater flow patterns within the confined aquifer.

STUDY LOCATION AND HYDROGEOLOGY
A recent groundwater flow and mass transport study on Long Island in Nassau County, New York evaluated plume transport in the confined Lloyd aquifer. The study was initiated following the detection of organic compounds of unknown origin at a public supply well that was screened in the Lloyd aquifer. The analysis objective was to evaluate whether a groundwater plume migration pathway existed from an identified potential source area, where cooling water injection wells could have introduced contaminants into the confined aquifer and impacted the public supply well. The relative locations of the cooling wells and the public supply well are shown in Figure 1.

The cooling water injection wells were installed in the 1960s. The affected public supply well was installed in 2002 as a replacement for an older well and organic compounds were detected at the new well shortly after start up. Limited operational information is available about the cooling water injection wells.  The groundwater system on Long Island consists of unconsolidated deposits of clay, sand and gravel underlain by Precambrian bedrock (Smolensky and others, 1989). Late Cretaceous deltaic clays, sand and gravel comprise the Lloyd sand, Raritan clay and Magothy formation.

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