In order to understand the role of aquitards at a site, it is important to understand what an aquitard is and how it affects the conceptual site model. Aquitards are geologic deposits, either unconsolidated (dirt) or consolidated (rock), of sufficiently low hydraulic conductivity and sufficient areal extent and thickness to impede groundwater flow between or to aquifers. Aquitards generally do not provide an economic quantity of water, which is a defining characteristic of aquifers, but aquitards do influence flow paths and act as storage units for groundwater contaminants.

If an aquitard is present at a site, it is important to understand how it influences contaminant migration. If an aquitard is underlying the impacted zone, it could be acting as an effective barrier to downward vertical contaminant migration, especially when dealing with a contaminant that is more dense than water (i.e. chlorinated solvents). If an aquitard is overlying an impacted zone, it is possible that upward vertical migration is present. These factors make it important to understand the nature of aquitards, as it is a very important aspect of the development of a conceptual site model and in understanding contaminant migration. Once it is determined that an aquitard is present, proper investigative techniques need to be applied to limit the potential of breaching the aquitard and creating a preferential pathway for contaminant migration.

Several investigative techniques are utilized when investigating sites with aquitards. It is extremely important when advancing investigative borings or wells through an aquitard that the borehole is sealed and no preferential pathways through the aquitard are inadvertently created.

Direct push investigative techniques, which are often used for the collection of soil and groundwater samples during initial site characterization and preliminary delineation sampling, often require the advancement of borings through an aquitard. By utilizing ‘dual tube’ technology during investigation, the borehole is sealed not only preventing overlying materials from collapsing into the borehole, but also preventing groundwater from vertically migrating through the borehole. The technique involves driving a hollow outer casing that effectively seals the borehole and allows sampling equipment to be advanced through the outer casing for sample collection. Once the desired depth of sampling is achieved, the borehole is sealed from the bottom up with an impermeable slurry while the outer casing is extracted, which, when properly executed, negates the potential for vertical migration of contaminants through the borehole.

Monitoring wells, which are often required by regulatory agencies for delineation sampling and confirmatory sampling, require proper construction in order to prevent cross contamination through an aquitard. Generally, an outer casing is installed to a depth within the aquitard and sealed with impermeable hydraulic cement. An inner casing is then advanced to the designated sampling depth through this outer casing, which serves to keep water bearing units overlying and underlying the aquitard from becoming hydraulically connected through the monitoring well borehole. This process is repeated for the investigation through multiple aquitards with a casing being set in each successive aquitard until the desired sampling depth is attained. Improperly constructed monitoring wells greatly increase the potential for contaminant migration through an aquitard. The application of proper investigative techniques allows for a better understanding of how an aquitard fits into a conceptual site model.