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Subsurface Investigation: Borings

BoringsAfter an environmental assessment, the last thing a property manager wants to hear is that monitoring wells have to be installed to further evaluate soil and groundwater. Not only does it disrupt day to day operations at the site, but it can be expensive. Knowing this, IES makes every effort to limit the number of borings that have to be made to evaluate a property. This is accomplished by understanding how groundwater flows and using that information to delineate the perceived threats from leaking tanks and documented spills.

Throughout New England, groundwater is stored in shallow, sandy aquifers that continually re-supply most of the water contained in lakes, streams and well fields.  The tops of these aquifers are typically located less than 10 feet below the surface and roughly mimic the overlying topography. Any contaminant released at or near the ground surface will inevitably seep down to the underlying aquifer, flow downhill with the groundwater and contaminate the nearest body of water.

borings2Fortunately, we can rely on this simple flow model to help predict where contaminants may move to, or conversely where they came from (such as an off-site, up-gradient source).  The only trick is determining which way the groundwater is flowing! On steeply sloping properties, it’s fairly easy to predict which way the groundwater moves under a site – down hill toward the nearest drainage.  However on flat properties, the only way to estimate groundwater flow direction is to install some monitoring wells (a minimum of three) that intersect the top of the water table, followed by a wellhead elevation survey. The wellhead elevation survey is necessary to define a horizontal reference datum from which the true elevation of the water table can be calculated. Of course groundwater may flow in two or more directions on very large properties, and therefore more wells may be required to define the water table, but in general, three wells will be enough to figure out which way the groundwater and entrained contaminants are moving.  (Diagrams from http://www.earthdrx.org/index.html)

 

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