Friday, January 9, 2015

What a Geologist Sees - Part 12

Just a brief lesson on water well construction and aquifers and other stuff like that.

The upper photo is of a poorly constructed/ poorly maintained residential drinking water well, probably on the order of 120-150 feet deep. It was located on the inner Coastal Plain, a few miles south of Augusta, GA, in Burke County.

Though we (the Georgia Geologic Survey Tritium Project) never subjected this particular well to a detailed examination by geophysical logging, it probably "bottomed out" and was drawing water from one of several formations in the unconfined Upper Three Runs Aquifer.  These could include the Late Eocene Utley Limestone Member of the Clinchfield Formation, Griffins Landing Member of the Dry Branch Formation, or the Irwinton Sand Member of the Dry Branch Formation. Overlying the Dry Branch Formation was the Late Eocene Tobacco Road Sand and that is what makes up the sandy soils seen surrounding the wellhead.

There are two areas of concern regarding the construction of this well. The first involves the apparent lack of a "grout seal".  When a drill rig drills a borehole, because of the sloughing of sediments, the borehole is rarely ever a perfect cylinder.  To prevent the sloughing of sediments that would fill the well, usually well casing is installed consisting of Schedule 40 or Schedule 80 (for deeper wells) PVC pipe that is from 4 to 8 inches in diameter.  With this well, the rust on the exposed casing suggests a steel or cast iron casing (it has been almost 10 years since I worked this area).

With the installation of the casing, the area between the borehole and the casing is called the "annular space" and it must be properly sealed to prevent surface pollutants from reaching the aquifer.  Around the "screen zone", where the water enters the well casing, coarse sand (sand pack or gravel pack) is usually introduced to keep the water flowing from the aquifer into screen zone. Above the "gravel pack" is where the first seal should be applied.  Below the water table, pellets of bentonite clay are introduced by way of a small pipe from the surface.  Once wet, the bentonite pellets will swell and seal the annular space, if properly applied.  Above the water table and all the way to the surface, a thin slurry of concrete seals the rest of the annular space.  At the surface, the well owner is supposed to pour a concrete surface pad to act as the first line of defense against surface pollutants reaching the aquifer.

That is how it should be.   A closer look at the surface area (amid the debris) surrounding the wellhead (in the upper photo) suggests that the area slopes inward towards the well casing.  This is not good.  This suggests that rainwater has been washing the sandy soil into the ungrouted (or poorly grouted) annular space.  Along with the sand, whatever else might be on the ground surface (chicken poop, etc.) is susceptible to being washed down the annular space, possibly reaching the aquifer if there is no grouting at all.

If you are ever in the position of buying a piece of property and the wellhead looks like this, at least get the water tested by the county health department or walk away from the deal.  Due Diligence applies to things more than land titles and termites.

In the photo below, these three GGS monitoring wells (one mile or so from the above well) exhibit properly installed surface pads to top off the annular space grouting. [There are three wells drilled into three separate aquifers at different depths to test for tritium among other things. Above background (but below EPA MCL) levels of tritium were found in the shallow aquifer well (the same one serving the well in the upper photo). One goal of our study was to make sure that similar levels of tritium had not reached the deeper aquifers (which it had not).

If the residential well (drilled to serve a couple of trailers) had at least a surface pad, that would have offered some protection from surface pollutants.

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