Coastal Plain
North Carolina's Coastal Plain has several aquifers that produce large quantities of high quality potable water. For a list of Coastal Plain aquifers and descriptions of their properties, please click here.Generally, the Coastal Plain aquifers are made up of interbedded sands and clays or limestone. However, DWR's monitoring well construction is typically the same, regardless of the aquifer lithology.
A typical Coastal Plain monitoring well consists of PVC or steel casing with a stainless steel well screen. Additionally, we install a five-foot tailpipe below the screen to keep settled material from blocking the screens. DWR installs a sand or gravel pack from the bottom of the tailpipe to five to ten feet above the top of the screen. Above the gravel pack, we emplace a ten to fifty-foot bentonite plug. This ensures that water from the aquifer does not travel up the borehole, affecting the integrity of the data we obtain from the well. From the top of the bentonite plug to land surface we emplace a neat cement or bentonite grout to seal and add structural integrity to the borehole. Diagram of typical coastal plain monitoring well construction (pdf).
Some of our older monitoring wells were installed using practices that we generally avoid today. One such practice is placing several discreet screen intervals in the same well. We avoid doing this today because this practice may impair the integrity of the data gathered at that well. As we have installed more wells and studied the Coastal Plain aquifers over the years, we have found that in many places there are more individual aquifers than we previously thought. The practice of installing multiple screens in monitoring wells can result in a situation where we have more than one aquifer open to the monitoring well in question. When this happens, the data is not nearly as useful than when we use discreet screen intervals in each monitoring well.
Another practice we avoid today is the once common method of installing a monitoring well with open holes in limestone aquifers. This method was formerly employed to cut material costs, as well screen is very expensive. However, many of the older wells that were installed with open holes have since seen those open holes partially or completely collapse. This greatly impairs our ability to gather useful data from these monitoring wells. We now install these wells with full casing to discreet screen intervals within the limestone aquifers. While this costs more initially, it saves much more money in the long run by greatly extending the useful life of the wells.