Excessive Gravel Pack
You can't have too much of a good thing, right? Well, we have found over the years that this statement is not necessarily true, especially when it comes to the gravel pack in a Coastal Plain monitoring well.A gravel pack is definitely a good thing, as it helps filter sediment from the aquifer before it can reach, and possibly clog, the well screen. However, too much gravel pack can render a monitoring well virtually useless.
It is a common practice in the water well drilling industry to emplace large amounts of gravel around the screens in a supply well. Often, the resultant gravel pack can extend fifty to one hundred feet or more above the top screen in the well. This can be a good thing, as it can make the well yield more water by exposing more of the aquifer material to transmissive media.
However, excessive gravel pack can render a monitoring well almost useless. Monitoring wells are most valuable when only a discrete portion of the aquifer is open to the screened interval of the well. Excessive gravel packs can result in more than one aquifer contributing water to the monitoring well. This means that any water level or water quality sample collected from the well is really a mixture of conditions in all the contributing aquifer. It is nearly impossible to distinguish which aquifer is contributing which water quality parameters or what pressure levels to the monitoring.
The solution to this problem has been that we very explicitly outline the importance for the smaller gravel packs we specify for our monitoring wells. We couple this with careful observation of the amount of gravel emplaced in each well.