Some considerations when drilling a dewatering well.

First, a correction: A few months ago, I wrote an article about geothermal loop installation. In it, I stated that the State of Georgia does not require bottom-to-top grouting of the loops. A friend of mine, Dan Elder, also a member of the Georgia Water Well Standards Advisory Council, and owner of Oconee Well Drillers, e-mailed me, saying that the law had been changed, and that now all loops that penetrate an aquifer have to be pressure-grouted from bottom to top. Thanks, Dan; I stand corrected. I do, though, stand by my comments concerning thermal conductivity. Now I wonder when they are going to do that for all casing.

Now, on to something else that drifted though my world: I was discussing a potential dewatering project with a friend the other evening, and I got to thinking about differences between a water well and a dewatering well.

In a normal-producing water well, we shoot for sustainable production, hopefully for years. Dewatering, on the other hand, requires deliberate overproduction with the goal of depleting the aquifer. This requires pumps designed to over-pump the well until the aquifer is exhausted, at least locally and usually temporarily. I say “usually” because there are situations that require permanent dewatering.

I once did a dewatering job for a new sewage plant. We dewatered the site so they could build some large, concrete holding tanks. They extended about 8 feet or 10 feet into the ground, and were about 60 feet in diameter and 30 feet tall. The water table at the site was a perched aquifer with a 3-foot static level. There was a clay aquitard at about 15 feet, so we designed and installed a system to lower the water to that point. After installation of the wells and pumping system, it took a lot of pumping to lower the water level enough to begin construction, and vigorous pumping lasted throughout the construction process. All went well. The wells and manifolds and pumps all were on the surface for ease of service because it was a construction site. It all worked as planned, and it was easy to abandon the wells and rig down when the job was done. The contractor built the tanks, filled them with water for testing, and put the plant into production.

This lasted several years until it was time to do routine maintenance on and clean the tanks. Since the ground water had long since returned to its original level, when the tanks were drained, they floated! It’s pretty amazing to see a round, cement, 60-foot boat come out of the ground, bending and breaking 30- and 48-inch steel lines. It looked like the launching of the Ark. (See, I told you it was gonna rain.)

It was time to dewater the site again. The problem was that it was a completed, working plant, and site access was a nightmare. Good dewatering often requires wells that are very close together – another difference between a production well and dewatering. In a production well, interference with the cone of depression of adjacent wells is avoided if possible. In dewatering, interference is necessary for the system to work well. If the cones of depression don’t overlap, there will be an area of influx that makes successful dewatering almost impossible.

On this job, we had all sorts of structures – parking lots, lines both buried and surface to deal with, not to mention getting the drilling equipment around. Some of the holes were drilled on various angles to get under obstructions, some were hand-jetted with 2-inch pipe and water pumps, and almost every hole had to be hand-excavated for the first few feet to check for buried lines and wires. We found things on almost every hole. For a fairly new plant, they sure didn’t have up-to-date as-built drawings worth a hoot. The job took almost four times longer than the original job, and because it was a permanent installation, we buried all the wellheads in vaults, and buried all the header lines to a new, permanent pump building. We installed a number of monitoring wells to keep an eye on the natural water table, which varies quite a bit during the year, according to rain and the season. When the water table rose, the pumps would run, often continuously, for weeks. When it fell, the pumps would be shut down. Tank draining also was done only after sustained pumping and a careful monitoring of the perched aquifer.

Another often-overlooked factor for dewatering wells is well efficiency. Often, a production well driller knows that a given aquifer may produce 50 gallons a minute, and he’s only going to use a 20 gallon per minute pump, so maximum efficiency is not high on his priority list. This won’t work on a dewatering project. Wells must be constructed and developed to produce the maximum amount of water with the minimum drawdown. Obviously, maximum drawdown is the goal, and anything less than maximum efficiency will prevent or slow down this process, leading to a less than harmonious outcome. Careful analysis of the formation sand – and the relationship between that and the gravel pack size and the screen size – is necessary. A simple field sieve analysis will go a long way toward proper sizing of the gravel pack and screen selection.

These are just some of the factors that come into play when designing a dewatering project; there are many others. But one thing is certain: It is the type of job most drillers are equipped for, and in these slow times, any job that will keep the rig running is worth looking at.