Monday, October 14, 2013

dry wells

I've made quite a few dumb mistakes over the years, but one type of mistake is particularly hard to minimize/live down: installing a dry monitoring well.

When you start an environmental investigation, you don't have all the information you need to site a monitoring well. That's why you're installing it in the first place. So the initial approach is to find the regional water levels (USGS, currently offline, is a good resource for this) and guesstimate the target zone. If you're in a relatively wet climate, which I usually am, the water level in nearby ponds and streams can give a good approximation of the minimum depth you need to get to.

Most of the time, we want to install wells that intercept the water table (for contaminants that may be lighter than water) or that sit on top of a confining unit (for contaminants that may be denser than water). The standard length for monitoring well screens is 10 feet. You need to get enough water in the well to sample, so ideally, our shallow well(s) would cross the water table 7-8 feet in, so you'd have 2 feet of play in case the water level rises.

Determining the exact depth of the water table while drilling isn't easy. The capillary zone above the water table is saturated, so you need to ensure that you've drilled deep enough that a well installed will actually yield water. The safest thing to do would be to keep drilling until you have free water in the borehole, and send the drillers out on a coffee break and wait until the water level stabilizes and then install the well accordingly. But in slow-producing formations, you run the risk of installing the well too deep. You usually don't have the luxury of waiting for hours or overnight to confirm the depth in a boring, so you guess and move on.

A decent shortcut is to check water levels in nearby wells (if you have them) and plan accordingly. The two wells that I installed that were utterly useless (entirely above the water table) used that reasoning. In one case, I was sighting off a well that was a couple hundred feet away, and although the water table was essentially flat in the area, I didn't realize that I was on a very gentle incline. In another case, I had another monitoring well about 30 feet away, but didn't appreciate that the water table had taken a nosedive because of wonky hydraulics around a nearby dam.

Of course, all this assumes that the well didn't get accidentally pulled up as you were removing the tooling or that you didn't run into heaving sand and have the borehole fill in 10 feet as the driller added the well material...

ETA: this is about overburden drilling, obviously. Bedrock is a whole 'nother ballgame...

1 comment:

C W Magee said...

I'm guessing you don't spend much time in Karst (which, in hindsight, is almost everything I drilled). Finding the water table was the least of our troubles- the only problem was one hole when we didn't have enough rods (big desert, porous rock).