In the old days, before well drilling technology became widespread, all rural homesteads had as their water source either a spring flowing out of the ground or a well that some industrious soul dug with a shovel and lined with rock.
When we bought our house in 1977, the latter was exactly what we had. Ours was about 10 feet deep and still had an operating hand pump mounted right on top of the well, but our predecessors had installed a pump in the dirt cellar of the house 50 feet away. Driven by a small engine, it was like trying to start a recalcitrant lawnmower every time we needed to pump a tank full of water, and then hearing and smelling the lawnmower in the cellar for several minutes until the tank was full. It was one of the truly charming features of the place.
Several years later we bought the adjoining piece of land, on which is a good and venerable spring that we piped about 1,000 feet down to the house. We know it is venerable because we came across a wooden pipe, or “pump log,” about five or six inches in diameter that had been bored out. The end was sharpened like a pencil to form a male end which fit into a corresponding female end of the next length. The log was headed in the direction of the stone foundations of the farm that once owned the land. Then in 1999, a bad drought struck. Both our water sources dried up and we debated drilling a well. But a friend pointed out we still had water in our pond and suggested digging down at that level, and we ended up with a very reliable dug well.
During this very dry fall, I heard several tales about springs, wells, dug wells, drilled wells, water tables, and ponds. By my rain gauge, it was drier than normal for three of the last six months (May, June, and September), with over three weeks without rain in September being the nadir. Our spring up in the woods got very low and we switched to the dug well that is more reliable. But why was there water in that spot when the spring up the hill was dry? Where did the water in the ground come from, why did it stop coming, and why did it come back without a whole lot of rain?
Surface water is obvious, running past during rain or snowmelt, running down streams, or sitting in ponds, lakes, and man-made reservoirs. Larger communities often use these water bodies as their water supply. St Johnsbury has Styles Pond, Boston has the Quabbin Reservoir, and New York City has a whole network of reservoirs dotting the map way up into the Catskills.
Here in the northeast, our scattered population usually relies on the water under the surface, accumulated in an aquifer, known as groundwater. It could be a shallow aquifer resulting from rain saturating a layer of soil only to seep out again in a few weeks or months somewhere down the slope. Or groundwater can infiltrate deep into the soil and/or into cracks in the bedrock. The top of the saturated soil or rock is known as the water table. In the hills of the Northeast Kingdom, the layer of soil is pretty thin but it mostly lies on top of fractured bedrock. Our rock is very old, tortured, and cracked and all those cracks can contain water. If the aquifer is confined by an impermeable layer above, it is called a confined aquifer. A drop of rain that sinks into a deep aquifer we don’t expect to encounter again for years, decades or even millennia.
In a water table aquifer, the water is not confined, is at atmospheric pressure, and may rise or fall depending on rate of recharge (precipitation) and rate of withdrawal (by us and by plants and trees). It could be a “perched” water table, meaning that the saturated soil is sitting on top of an impermeable layer, under which the soil is dry. A perched aquifer usually contains a pretty small amount of water and is vulnerable to dry up in a short term drought such as we have had this fall. The more substantial unconfined water table is the top of an aquifer saturating the soil and fractured rock down to solid bedrock. In a valley where there are many feet of sand and gravel before you hit bedrock, the aquifer can hold prodigious quantities of water. Wells drilled in a valley bottom can be highly productive at shallow depths. Where the water table is not far below the surface of a sandy valley bottom, a driven point, or “sand point” well is possible. This is a two-inch steel pipe with a hard steel spear tip and a screen. The pipe is simply driven into the ground directly into the aquifer.
If your water source is a stack of 2, 3, or 4 round concrete well tiles dug into a wet spot on the landscape, then you have a “dug well,” dug into a water table aquifer. Ours consists of 4 well tiles, a total of eight feet deep, with a submersible water pump hooked up to a pressure tank in the basement about 200 feet away. It is very reliable, only dropping a foot or so in this fall’s drought, so I think it is dug into the top of a substantial water table aquifer.
In a deeper, confined aquifer, the water is overlain by a relatively impermeable layer of solid bedrock or clay and may be at a pressure greater than atmospheric. The pressure results from the difference in elevation between the recharge area (where precipitation soaks into the ground up the hill) and the discharge point. The discharge could be a well drilled through the impermeable layer, or spring at a spot where the confined aquifer meets the ground surface. In either case, there is a volume of water in the aquifer upslope of the discharge, resulting in water pressure. When the confining layer is penetrated, such as by a drilled well, the water rises into the well and the aquifer is said to be artesian. Water may flow out the top of the well. Many artesian wells do not flow freely at the ground surface, but the water rises into the well which reduces pumping requirements.
If your water source is a six-inch metal pipe that sticks up out of the ground with a cap on it, you have a drilled well (as opposed to a dug well). If you are up in the hills, the well is drilled into bedrock. You have likely seen the trucks with their drilling rigs working; the powerful bit pounding its way down, first through whatever soil is above the rock and then into the hard stuff. Out of the hole comes powdered rock and the water that is used to lubricate the bit as it grinds away down through the rock. There are some good videos of the process on YouTube if you’re interested.
Dowsers can often find shallow groundwater but are less reliable in predicting a good spot for a drilled well. Where to drill is a crap-shoot. In our bedrock, wherever the hole is drilled, at some point the drill will encounter a fracture in the bedrock that is full of water. Sometimes the water is encountered in the first hundred feet, but often the drill goes down hundreds of feet, and sometimes no water is found. When that happens, the driller moves to a new spot and tries again. It is truly hit or miss. If the water in the fracture is under pressure, the well is artesian and water will rise up in the drill hole.
Our spring in the woods flows out of the ledge at the foot of the slope. It yields excellent water, as does every bedrock spring I know of. The water comes from precipitation falling on the watershed above the spring. Such springs can be unconfined, flowing more or less according to the rate of recharge in the hills above, or confined and flowing under artesian pressure.
Water tables fluctuate with the seasons. A wet spring after the ground thaws and before the leaves come out results in the highest water table of the year. As the trees leaf out and begin to grow, they suck huge quantities of water out of the ground which transpires into the air. Our normal precipitation of three or four inches per month just about keeps up with the transpiration. Last May and June, our aquifers were taxed by lower than normal rainfall while the plants kept on transpiring. Normal rain in July and August kept up with the trees but did not recharge the groundwater. Then September’s drought struck while the trees were still sucking water and the water table dropped precipitously. As soon as the leaves dropped, the rain of October and November sank into the ground and continued down to the aquifer, raising the water table again. If the ground freezes solid this winter, thus causing snowmelt and winter rain to run off instead of soaking in, the water table will drop through the winter. We often see very low water tables in March for this reason.
This dance of precipitation, transpiration, recharge, and discharge is known as the hydrologic cycle. It’s a cycle that’s easily ignored until your spring, dug well, or drilled well runs low. It’s a not-so-gentle reminder from Mother Nature to pay attention to her rhythms.