Groundwater wells may be the most important, and often only, source of water on your farm Having some knowledge of where that water comes from may help you understand, utilize, protect and conserve your water supply.
Groundwater wells on your farm.
What is groundwater?
Groundwater is water found underground in the cracks and pore spaces of sand , soil and rocks. Many of these materials will allow water to move through them – and we call these permeable. Sometimes there are layers of material, such as thick clays or the rock equivalent – shale, that water can’t flow through. We call these impermeable layers.
Where does the water come from?
Water gets into these underground materials by soaking down into the ground when it rains, snow melts or even by leaking into the ground from ponds, lakes, creeks and rivers. Sometimes the source of the groundwater that is underground at your farm could be many miles away. When we take water from the groundwater “pool” it is replenished by new groundwater from the source area. We call this groundwater recharge. How fast it will recharge depends on the way the groundwater moves through the ground and how much water is or has fallen on the source area.
Most groundwater has travelled long distances through rocks and sand etc, and so it is naturally filtered. That said, you should also have groundwater tested for any contaminants before you use for yourself or your animals. Some towns/states insist on this when you buy a place.
Where do you find groundwater?
Groundwater is almost everywhere! How much and how deep is always the question for people who want to use it. Groundwater moves through permeable materials due to gravity – it moves downhill! So most of the recharge of groundwater below your farm has come from upslope. The amount of fractures and spore space and how well all those are connected influences the speed which the groundwater moves. This is important, because if you take water out of the groundwater ‘pool’ the time it takes to refill that ‘pool’ will affect how long a time you have to wait to be able to take more water out of the ‘pool’.
Please don’t think that there is an underground lake , ocean or swimming pool of open water underground. There is not! It is contained in rocks a little like water in a sponge. I am using the term ‘pool’ just to explain the process
The top of the layer of groundwater is called the water table. How far you need to dig down to find the top of the water table will change from place to place and even season to season. There is a great website for the USA that has data on the depth below ground for monitored wells across the country. Its a great tool to see how deep you would have to go to find water in your region (local variations obviously happen).
How are springs formed?
A spring happens when the water table level is at the surface…and so the groundwater just bubbles out of the ground. In some places the groundwater is under pressure as it is trapped under a layer of impermeable rock. Here the water can really flow quickly. We call these artesian springs.
For most of us, we need to dig down to find groundwater. In some places this can be quite shallow – a matter of just a few feet. In other places it is hundreds of feet or more. (Again…go look at the depths of the wells in the USGA data here for your state).
Three types of groundwater wells.
There are three basic types of groundwater wells – dug, driven and drilled.
Dug groundwater wells – these are hand dug wells that people used picks and shovels to dig a hole below the water table. They kept digging until the water flowed in faster than they could bail water out and keep digging. These wells were then lined with stones, brick, tile, concrete etc to stop the sides falling in. They are shallow and so can dry out in droughts, be easily pumped dry or can be polluted by surface water running into them.
Driven groundwater wells – these are wells created by driving or pushing a pipe into the ground until its below the water table. Some driven wells are put in by hand and more modern are put in using a pole driver. They are all still very shallow and suffer from similar issues as hand dug wells.
Drilled groundwater wells – there are drilled into the ground and through rock using a rock drill. There are various types of drills, but the process ends up the same – a deep well. The outside of the hole has to be ‘cased’ to stop surface water contaminating the water. These are now very common well types and can be used in even the driest areas as they can obtain water from very deep water tables.
What is ‘draw down’?
When you pump water from any type of well you basically ‘suck’ water from the surrounding water table. The height of the water table drops around the well and this is called draw down. Depending on the speed in which groundwater flows back to fill this area the draw down can be recharged in minutes, hours or days. In drought conditions, or in places where there are just too many wells into the water table, the drawn down can be permanent.
So what about well care?
Caring for your groundwater well is one of the most important things you can do on your farm. You need to ensure that surface water does not contaminate your well water. This means you need to stop surface water flowing into or even around your well head/casing. You also need to make sure that you keep away activities around your well head that could contaminate your water including:
- Composting/dung heaps
- Oil/hydrocarbons – so don’t park your tractor upslope of your well!
You should consider getting your well water tested every few years to make sure that contamination is not happening to that water OUTSIDE of your farm activities!