Hydrogeology is the study of groundwater -- water found below the surface of the earth. Believe it or not, about half of Maine's homeowners are practicing hydrogeologists, because they have, with the help of a well driller, explored for, found, and are actively utilizing groundwater. These homeowners derive their drinking water from private water wells located on their property. All of these wells, regardless of the type or depth, get their water from groundwater. Here are a few basic geologic concepts you should be aware of when considering having a well installed or evaluating an existing well during a home purchase.
Infiltration
All groundwater originates as surface water, either in the form of rain, snow or other precipitation, or as water collected in surface impoundments like rivers, streams and lakes. Water that falls on the ground, or runs over it, infiltrates into the spaces between particles of soil, and continues down into cracks and fractures in the bedrock below. Have you ever poured a bucket of water on the ground and watched it disappear? If you have, you've conducted a successful infiltration experiment.
Infiltration can also act as the mechanism for contaminating groundwater. Never dispose of petroleum, solvents, or other chemicals by dumping them on the ground. If you've spilled gasoline, paint, household cleaners, or any other chemicals on the ground and watched them disappear, you've not only conducted a successful infiltration experiment, but there's also a good chance you have contaminated your groundwater.
Aquifers
All wells derive water from aquifers. An aquifer is a zone below the earth's surface that can provide a usable quantity of water. In Maine, two types of aquifers are commonly used for water wells: bedrock and overburden aquifers.
Bedrock Aquifers
Bedrock aquifers have water in the cracks and fractures in the rock. In other states, sedimentary rocks (rocks made up of soil particles packed together) have pore spaces in the rock that can transmit water. Maine's bedrock is predominantly crystalline, either in the form of igneous rocks, which formed from the hardening of molten materials, or metamorphic rocks, formed by millions of years of being buried and crushed, heated up and melted, cooled down and solidified again, and lots of additional general abuse. Both types of rocks have no pore spaces usable for transmitting water. So, well drillers attempt to find large fractures in the bedrock that are full of water to supply the well being drilled. Luckily, the same havoc that eliminated the pore spaces in Maine rocks created lots of fractures. Most home lots in Maine have the ability to provide usable quantities of water from a bedrock well for domestic use.
Overburden Aquifers
Overburden wells are quite different from bedrock wells. They tap aquifers located in the loose, unconsolidated materials on top of bedrock. The most common are wells drilled into sand and gravel deposits left by the last period of extensive glaciation, 12,000 to 18,000 years ago. These deposits are the remnants of glacial rivers, material that dropped out of the ice as it melted, and the debris bulldozed up by the advancing ice sheet. The water from these deposits comes from the spaces between particles, something geologists call porosity. The higher the porosity, the more water that's available for a well to pump. The deposits in overburden aquifers act as natural filters, removing bugs, small particles, and other contaminants from the infiltrating water before they get to your well. Drilled wells, dug wells and driven well points are all examples of wells located in overburden.
The DWP recommends drilled wells rather than dug wells or driven points because they are constructed by licensed professional well drillers, and because they experience far fewer problems with contamination by bacteria. They are also much less likely to go dry during the hot summer months.
Driven-Point- and Dug Wells tend to be shallow (less than thirty feet), resulting in easier contamination (Mother Nature's filter doesn't have time enough to work), and they are the first wells to go dry when water shortages occur.
Well Types
There are three types of wells commonly used in Maine: drilled bedrock wells, drilled overburden wells, and dug wells/springs. Of the three types, drilled bedrock wells are by far the most common source of drinking water for Maine homes.
Bedrock Wells
The typical bedrock well is six inches in diameter, one hundred to five hundred (or more) feet deep, and has a yield of less than 10 gallons per minute (GPM). Bedrock wells generally have steel casings, driven through the overburden, and 'set' into bedrock. A pump is placed in the well at a depth where sufficient quantities of water exist to handle anticipated use.
A couple of important points to make about bedrock wells:
- Not all bedrock wells are 'artesian' wells. An artesian well is a particular type of well that is free flowing at the surface under natural conditions.
- There is no water in bedrock. You can squeeze it all you want, but you'll never get water out of a rock.
So, if there is no water in bedrock, where does all the water in a bedrock well come from? It comes from cracks and fractures in the rock. When a well driller is drilling a well, he/she is searching for fractures that are full of water. Naturally, the bigger the fracture, the more water it can hold, but, just as important, the fractures have to be connected to some source of recharge -- additional water that will refill the fractures as water is pumped out. The safe yield of a well is the rate at which water can be pumped out and at the same time be replenished by these sources of recharge. Keeping water use at or below the safe yield will ensure that your well will never run dry.
Following are some considerations a homeowner should be aware of when having a well drilled:
- Is the driller licensed and in good standing with the proper regulatory agency?
In Maine, well drillers and pump installers are licensed by the Maine Well Drillers Commission (207-287-5699). - Is the location you've chosen safe from potential sources of contamination? In particular, is it a safe distance away from any septic systems?
The Drinking Water Program recommends at least 100 feet of horizontal separation between a private well and a private septic system(s), with greater setbacks for public wells and/or large septic systems. The well should also be as far as possible from oil and fuel storage, parking areas, sheds with equipment or chemicals inside, and agricultural fields (including home gardens). - Because of the above, the well should also be as far from property lines as possible. Your neighbor's septic system or fuel tank can also contaminate your well.
- Has the driller properly driven the casing into bedrock, and sealed it as required in your state?
- In Maine, a minimum of 10 feet of casing driven into bedrock is recommended.
- Is there enough casing extending above the ground to ensure surface water will not enter the well?
If surface water (rain runoff, snow melt, etc.) is allowed to pool on the ground around the wellhead, it may enter -- and contaminate -- the well and the drinking water. - Is the casing properly capped and any vents screened to prevent bugs and animals from getting into the well?
- Has the well and water supply system in your home been properly disinfected?
- Has the water from the well been tested for bacteria, nitrate & nitrite, arsenic, radon, and any other potential contaminant that may have entered from groundwater?
Long Term Care of a Bedrock Well
Know the limits of your well: don't allow it to be pumped dry. This is bad for you and your well, and especially the pump.
Do not allow contaminants to get into your well. This includes parking cars or refueling equipment near the wellhead. Never store or use dangerous chemicals near your well. Also, only apply fertilizers or pesticides as directed by the manufacturer, and if possible, never apply them within 100 feet of your well. Your drinking water is much more important than a weed free lawn.
Keep all of the information your well driller gave you about the well, such as the depth drilled, the length of the casing, the location (depth) of any fractures encountered, and the safe yield. Be sure this information is kept in a safe place. It might come in handy in the future.
Definitions
Artesian Well: A well that is free flowing at the ground surface due to natural pressure, usually a confined aquifer, which causes the water level in a well to be above the ground surface.
Bedrock: The solid rock that underlies unconsolidated overburden materials.
Casing: The 'pipe' used in well drilling to maintain a hole in the overburden. For most wells, the casing is 6 inches in diameter and made of steel.
Fracture: Breaks in bedrock, some of which transmit usable quantities of water.
GPM: Gallons-per-minute -- The most common unit of measurement for describing the yield of a water supply well.
Overburden: Unconsolidated materials such as clay, silt, sand, gravel, and boulders that overlie bedrock.
Recharge: Water that originates as precipitation, infiltrates through overburden and then into fractures in bedrock, which is then available to replace groundwater removed by pumping.
Safe Yield: The maximum amount of water that can be pumped safely from a well without risking damage to the well or the pump. The safe yield equals the amount of recharge.
Yield: The amount of water being drawn from a well.