Division of Environmental and Community Health

Maine Center for Disease Control & Prevention

A Division of the Maine Department of Health and Human Services

DHHSMeCDCEnvironmental and Community HealthDrinking WaterWater Resources → Hydrogeology


Hydrogeology is the study of groundwater. Groundwater is water found below the surface of the earth. 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: How did the water get down there?

All groundwater originated 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. Unfortunately, if you’ve spilled gasoline, paint, household cleaners, or other chemicals on the ground and watched them disappear, you’ve not only conducted a successful infiltration experiment, YOU’VE CONTAMINATED YOUR GROUNDWATER!!! Infiltration is the mechanism for contaminating groundwater. Never dispose of things by dumping them on the ground.

Bedrock Wells: Is there really water in solid rock?

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 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 are trying to find large fractures in the bedrock that are full of water to supply your well. 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 Wells: Mother Nature's Filter

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 glaication, 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. These deposits 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 Maine Drinking Water Program 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 points and dug wells tend to be shallow, 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.