HOW OLD IS YOUR WELL WATER?
This article, written by the American Ground Water Trust was originally published in THE AMERICAN WELL OWNER, 2001, Number 3]
HOW OLD IS YOUR WELL WATER?
The water you drink may be composed of the same water molecules that have been around since life started on this earth 4.6 billion years ago. We could be drinking the same water dinosaurs swam in millions of years ago. The age of ground water pumped from a well is taken to be the length of time it took to reach your well since it infiltrated the ground surface. Most drilled wells may contain water of different ages and the pump may deliver a “water cocktail” from different rock layers with the older water usually deeper in the well.
The long travel time underground, away from risks of surface contamination, is one of the reasons that most well water is fit to drink without treatment. Most home wells are likely to pump relatively “young” ground water that is less than ten years old. However, wells tapping deeper aquifers may have water that last saw the light of day many centuries ago. The following information, taken from a 1994 Trust publication* helps explain subsurface flow of ground water.
Groundwater generally moves very slowly. Rates of a foot per day are considered fast, although rates of a few feet per month are more common. It is hard to imagine these slow rates of movement when compared with the apparent fast flows from a large spring or a pumping well. However, at a well or spring, the flow of water is concentrated and may represent the net combined input from many deep bedrock fractures and recharge derived from a large area. For a given aquifer system, ground water flow rates generally increase as the water table level is raised following recharge.
Virtually all aquifers are dynamic, and flow is constantly occurring although rates of flow will vary within the aquifer with deeper water tending to move more slowly. The direction of flow within the aquifer is generally from the point of recharge to the point of discharge. These places may be some distance apart and ground water discharging into a wetland (or well) may have received most of its recharge some distance away. With slow travel times, and long travel distances, the age of ground water may be tens of years or more between recharge and discharge. Some deep confined ground water is thousands of years old and yet is still slowly on the move.
Although often spending a long time beneath the surface, ground water can respond almost immediately to recharge because the “new” water can have the effect of pushing out the “old.” Geologic structure and rock types can influence ground water flow directions but even in relatively simple geologic situations, movement does not occur in straight lines. Most ground water has curved flow paths.
[*Wetlands and Ground Water in the United States, a joint production of the Trust and the Audubon Society of New Hampshire, is available for online purchase at www.agwt.org]
[© American Ground Water Trust. This article may be reprinted for non-commercial educational purposes provided it is used in its entirety and that reference is made to its source as an article in THE AMERICAN WELL OWNER, 2001, Number 3]
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