INTRUSION OF SURFACE, SALT OR CONTAMINATED WATERS INTO POTABLE WATER WELLS
Cone of Depression related problems

As constructed today, a well is a hole drilled or bored in the ground into which water flows or dribbles depending on where you drill it. When a well is pumped water from the surrounding area flows in to replace what was withdrawn. This creates a depression in the aquifer around the well which is deepest at the well and grows shallower as you get further from it. This is because water immediately adjacent to the well flows into the hole faster than water coming from a distance can replace it. The depression around a single residential well isn’t nearly as large as that around a development of a hundred homes or a municipal well that feeds the water system for an entire town.

The depression, called the cone of depression, varies in size depending on the rate at which water is withdrawn and the length of time the pumping continues. The time required for the depression to fill back in after pumping ceases is directly related to the rate at which water flows through local geology.

Water in an aquifer flows from areas of high pressure toward areas of low pressure. Water in the ground near exit points like stream and lake banks or bleeding fractures in a cliff face flows faster than water from a distance can move In to replace it so a depression, similar to that found around a well, forms creating the pressure differential that keeps water flowing in that direction.

The rate at which water moves horizontally through an aquifer depends on the difference in pressure and the size of the pores and fractures in the geology through which it is moving. When wells are pumped down below the fractures or points that feed them, pressure in the well drops to zero and the rate at which water from the immediate area flows to it accelerates. This unnatural acceleration causes dirt and rock chips that have settled to be picked up and carried toward the well.

If the well is drilled in hard rock and the fractures that lead to the well narrow in that direction this debris may collect and choke off the fissure feeding the well. If they get larger or stay the same size the grit may enter the well and show up in filters, screens and orifices in the plumbing or irrigation system.

 If the well is constructed in ground that is not self supporting it may be cased all the way down or a portion of the bore that will not support itself may be cased. In this type of well water enters the bore through holes in the casing or sections of the casing in which well screens are installed. Debris that moves toward the well as a result of over pumping can also cause holes and screens to become clogged resulting in decreased well yield.

We use this relationship between pressure and flow direction to advantage when cleaning up underground storage tank (UST) leaks. Fuel leaking from a buried gas or oil tank spreads out in a plume that follows the natural flow of water in the aquifer. It has become commonplace to drill wells at a UST leak site and then over pump them creating an area of low pressure which can reverse the natural direction of flow and return the spilled petroleum to the site from which it originated. Water removed from the wells is then treated to remove the petroleum products.

So you see, a large area of low pressure created by over pumping wells can create a flow in an aquifer toward the low pressure point. A development of a hundred homes, for instance, can create a large cone of depression that will exert a large influence on the aquifer when all of those pumps start up to satisfy simultaneously occurring peak demands.

When you create a point in the aquifer which attracts flow toward it from every direction, you create a situation that attracts pollutants to the one place you do not want them to be; potable water wells.

The effects of over pumping disturb the natural balance that keeps nature’s infrastructure working correctly. In coastal areas where fresh water aquifers adjoin those containing salt water this balance keeps the two separated. As the tides rise and fall, the direction of flow in the nearby aquifer changes. But each time the salt water advances the falling tide causes the pressure on the sea side to drop so the fresh water pushes it pack. This goes on every day but the net result is that the interface between the salt water and fresh ends up at about the same spot every day.

When we construct a large well close enough to influence this relationship, as happens in coastal communities, the resulting cone of influence can alter the relationship between fresh and salt water to the point where the interface between the two advances inland a little each day until one day the water from the well begins to show just a hint of salt taste. Once that happens there is an excellent chance that all of the wells that lie between the well that is being over pumped and the body of salt water will also show signs of salt intrusion.

Reduction of the size of the cone of depression around a well or wells also reduces its influence on the aquifer. The accepted standard in water well operation today is to guzzle directly from the aquifer to meet peak demand. A change in that behavior would reduce the influence one well has over another, their collective influence on an aquifer and result in better quality and increased quantity of water harvested from a well.

Time Based Pumping Can Reduce the Cone of Depression  

It is possible to reduce the cone of depression by reducing the rate at which you withdraw water from a well and by not pumping it down too far. This can be done with equipment that can control pump speed or by installing a flow restrictor in the pump discharge line and thus reduce the rate at which water is withdrawn from a well or it can be accomplished using timers which allows the use of any pump that happens to be installed in the well.

Systems that use speed controls or flow restrictors must run the well pump constantly to harvest the wells full potential whereas the use of time based pumping will require only a few minutes of pump run time per hour.

Using time control it is possible to adjust withdrawals to match well yield without running the well pump very long at all and you get to decide how much water will be withdrawn at a time. For instance, a one gallon per minute well can produce 60 gallons of water per hour. A 6” diameter well holds about 1.5 gallons of water per foot. If you pump 60 gallons from a well once every hour you may draw the well down (60÷1.5 = 40) 40 feet.

If, instead, you pumped 20 gallons three times an hour you would still withdraw 60 gallons per hour but the well would only be pumped down (20÷1.5 =13.33) 13.33 feet thus reducing the size of the resultant depression in the water table and the adverse influences that accompany it. More frequent pumping for shorter durations reduces the overall impact on the aquifer as long as the total amount taken per unit of time does not exceed well production.

Time based pumping can also produce more water over time. Wells only produce water when they are not full. If a well is left full, no water will flow in. If you remove some water more will flow in to replace that which was withdrawn.

The maximum production of any well is the product of the rate at which water flows or dribbles into it (well yield) times the number of minutes in a day. There are 1440 minutes in a day so a one gallon per minute well can produce 1440 gallons per day. But, to do that the well must be kept producing. If the well is left standing full for 12 hours then the maximum production for the day is reduced by half to 770 gallons. Most water collection systems in use today collect only a small percentage of a well’s potential because they only collect if someone uses a prescribed amount of water first. In other words, they waste time!

Time based pumping can keep a well producing by making regular withdrawals so it becomes possible to get the entire daily production of a well while limiting the cone of influence to the point where adverse affects are negligible.

Drawdown of the well level is controlled, keeping the feeders submerged. You are now withdrawing water at a rate that is the same as or less than the rate at which water is traveling through the aquifer toward the well. There is no large reduction of pressure and no accompanying acceleration of water flow so debris is not carried to the well. Pumping a well in this way often causes water that was muddy looking or full of grit to clear up and, because the fracture’s or well screens tendency to clog is reduced, extend its life.  

Timed pumping allows the use of wells that would normally be considered unusable. The same advantages that allow reduction of the cone of influence around a well also permit collection of the well’s entire production when there is very little standing water in it. For instance it is possible to collect the entire output of a quart per minute well with only 5 feet of standing water in it by collecting 5 gallons (3.33 feet) of water three times per hour. AND it is possible to do the same with a 1/10th gallon per minute well with 4 feet of water in it by collecting 6 gallons once per hour.

It is also possible to insure performance of a connected plumbing system into the future by restricting the amount of water that can be withdrawn to a portion of a well’s yield.

SEE: INSURING SUSTAINABILITY OF PERFORMANCE IN WELL SUPPLIED SYSTEMS BY LIMITING WITHDRAWALS