If a well doesn't produce much water, it's typically called a low-yield well. For household water use, this would be a well that could only produce a few gallons a minute or less. There are a few reasons why a well might be low yielding. The well may be in an aquifer that just doesn't allow water to move freely to the well, meaning the aquifer around the well has low permeability. There may be plenty of water in the aquifer, but the water cannot travel through the aquifer material quickly enough to replenish the water being removed by the pump in the well.

For example, even clay can hold up to 30% of its volume as water, but water moves through clay very slowly — too slowly to be considered an aquifer. Water in a well can only be pumped out at a rate equal to the rate the water can recharge the well. If pumped at a rate higher than that, the water level in the well will decline until it reaches the intake for the pump, and then the pump will begin pumping air or lose its prime. In either case your water system will lose pressure and if the pump continues to run it could overheat.

Another possibility is that the aquifer itself may have low porosity, meaning that there just isn't a lot of water in the aquifer near the well. This would typically occur in a bedrock well where the well only intercepts a few small fractures. For example, granite is not a good aquifer material, but in the mountains it may be your only choice. Typically the fracture volume that will hold water in granite is only a few percent, so many wells in granite are low yielding.

Rather than relying on the pump to keep up with water use, these systems typically rely on large pressure tanks or multiple pressure tanks that can store water for when it's needed. The pump is sized small enough (say, one half to two gallons per minute) to continually fill the pressure tanks while maintaining the water level in the well above the pump intake. It slowly fills the pressure tanks at times when the water is not being used.

For example, a well owner on the western slope of the Rockies in Colorado has a granite well that only pumps a half gallon a minute. Pumping any more than that will draw the water level in the well down to or below the pump. Instead, it pumps continuously at this rate into a 3,000 gallon cistern, so that when they need water for showers, laundry, etcetera, it is available by pumping from the cistern. It's not an ideal situation, but a workable solution when water is not readily available.

If your pump capacity is too large, the water level in the well will drop to the pump intake. When this happens, air can get into the system, which prevents water pressure build-up in your tank. Since shut off for the pump is based on reaching a high end pressure in your tank, this can keep the pump on and cause it to overheat, damaging the unit. You can protect your pump by installing a low pressure cutoff switch. This switch will shut off your pump when the pressure in the system drops too far below the normal cut out pressure. This way your pump doesn't continue to run when water isn't available. Before considering this, you should talk to your contractor for advice, but when you have a low yielding well, this switch is a great way to ensure your pump doesn't run when the water level in the well drops to the pump intake.

Low-yield wells can be problematic for other reasons., even if you have a very large pressure tank and can manage to provide an adequate supply of water to your home. These wells can have significant variation between the non-pumping and pumping water levels, especially in bedrock wells that are open-hole, meaning they have only a little casing in the bedrock. The repeated introduction of air on the rock as the water level drops in the well can cause changes in chemistry.

Many metals form precipitates when exposed to air or oxidized. These changes can release metals into the water, cause sediment formation in the well, and possibly cause additional scale build-up on your water system components. They can also provide a better environment for biofilm growth. If you have ever noticed the rusty color that iron can cause on porcelain, that is iron oxide, which comes from the iron in the water that has been oxidized when exposed to air. The same can happen with arsenic and other metals that might be a health risk.

A low-yield well in Pennsylvania was sampled throughout the day as the well was used. The water level in the well dropped and recovered by nearly 100 feet. Hourly samples were collected and the data showed significant changes in water chemistry on an almost hourly basis. Our suggestion is that if you have significant drawdown in your well when it's being pumped or if you have a low yielding well, contact your local health department to find out if there are any naturally occurring contaminants of concern in your area.

This is important health information for any well owner, but will be particularly significant given the characteristics of your well. We suggest you regularly sample your well for the metals listed by your health department to better understand if changing water levels affect the chemistry of the water coming from your well.

The Private Well Class is a collaboration between the Rural Community Assistance Partnership and the University of Illinois through the Illinois State Water Survey and the Illinois Water Resource Center, and funded by the United States Environmental Protection Agency. The views expressed here are solely from the class authors and not endorsed or reviewed by US EPA. For more information on private wells, sign up for our free 10-week email course at www.privatewellclass.org.