Domestic Private Wells

Well Operation and Maintenance

Well owners should test their well water (see Section 8) as there are no state or federal regulations that require routine testing or monitoring of private domestic wells. Domestic well owners must also repair and maintain their own wells to assure a reliable water supply of consistent quality.

Arizona has stringent permit requirements for the installation of new wells, and the construction diagram and geologic log of all modern wells in the state are recorded with the Arizona Department of Water Resources (ADWR). In addition, a private domestic well is exempt from regulation, and is thus referred to as an “exempt well.” The ADWR website—www.azwater.gov/dwr—provides a wealth of information for the private domestic well owner.

Well Construction

For the proper maintenance of wells, it is important to have a basic understanding about the different materials that comprise your home water supply system based on a domestic well. The following sections present some information about well casings, well caps, well screens, and pitless adapters; basic materials that combine with a pump to provide water for a household. Please refer to Figure 15 for the location of these well components.

Figure 15: Domestic well diagram

Well Casing

A modern domestic well has two well casings—the outer casing is a tubular structure or large diameter steel pipe that encircles the actual well casing, and is used as a surface seal. The length of this surface seal well casing is specified by Arizona Statutes and Rules, as regulated by the Arizona Department of Water Resources (www.azwater.gov/dwr). The surface seal casing must be a minimum of 20 feet, one foot of which must extend above land surface. The final length of the surface seal casing is dependent on the local geology and may extend to greater depth to seal the well from contact with a shallow aquifer. The intent of the surface seal casing is to prevent surface contaminants from entering the well.

An example of a surface seal casing is a concrete apron, sloped away from the pipe, to reduce the potential for standing water to pool at the well head. At a minimum, the land surface or soils near the well head should slope away from the surface seal casing if a concrete apron is not present.

The well casing is placed in a drilled borehole to maintain the well opening and contain the drop pipe and electrical wiring to the pump. Along with cement grout that seals the upper portion of the well to the surface casing, the well casing prevents mixing of multiple aquifer zones and may extend to the full depth of the well. In rock aquifers, the well casing may only extend a hundred feet or more through broken rock, leaving an open rock borehole as the well.

The most common materials for well casings are carbon steel, plastic (PVC is commonly used), and stainless steel. PVC is lightweight, resistant to corrosion, and relatively easy for contractors to install. (Note: To minimize exposure to residual solvents, PVC casing sections should be joined without glues that contain solvents.) Although more expensive, when possible mechanic coupling or threaded pipe fittings are recommended. Steel, although stronger, is susceptible to corrosion, can develop scale in hard waters, and is more costly. Some well casings may also be constructed of concrete, fiberglass, and asbestos cement. Older wells may be hand-dug and cased with hand-placed bricks or stone.

Caps

On the top of the surface seal casing, and sometimes on the well casing itself, should be a wellhead seal or cap. Well caps are usually aluminum or a thermoplastic, and include a vented screen so that the pressure difference between inside of the well and outside atmospheric pressure may equalize when water is pumped from the well. The cap should fit snugly so debris, insects, or small animals can’t find their way into the well system.

Well Screens and Gravel Packs

Well screens are filtering devices used to prevent excess sediment from entering the well. Attached to the bottom of the well casing, the screens allow water to move though the well while keeping out most sand and gravel. The most common screens are slotted or perforated pipe.

Perforated pipe is a length of casing that has holes or slots drilled into the pipe. It is not efficient for aquifers that contain fine-grained materials because it has wide openings that allow sand to fall into the well. A continuous slot screen is made of wire or plastic wrapped around a series of vertical rods, whereas slotted pipe features machine-cut slots into steel or plastic at set distances.

Well screens are manufactured with specified openings and hole diameters to match the screen filtering capability to the geologic conditions. Well screens are designed to be placed only within the saturated portion of the aquifer. If the ground water elevation drops and air is allowed to enter the well screen, the well may be damaged by oxygen-induced metal corrosion.

During well design and installation, a gravel pack is typically placed in the annular space outside the screen casing yet within the drilled borehole. The gravel pack consists of sand or gravel that has been designed with a grain size finer than the adjacent soils or unconsolidated aquifer material, yet larger than the screen slot size. The gravel pack acts as a filter to prevent sediment from entering the well, and also to manage the velocity of the water passing through the aquifer and into the well. High-speed water velocity, due to excessive pumping or improperly sized gravel pack, results in erosion of the aquifer as sediment is pulled into the well. Above the gravel pack and the well screen, the annular space between the well casing and borehole wall is backfilled with grout and/or concrete to prevent surface water from draining into the aquifer.

It is common for wells constructed in hard, stable bedrock to remain as an open borehole. In these cases, a screen or gravel pack is not necessary. Since ground water entering an open borehole in a bedrock well typically travels through narrow cracks and fissures, no sediment filter is necessary.

Pitless Adapters

In higher elevations where frost may penetrate the ground, pitless adapters provide wells with a sanitary—and frost proof—seal between the well casing and the water line running to the well system owner’s house.

After a frost depth is determined for the area where the well is being installed, the adapter is connected to the well casing below the frost line. Water from the well is then diverted horizontally at the adapter to prevent it from freezing, and the plumbing continues beneath land surface to the well system owner’s house.

Storage Tank

Most home-owner water well systems include a pressurized storage tank to store water for use during periods of heavy usage. The pressure tank is designed to have extra water on reserve so that small demands don’t require the pump to switch on. However, a tank cannot compensate for demand greater than your pump or well capacity.

Well Log/Report

Every modern well in Arizona is required to be registered with the Arizona Department of Water Resources (ADWR) and a well log must be submitted by the well driller. This log is available to the well owner through ADWR. The well log identifies the type of geology of the aquifer, the construction materials used to construct the well, the well depth, casing length, screen length, the presence (or absence) of a gravel pack, depth to ground water at the time of installation, and the capacity of the well at the time of well installation. Every well owner should have a copy of their well log.

At the time of construction and pump installation, the licensed well driller pumps the well to test the capacity of the well to yield water, and to also remove any fluids (such as chemical drilling muds to facilitate drilling) from the aquifer. This pumping also develops the gravel pack around the well, flushing out fine-grain silts and sands from the pack to allow water to flow freely into the well. For an exempt domestic well, well pump capacity is restricted to 35 gallons per minute (gpm), but some aquifers are not able to yield water at that rate. It is not uncommon for wells constructed in consolidated bedrock or finer-grained alluvium to yield 3 to 5 gpm.