Irrigation Wells and Contamination
Even though the water from an irrigation well might not be used for drinking water, contamination from it can affect the quality of water from nearby drinking water wells. For this reason, it is important to make sure that all wells are properly constructed and protected as much as possible.
Wellhead Protection AreaWellhead area refers to the area in which surface water recharges the ground water supply that feeds a well. There are a number of steps in protecting a wellhead area. One of the first steps is to determine the size and shape of the wellhead area. This can be a complex process, affected by the geology and topography of the area, the rate of pumping from the well, and the time frame for needed protection.
As water is pumped from a well, ground water flows from the surrounding aquifer into the well. This causes a cone-shaped depression in the ground water surface around the well where the water has been pumped out. If the water in the aquifer were not moving, this cone of depression would be a circle. However, ground water usually is flowing very slowly, and, as a result, the water table usually is sloped slightly. The slope of the ground water surface generally follows the slope of the ground surface, with ground water flow from highland areas toward river valleys, lakes and the ocean. Because of this flow, the cone-shaped depression in the ground water surface around a pumped well usually is distorted.
Once you get far enough downhill from the well to escape the zone of contribution, water and contaminants carried by surface water percolating into the ground are carried away from the well by the natural ground water flow and will not affect your well. Directly uphill from the well, however, nearly all contaminants eventually will reach the well - and the closer the contamination occurs, the more quickly it will reach the well.
Because ground water moves very slowly in most aquifers, a spill of contaminants a few hundred feet from a well might take a year or more to reach the well. During that time, contaminants are constantly being decomposed and diluted; therefore, their effect on the well will be reduced. The change in the contaminant depends on its makeup and conditions in the subsurface environment. Some contaminants break down in a relatively short time, while others may be unchanged over a period of many years. Products that break down very slowly have a long life and should be avoided or used with extreme care in the zone of contribution of water wells.
Well LocationLocation determines, to a great degree, a well's potential for ground water contamination. If there are possible sources of contaminants in the area, the well should be located uphill from these sources. Locating the well uphill means the natural flow of ground water will reduce the chance of leached contaminants being in the water pumped by the well. The uphill location also ensures surface water runoff will carry contamination from possible pollution sources away from the well.
Moving high-risk activities outside the wellhead protection area will reduce the risk of well contamination. However, if spills or other accidental releases of contaminants occur, the contamination still can reach the ground water and cause pollution in neighboring wells. Separation from sources of contamination is important to protect wells from pollution. If a well is a long distance from a source of pollution, the contaminant may degrade a great deal due to exposure to air, sunlight and biological activity before it reaches the well. Any degradation and dilution of the contaminant reduces its potential as a health or environmental hazard.
Housekeeping IssuesIt is important to keep the wellhead area clean and environmentally safe. There are many products that may be used around irrigation wells that can potentially cause contamination and rapidly enter the ground water through a damaged well casing or through coarse topsoil overlying a shallow water table. Well houses often are seen as convenient places to store items such as fertilizer bags and pesticide containers. In no case should the well house be used as a storage area.
If an accidental release of a contaminant occurs in the wellhead area, it immediately should be cleaned up as completely as possible. If the contaminant is allowed to remain in the soil of the wellhead area, much of it eventually will be carried into the ground water by rain and surface water percolating through the soil to recharge the ground water. From there it may be pumped out in the irrigation water or drinking water of your well or a neighboring well.
Backflow PreventionMany operators use irrigation systems to apply fertilizer and chemicals, a practice called chemigation. Certain precautions must be taken to prevent contamination of the irrigation well should an unscheduled shutdown of the irrigation pump occur during chemigation. If the pump stops while chemical products are within the irrigation pipeline, backflow of contaminated water into the well can contaminate the ground water supply. Federal law requires backflow prevention devices when toxic chemicals are applied through irrigation water. The main device required is a chemigation check valve. This consists of a spring-loaded, positive-seating, chemically resistant check valve with an atmospheric vacuum breaker and a low-pressure drain. The chemigation check valve is placed in the irrigation pipeline between the pump and the point of chemical injection, preventing backflow of contaminated water into the well.
An approved alternative backflow prevention device is a gooseneck pipe loop. This is a loop in the irrigation pipeline that rises at least 24 inches higher than the highest outlet in the irrigation system and has an atmospheric vacuum breaker at the top of the loop. The rise in the loop prevents backflow due to the backpressure in the sprinkler system, and the vacuum breaker prevents formation of a siphon down to the bottom of the well.