Safeguarding Wells

Groundwater Bulletin No. 2, Winter 1995/96

Recommendations for Safeguarding Shallow, Large-Diameter Wells

The Illinois State Water Survey recently wrapped up the IGC-funded sampling study of private wells described in the first issue of the Groundwater Bulletin. This project focused on agricultural chemical contamination in shallow, large-diameter (dug or bored) wells, which are in widespread use for drinking water in many rural areas of Illinois and other states. When the project began in 1991, little research had been done on water quality in these types of wells.

The ISWS team collected water samples from 75 shallow, large-diameter wells in Christian, Edgar, and Sangamon counties over a three-and-a-half-year period. Samples were analyzed for nitrate and for 23 pesticides. Some wells were tested as many as 21 times during the study in order to track seasonal or yearly differences.

Final sampling data confirmed the findings reported earlier. The team's primary conclusions:

* Shallow, large-diameter wells are especially susceptible to contamination by agricultural chemicals. In large part this is because they store water that has seeped from the surface rather than draw water from aquifers like drilled wells do. Consequently, contamination in shallow wells is a site-specific rather than a regional problem. Local land use, chemical use, well characteristics, and surface geology, such as the type of soil in the area, determine contamination levels.

* Dug wells are more likely than bored wells to be contaminated. Dug wells often are not properly covered at the land surface. They also tend to be older and in poor repair.

* Nitrate contamination is of greatest concern. Virtually every well sampled had detectable levels of nitrate. Levels frequently were several times higher than the acceptable limit of 10 milligrams of nitrogen per liter of water.

* Fourteen pesticides were detected in the well-water samples. Atrazine was the most commonly found chemical, followed by dieldrin and chlordane. Other chemicals detected were acifluorfen, alachlor, butylate, cyanazine, diazinon, dinoseb, ethalfluralin, metolachlor, metribuzin, simazine, and trifluralin. Few wells exceeded government safety thresholds for pesticides, but about 15% of the wells contained unacceptable levels of at least one pesticide on at least one occasion.

* Contamination was persistent. The same wells consistently tested high in nitrate or turned up positive for a given pesticide. In most of the wells tested, contamination levels did not fluctuate greatly from season to season.

* The presence of nitrate did not predict the presence of pesticides or vice versa.

How to minimize contamination?

* Make sure all wells (especially dug wells) have a tight-fitting cover at the land surface and no cracks in the concrete or brick casing above the land surface.

* Make sure there are no cracks or gaps in the soil around the casing; they can lead to direct runoff of water into the well.

* Make sure that the top 10 feet of casing below the land surface is watertight. Bacteria and other contaminants can enter wells when surface water seeps down along the casing. Bacterial contamination is unlikely, however, if water entering the well has filtered through at least 10 feet of normal soil first, according to the Illinois Department of Public Health.

* Replace dug wells with bored or drilled wells whenever possible.

* Site wells used for household water as far as possible from farm fields, feedlots, and barnyards. Never locate wells closer than 10 feet to sewers, 50 feet to septic tanks, or 75 feet to sewage septic fields. Surface water should not accumulate within a 15-foot radius of the well.

To obtain detailed guidelines on proper location and construction of shallow (bored) wells, or to obtain a copy of the Illinois Water Well Construction Code, call the Illinois Department of Public Health's Environmental Health Division at (217) 782-5830. All new private wells must conform to the code specifications, and old wells must be brought up to code when modifications (including repairs) are made to them.

* Increase the setback zone from your wells in applying fertilizer and pesticides, and cut back on chemical use where feasible.

* Have your well water tested at least once a year for nitrate and bacteria. Call your local public health department or the ISWS for more information.

* Identify and properly seal all abandoned wells on your property. Old, unused wells can fall into disrepair and can provide a direct pathway for contaminants to reach groundwater supplies. Contact the ISWS or a licensed well drilling contractor for information about how to properly seal an abandoned well.

For more details about the ISWS research, contact Steve Wilson at (217) 333-0956 or Joe Karny at 333-8466.