COLUMBIA, Mo. - With spring and warmer weather soon drawing children outdoors, it is a good time for farmers and rural landowners to check abandoned wells.

"There are an estimated 150,000 to 300,000 of these wells in Missouri. If left unplugged, these are a physical threat to children and a potential source of groundwater contamination," said Bob Broz, a University of Missouri water quality specialist.

"If a well is not properly sealed, children or even adults can be at risk of falling into a well," said Broz.

Plugging a well is cheap insurance for the landowner and may prevent a liability claim, he said.

Unplugged abandoned wells can be a conduit for contaminants such as pesticides, petroleum products, human or animal waste and bacteria entering groundwater supplies.

Bacteria and nutrients entering abandoned wells may also contaminate water in nearby newer wells of similar depth.

It is the landowners' responsibility to plug any abandoned wells on their property. Landowners can plug their own wells if they follow state rules, Broz said.

To plug an abandoned well, the landowner needs to know the total depth, length of casing and depth to water. If these details are unavailable, landowners should use a well installer or pump contractor with a state-issued permit.

Dug wells and cisterns are typically 3 to 6 feet in diameter and 15 to 50 feet deep. To plug these wells, push in the upper few feet of well lining then fill the well to within 3 feet of the surface with a chlorinated clean fill such as coarse gravel or rock. A mound of clay-rich soil over the rock or gravel completes the job.

Wells bored with an auger, scoop or dragline also usually have large diameters, making them as dangerous as abandoned dug wells. Bored wells can be plugged using the same methods for dug wells and cisterns.

Drilled wells are not usually a safety hazard, but when abandoned they can admit contaminants into the groundwater system.

To plug a drilled well, cut the casing off 3 feet below the surface and fill the well with neat cement, bentonite grout or chipped bentonite. Neat cement is a mix of Portland cement and clean water combined at a specified ratio. This material should not be poured from the surface. Instead, it must be inserted through a grout pipe placed into the bottom of the well.

Clean fill at the bottom of the well should be chlorinated to prevent bacteria from entering the aquifers.

"It's best to have a permitted installer do this work to make sure it's done correctly," Broz said.

Missouri requires landowners to register the plugging of abandoned wells by submitting a form to the Missouri Department of Natural Resources' Geological Survey and Resource Assessment Division.

For state regulations and the process of plugging an abandoned well, a fact sheet is available on the Missouri Department of Natural Resources, Division of Geology and Land Survey Web site at http://www.dnr.mo.gov/pubs/pub682.pdf.