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Pesticide Safety Education Program (PSEP)
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Back Siphoning Pesticides


Many people are busy at this time of year spraying fruit trees, vegetable gardens, and lawns with pesticides and herbicides. Most people use small hand-held sprayers attached to a garden hose. We have seen several situations where those pesticides have siphoned back into the home water system, and people have become ill. In one case, a man died from drinking contaminated water from his home. We would like to take a moment to tell you about how some of these incidents occurred, and how you might prevent that happening to you.


In Nampa, Idaho two homes and a large acreage all take their water from one well. The acreage is primarily alfalfa, and the well is used to supply water for sprinkler irrigation. 2,4-D is a herbicide which is used to kill broadleafed weeds. It is relatively low in toxicity to humans, but can be fatal if taken in very large quantities. As 2,4-D was being injected into the sprinkler system for weed control in the alfalfa, the pump failed. Water fell back down the well, and that created a back-siphon which pulled the 2,4-D backward in the water line. Most of it ended up at one home. The safe level for 2,4-D in a domestic water system is 0.1 parts per million, or one drop in ten 55-gallon drums (that is a high safe level for a pesticide). The accident happened Saturday. The people could still taste the chemical Tuesday, so they had the Health Department test the water. On Tuesday the 2,4-D concentration was 1.3 parts per million, or 13 times the acceptable level. The people had to carry in water for some time, and the well still has not cleared up. So far, the people have had their water tested 10 times to try to isolate the problem and see if it has cleared up. Those samples cost $75 each.

The Poison Control Center in Pocatello recently received a call from an individual in Weston, Idaho. The person said they were spraying fruit trees with a hand-held sprayer on a hose, using 2,4-D. There was some problem in the well, and the chemical siphoned back into the house line. They ran the well 12 hours and could still taste the chemical, so they wanted advice on how to clean the line.

In actuality, that individual had back-siphoned 2,4-D into their water system twice, and had tasted the chemical in their water for 5 days! The Poison Control Center gave them limited advice, and gave them the phone number of the State Pesticide Lab where they could get more specific information on clean-up procedures. The people in Weston have never even bothered to call the Lab. They are potentially drinking contaminated water, and they could correct the problems with relatively little trouble.

In Blackfoot, Idaho a farmer was filling his stock tank. He left the hose in the full tank, turned off the hydrant, and went to town for coffee. Two hours later, the tank was empty. Two check valves in the hydrant had failed and the entire tank had drained down into the well.

That was six years ago. The farmer is well aware of water supply systems and their problem, but has not yet replaced the check valves!

In Utah a man was spraying malathion on fruit trees. He set the hose down to answer the phone. At that time, there was a fire two blocks away. Fire hydrants placed such a demand on the water system that malathion was sucked into the water line. Pressure built up and the water flowed again. When the man came back out, the water was flowing so he thought there was no problem. Actually, the water leaving the hose was malathion that had first entered the hose a few minutes ago. He took a drink of the clean water. He was dead in 20 minutes.


These problems are examples of what is called a cross-connection; a place where contaminated water can get back into the supply. In all cases, the chance for a problem is very slim, but the effects of the problem are usually major.

In the example of the 3 families on the well in Nampa, the pump had pulled a long column of water up from the ground. There is supposed to be a check valve that lets water flow from the well to the line but not flow the other way. In this case, the check valve failed, the water fell down the well, and created a siphon from the irrigation system. When someone turned on the water in the house, they got water flowing from the irrigation line and not from the well. The problem was similar in the other cases; a little piece of the system failed and someone got badly hurt.



The solution to cross connection control problems is to prevent them from occurring. Municipalities are now required to have cross connection control ordinances. Those laws must specify certain backflow prevention devices for different industries. Car-washes, hospitals, and mortuaries are examples of facilities that must have backflow prevention devices. It is a much more common problem for pesticides to be taken into private or municipal water systems. Homes do not have backflow prevention devices. A fire on your block might put so much drain on the line that water from your neighbor's house gets back into the line and out of your tap. A failed check valve in your well or hydrant may put contaminated water into your water supply line. The solution is to maintain your water supply in good repair.

Replace failed check valves and be aware of water supply problems. Self-contained, pressurized sprayers that are not connected to hoses are the safest. Hose-connected pesticide sprayers are now available with backflow prevention devices built in. They prevent any water or other material from flowing back into the water line from the spray gun. All spraying should be done with sprayers clearly marked as having backflow prevention devices. Even that will not totally ensure safety. Use of self-contained pressurized sprayers is recommended.


If you know that pesticides, or other materials, have been siphoned back into your water supply, ask for help. There are several ways to try to clean the contaminated material out of the system.

The easiest solution is to pump the water for an extended period. If you run the water from all the taps in the house for 24 hours, you will often draw all contminated water out of the system. If the water does not clean up, try leaving the system alone; no use at all, for 12 hours, then pump for 2 hours. Charcoal filters placed on the tap will often absorb many organic compounds from the water. The only way to be sure the problem is cleared up is to have the water tested, but the samples have to be collected in special containers which are pretreated with a solvent. A mayonnaise jar of water is of no use to you or the lab. Let your County Health Department representative collect the samples, and advise you on clean-up of the well.

Adapted from Idaho Safety News

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