Tap Water for the Garden...is it Safe?

I had a call from my friend Ed today.  

A couple of weeks ago we were talking about tap water and discussing how safe it is to drink. Here, we are on well water, but I mentioned to him that when we travel we leave a pitcher of tap water on the counter. This, overnight or for some hours allows the chlorine to dissipate. Ed, a city dweller, had an “ah ha” moment. He mentioned that his cat goes out each morning and drinks from yesterday’s water dish on the porch and won’t drink from the fresh bowl in the kitchen. 

Today Ed brought it up that he has been leaving an open container of water on the kitchen counter overnight. He has been blown away by the taste difference. “Tea has never tasted better”, he said. Then we began to wonder whether that same chlorinated water from the tap could have negative affects on the vegetables in our gardens.

There have been many studies about the effects of chlorine in tap water. Studies by the Environmental Protection Agency, National Academy of Science, U.S. Council on Environmental Quality, and by universities and physicians have identified chlorine as a carcinogen that elevates the risk of bladder and rectal cancers. The introduction of chlorine into the public water supply was to combat waterborne diseases such as typhoid fever, cholera and dysentery. Chlorine was also used in mustard gas in World War 1 and in chemical weapons in more recent wars.

Chlorine is not dangerous in and of itself, but when it comes into contact with organic sources it takes on whole new form. When chlorine combines with humus and other organic matter it creates trihalo-methanes (THMs), a known carcinogen. Considering that organic gardens are full of humus from composting, is it dangerous to add tap water to the mix? Will it affect us in some way? These questions go unanswered in my inquiries into this topic.

Because there is no ready answer, it seems best to keep chlorine out of the vegetable garden. 

One method may be to use a rain barrel.  However, rain water that has come over asbestos shingles may have more chemical compounds than the tap water we are trying to avoid. 

Another method, suggested by a friend, Tim, is to run water from the hose into a barrel and let it sit overnight. This corresponds to the drinking water left on the counter to let the chlorine evaporate. Then, use a galvanized, non-plastic watering can or non-toxic hose to take water from the barrel for the garden. 

More research needs to be conducted on the relationship between tap water and its use for irrigating organic produce. Meanwhile we may want to be cautious about the possible effects of chlorine in our food supply. -G.H. 

Some references:

A study showed that “the cancer rate among people drinking chlorinated water is 93% higher than among those whose water does not contain chlorine". -U.S. Council of Environmental Quality

"When natural waters are chlorinated and come in contact with humic substances (decaying vegetation, etc.) it produces trihalomethanes." -National Academy of Sciences.

"Trihalomethanes in general, and chloroform, a known carcinogen in particular, are found in drinking water as a direct consequence of the practice of chlorination, a long established public health practicer for the disinfection of drinking water." -Francis T. Maho, Director of Municipal Environmental Research Lab.