Free Chlorine in tap water - why is it a concern for gardeners?

  1. A fellow faculty member (Math/Biology) at the high school where I teach (Physics/Chemistry) recently asked me this question:

    How long does it take the chlorine to evaporate out of my vase that I use to water my plants?

    My physics background kicked in and I responded that how warm the water was as well as the surface area of the meniscus of the water level would contribute to the answer.... he went away unsatisfied as most people do when they ask me what they consider to be a simple question.

    I have some questions of my own now...

    1. My chemistry background got me to thinking, wouldn't chlorine in tap water eventually be bound-up/reacted with other compounds floating about in the municipal water system....Or more to the point, does chlorine evaporate from standing water at all?

    2. The root of his question - Why is chlorine a concern for gardeners?

    3. Would the CO2 dissolving into the standing water negate any benefit that evaporating the chlorine from the water would provide? Assuming that the answer to question 2 is related to pH....
  2. jcsd
  3. turbo

    turbo 7,366
    Gold Member

    For #2, which is an important one: The soil in your garden is not just sterile dirt. It contains bacteria, fungi, and other microorganisms, many of which are beneficial to the plants in your garden. Even if you put organic materials in your garden soil like bone meal, blood meal, rotted manure, and compost those materials need to be broken down by microorganisms to free up the nutrients so that your plants can make use of them. Chlorine is added to drinking water to kill pathogens, and any residual chlorine left after that process is capable of killing the good microorganisms in your soil, if you water your garden with it.
  4. Astronuc

    Staff: Mentor

    As far as I know, chlorine affects the absorption of nutrients, and pH was an issue. The blueberry plants I grow need a pH of around 4-5 for optimum nutrient absorption, and raspberries and blackberries need pH = 5.5-6. I'm not sure if it has to do with an effect on the microbes in the soil or the chemical interactions with the neutrients.

    I worked at a municipal water production plant which chlorinated water as it was released to the city main. We targeted about 2-2.2 ppm, but it sometimes went as high as 3 ppm, but that was to ensure that there was chlorine in the water at the farthest boundary. The storage tanks, both ground and elevated were ventilated to the atmosphere.

    Some chlorine would be bound in chloramine and other organics, but we ensured that organics were very low, IIRC in the ppb range, and well below allowable limits.
  5. "The storage tanks, both ground and elevated were ventilated to the atmosphere."

    Do you mean tanks for the city water before it was distributed or for something else?
    I am curious to know if simply leaving the water open to the atmosphere is all that is necessary to remove the chlorine in the water.
  6. Astronuc

    Staff: Mentor

    There was a ground storage tank at the main pumping station. That was a 2 or 3 million gallon reservoir to store water if we took the production wells down for maintenance, which would happen sometimes. There was also a smaller storage tank at the production field. The ground storage tanks were before the chlorine was added.

    There was one elevated tank with a capacity of about 1-2 million gallons, and that tank provided a pressure on the system, since it was elevated. The tanks were vented (with a filter to prevent contamination) in order to prevent compression (when filling) or vacuum (when emptying). The elevated tank was on the city system (roughly in the center of the city), so it's water had some residual chlorine.

    Also, I think the issue with Cl (or chlorides) has something to do with pH.
  7. Astronuc, would the chlorine levels in the water be reduced between the initial chlorination stage and the distribution stage due to the water being stored in open tanks?
  8. Astronuc

    Staff: Mentor

    The main storage tanks were before the chlorine injector, i.e. they were storage on site before distribution. The chlorine in the vertical storage tank probably did dissipate to the atmosphere somewhat, but the concentration in the water was very low, probably ~1 ppm or less. There was another ground storage tank that was part of older original system when the city received water from another system. That had it's own chlorination facility, and that was probably on the order of 1-1.5 ppm.

    The objective was to minimize the use of chlorine (keep the cost down), but high enough to prevent harmful bacteria. I think there was a natural quantity of fluride in the water since it came from a particular acquifer formation, and in fact, the water was naturally soft.
  9. hi, i am not sure of the in and outs of the chemistry but the end effects i can testify to!
    my wife is a certified master gardener..
    i did not think that it made much diff using tap water or not..
    at her instance i set up a 200 gal rain tank from which she watered her plants
    and i used tap water to water mine..
    this on the deck plants or other potted plants ..
    results were clear- rain watered plants were fuller and had more fruit
    it was also clear that abundant tap water was better then spotty or no rain water!
    another result is that fertilizer of some kind will make a diff..
    more so then the water alone..
    the best result was with an compost tea made from using rain water or treated water and cloth sack of compost in a 5 gal bucket for a few days..

    "Our" actions now for all potted plants are we use rain water when we can
    that if it is a plant we want to be at its best or a sick plant we are trying to save we add compost tea and fish emulsion !!!
    if rain water is lacking ...
    we will use tap water with anti-chlorine such as used for fresh water fish tanks added..

    for yard grass .. water of any kind was OK :)
  10. turbo

    turbo 7,366
    Gold Member

    I have two wells. A dug well that is fed by surface/shallow water, and a drilled well that is fed from a rocky aquifer. I prefer the taste of the water from the drilled well (just a *bit* of iron in it, it seems) but I water the garden with water from the dug well. It seems to be a good balance. If we don't get enough rain to keep the garden watered, use water from the dug well that is fed by surface water, shallow springs, etc uphill of me. The guy who built this place made certain that either or both wells (each with their own pump, isolation valves, etc) could be used to supply either the domestic water, the sill-cocks, or both. We don' need no steenkin' chlorine!!!
    Last edited: Jan 13, 2009
  11. oh i agree !! in fla i live in horse country out side of ocala..
    the well water there is sweet and cold from 130 feet down !
    would only have to filter fine partials out for bottling!
    after well water city water taste bad..
  12. DaveC426913

    DaveC426913 16,244
    Gold Member

    That has certainly been the lore in the aquarium hobbyist world. Chlorine affects aquarium denizens much more than garden denizens and the cure for chlorinated water is simply to leave it in a an open bucket for 24 hours.
  13. turbo

    turbo 7,366
    Gold Member

    Free chlorine will off-gas when left standing and exposed to air. I'm not sure of the magnitude of the effect (how much it could effect nutrient up-take and hinder plant growth), but irrigating with tap water that has excess chlorine should result in an almost immediate formation of chlorine salts that you might prefer than your plants could absorb. Magnesium, Iron, Calcium, etc could all be captured by the chlorine, reducing their availability for uptake.
  14. this may not work if your city is sterilizing its water with chloramine instead of chlorine.

    another thing about rain water versus tapwater for gardening... our city water here is very hard. rain water is very soft. may be the same for turbo's shallow vs. deep wells. and this is another confound when comparing water for gardening. differences are more complicated than just chlorine.
  15. I have a friend who is a botanist and, interestingly, he suggested that small amounts of chlorine is actually GOOD for plants.

    But, this is for plants in standing water, not planted in soil (ie in a vase). A drop of chlorine will keep them healthier for longer. Not sure if thats true, but that's what he told me.
  16. What I've heard is that water sitting in an open water bottle will naturally dechlorinate within 24 hours.
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