What amount of pool hypochlorite would damage an adjacent lawn?
September 11, 2013 10:29 AM   Subscribe

My neighbor believes I am killing his lawn by emptying small amounts of pool water into a pond on my property which borders his property. I do wish he would stop yelling at me. How much pool chemistry (say, calcium hypochlorite) would I need to add per gallon to my pool to damage his lawn?

Let me outline a couple of scenarios. Most of the water relased from my pool is via cleaning the filter. When I backwash the filter, I release 100-200 gallons of water into a small "pond" on the property line. I arrived at that figure by working out how many gallons are in a 10' x 20' x 2" volume of water. The "pond" is maybe a 10' diameter by 2' deep muddy hole full of cattails.

The worst case scenario I can think of is that I would add the hypochlorite to my pool and immediately cackle sadistically as I spray 200 gallons of that water onto his grass, thereby applying the maximum "available" chlorine directly to the grass. How much calcium hypochlorite would kill his lawn in this case?

In practice I never shock my pool and then immediately clean the filter. I always clean the filter first. In this case, my understanding is that the available chlorine is negligible and that all the chlorine is present as ... I don't know what, chloramines and wherever else it goes. And again, I don't spray it on his lawn, I empty it into a pool, so now we're talking groundwater leaching. His lawn is very roughly on a 2% grade downhill from my pump. Can anyone walk me through a back-of-the-envelope calculation for destroying his lawn in this scenario?

Parenthetically, I am not attempting to convince my neighbor of anything. I'm just curious!
posted by mindsound to Science & Nature (13 answers total) 4 users marked this as a favorite
I do not know the calculation, but why not simply dump the backwash onto your lawn the next time and see what it does? Also, if it was going to kill things, wouldn't your pond and the cat tails be toast?
posted by JohnnyGunn at 10:40 AM on September 11, 2013

You shouldn't be releasing chlorinated poolwater directly into groundwater! That's bad and your neighbor is right that you shouldn't be doing it. You're supposed to release the water into a sanitary sewer or, failing that, onto vegetated ground on your property. The more vegetated the better. Not into water which is the worst thing you can do!
posted by Justinian at 10:56 AM on September 11, 2013 [13 favorites]

Can anyone walk me through a back-of-the-envelope calculation for destroying his lawn in this scenario?

No, because you don't mention the frequency with which you do this. And even if you did, it would depend upon soil chemistry, passage rate, temperature, incoming UV, species composition and a bunch of other stuff. Numbers won't help you here. But if you've paid for a pool, surely you can pay for someone who knows what they're doing to fit the hardware and piping to dispose of the effluent to code?
posted by cromagnon at 11:07 AM on September 11, 2013 [2 favorites]

Best answer: This fact sheet from the Maryland Department of the Environment may be of assistance; scroll to the end for some best practices.
posted by Empidonax at 11:13 AM on September 11, 2013

Response by poster: Justinian: it releases into said ten-foot "pond", which is entirely filled with cattails and some other wet-loving plants, and the pond is surrounded by small trees, and native shrubs and ground cover. It's usually just a mud hole in the summer.

cromagnon: I dunno about code? The installation is from the '70s. Our neighbor actually called someone from the county to complain. They came and looked at it and didn't have a problem with it.

What I wanted to know was the amount of chlorine, as provided by the hypo, that would be needed to definitively kill grass...
posted by mindsound at 11:13 AM on September 11, 2013

Response by poster: Empidonax: That's great, it's got figures I can maybe use. "...acute aquatic toxicity in fresh (19ug/L) and salt water (13 ug/L)..."
posted by mindsound at 11:20 AM on September 11, 2013

Response by poster: 25,000 gallons ~ 100,000 liters

100,000 liters * 0.1 grams/liter ~ 10 kilograms of chlorine would exceed COMAR, per the statement, "Compliance for results less than 0.1 mg/L is specified in COMAR and produces the most meaningful results." The brand of calcium hypochlorite we use claims a little under 75% hypo by volume... so to be, as I understand the Maryland regulations for practical purposes, illegal, we would have to shock with about 13 kilos of hypo product and immediately flush. We use about 1 kilo.

If I wild-ass-guess that one or two orders of magnitude more than that would be immediately lethal when directly applied to plants, we're looking at 100-1000 kilos of hypo product.

Works for me!
posted by mindsound at 12:00 PM on September 11, 2013

Errr... 0.1mg/L puts you at 0.0001 g/L * 100,000L = 10 g, not 10kg. Using 1 kg of hypo product at 75% puts you at 7.5mg/L (1 kg = 1000 g *0.75 = 750g / 100000 L = 0.0075g/L = 7.5mg/L
posted by defcom1 at 12:15 PM on September 11, 2013

Yeah, your math is wrong mindsound.
posted by Justinian at 12:37 PM on September 11, 2013

Response by poster: Dammit, of course you folks are right, I read it as grams not milligrams.
posted by mindsound at 12:39 PM on September 11, 2013

Also, and sorry to reiterate (but "it's got figures I can maybe use" is worrying) - if you don't know the difference between acute aquatic toxicity and (sub)chronic aquatic toxicity then your figures will not be useful. Try this if you really want. The approximate concentrations you are creating are those that will kill an individual over the course of a day, not a year.
posted by cromagnon at 2:03 PM on September 11, 2013 [2 favorites]

Response by poster: cromagnon: Wow, so okay. It's useful to understand how ill equipped I am to actually answer this question.
posted by mindsound at 6:58 AM on September 12, 2013

Yeah, sorry: that wasn't really in the spirit of AskMeFi. It might also be that your neighbour is killing his own lawn by over-applying lawn feed, or your other neighbours' dog likes that bit more than is healthy, or whatever. But you won't get the proof you're looking for in the way you were suggesting - and there might well not be any proof one way or the other anyway. I'd say mediate a solution if you can.
posted by cromagnon at 5:12 PM on September 12, 2013

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