# After 10 years, is any of the original water still in my pool? **URL:** \ **Category:** Factual Questions\ **Created:** [June 28, 2013, 6:39pm UTC](https://boards.straightdope.com/t/after-10-years-is-any-of-the-original-water-still-in-my-pool/662148 "2013-06-28T18:39:34Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![cainxinth](https://avatars.discourse-cdn.com/v4/letter/c/7bcc69/32.png) [@cainxinth](https://boards.straightdope.com/u/cainxinth)\ **Post date:** [June 28, 2013, 6:39pm UTC](https://boards.straightdope.com/t/after-10-years-is-any-of-the-original-water-still-in-my-pool/662148/1 "2013-06-28T18:39:34Z")
There are probably too many unknown variables involved to answer this question but after 10 years of rainwater and evaporation and hose top offs I’m wondering how much of the original water is still in my pool? ---
**Author:** ![Darth\_Panda](https://avatars.discourse-cdn.com/v4/letter/d/ee7513/32.png) [@Darth\_Panda](https://boards.straightdope.com/u/Darth_Panda)\ **Post date:** [June 28, 2013, 6:43pm UTC](https://boards.straightdope.com/t/after-10-years-is-any-of-the-original-water-still-in-my-pool/662148/2 "2013-06-28T18:43:43Z")
Using weighted average or LIFO accounting, there would be, but not under FIFO. I’d stay away form specific identification, it isn’t recommended for interchangeable goods. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [June 28, 2013, 6:54pm UTC](https://boards.straightdope.com/t/after-10-years-is-any-of-the-original-water-still-in-my-pool/662148/3 "2013-06-28T18:54:04Z")
Possibly none, but the point of the glass of water in the ocean tale (see below) is to point out how tiny and abundant water molecules are. So out of the billion trillion gazillion brazilian molecules of water originally in the pool, it wouldn’t be two surprising if some were left. If you were to toss a glass of water in the ocean, and after some number of years when hypothetically the molecules of water from the glass became evenly distributed across all the world’s oceans, you took a glass of water out of the ocean, it would contain some of the water molecules that you threw into the ocean years before. That’s because there are many more molecules of water in a glass of water than there are in all of the oceans. Or so it has been said. ---
**Author:** ![newme](https://avatars.discourse-cdn.com/v4/letter/n/e0b2c6/32.png) [@newme](https://boards.straightdope.com/u/newme)\ **Post date:** [June 28, 2013, 7:17pm UTC](https://boards.straightdope.com/t/after-10-years-is-any-of-the-original-water-still-in-my-pool/662148/4 "2013-06-28T19:17:45Z")
Assume you lose 1% of the water each week. After one week, you still have 99%. After 2 weeks, you have (.99)x(.99) = 98.01% After 10 years = 520 weeks, you have (.99)^520 = 0.54 % ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [June 28, 2013, 7:18pm UTC](https://boards.straightdope.com/t/after-10-years-is-any-of-the-original-water-still-in-my-pool/662148/5 "2013-06-28T19:18:17Z")
There are about 1.3 \* 10^26 molecules of water in a gallon, 2 \* 10^4 gallons in a typical pool, and 1.5 \* 10^16 poolfuls of water in the oceans. Suppose that all of the original water has evaporated, or otherwise left the swimming pool. Each original molecule has a 6.6 \* 10^-17 chance (poolful divided by total) of having returned and being there now. (This assumes the oceans’ and atmosphere’s waters are completely mixed; instead the chance might be higher.) 6.6 \* 10^-17 \* 2 \* 10^4 \* 1.3 \* 10^26 = 1.7 \* 10^14 Even if all the original water left the pool at some point, about 170 trillion molecules have returned and are there now. That’s more than four milli-micrograms, if my arithmetic is correct. ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [June 28, 2013, 7:57pm UTC](https://boards.straightdope.com/t/after-10-years-is-any-of-the-original-water-still-in-my-pool/662148/6 "2013-06-28T19:57:43Z")
There are likely molecules from \*the first swimming pool ever made \*in your pool. ---
**Author:** ![TimeWinder](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@TimeWinder](https://boards.straightdope.com/u/TimeWinder)\ **Post date:** [June 28, 2013, 7:59pm UTC](https://boards.straightdope.com/t/after-10-years-is-any-of-the-original-water-still-in-my-pool/662148/7 "2013-06-28T19:59:54Z")
For more, see the truly epic nitpicky hijack here: > **[Great science facts](https://boards.straightdope.com/sdmb/showthread.php?t=689797)** > > I thought it might be fun to start a thread of interesting ways to describe the wonders of the universe in enjoyable and digestible ways. So let me begin with a few examples that I believe are true and a hope this thread doesn’t die on it’s ass. ... ---
**Author:** ![Cartoonacy](https://avatars.discourse-cdn.com/v4/letter/c/848f3c/32.png) [@Cartoonacy](https://boards.straightdope.com/u/Cartoonacy)\ **Post date:** [June 28, 2013, 8:34pm UTC](https://boards.straightdope.com/t/after-10-years-is-any-of-the-original-water-still-in-my-pool/662148/8 "2013-06-28T20:34:14Z")
> [@TriPolar](#): > > That’s because there are many more molecules of water in a glass of water than there are in all of the oceans. Or so it has been said. Ummm… _what?_ ---
**Author:** ![RobDog](https://avatars.discourse-cdn.com/v4/letter/r/f07891/32.png) [@RobDog](https://boards.straightdope.com/u/RobDog)\ **Post date:** [June 28, 2013, 8:45pm UTC](https://boards.straightdope.com/t/after-10-years-is-any-of-the-original-water-still-in-my-pool/662148/9 "2013-06-28T20:45:02Z")
> [@Cartoonacy](#): > > Ummm… _what?_ More molecules in a glass of water than glasses of water in all the oceans, perhaps? ---
**Author:** ![Munch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/munch/32/5281_2.png) [@Munch](https://boards.straightdope.com/u/Munch)\ **Post date:** [June 28, 2013, 8:48pm UTC](https://boards.straightdope.com/t/after-10-years-is-any-of-the-original-water-still-in-my-pool/662148/10 "2013-06-28T20:48:22Z")
How much is his pool’s water worth if he sold it all to a homeopathy company? The resonancy of that shit must be off the chart! ---
**Author:** ![Tom\_Tildrum](https://avatars.discourse-cdn.com/v4/letter/t/e95f7d/32.png) [@Tom\_Tildrum](https://boards.straightdope.com/u/Tom_Tildrum)\ **Post date:** [June 28, 2013, 8:52pm UTC](https://boards.straightdope.com/t/after-10-years-is-any-of-the-original-water-still-in-my-pool/662148/11 "2013-06-28T20:52:22Z")
> [@DrDeth](#): > > There are likely molecules from \*the first swimming pool ever made \*in your pool. Urine molecules! ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [June 28, 2013, 9:25pm UTC](https://boards.straightdope.com/t/after-10-years-is-any-of-the-original-water-still-in-my-pool/662148/12 "2013-06-28T21:25:52Z")
> [@Cartoonacy](#): > > Ummm… _what?_ > [@RobDog](#): > > More molecules in a glass of water than glasses of water in all the oceans, perhaps? Yes. My mistake. I was trying to compact that statement, a wee bit too much. More molecules of water in the glass than glasses of water in all the oceans.