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MO question via email
#531519 03/05/21 11:30 AM
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Here is the question I received by email - assume it came from a viewer on PB. You guys take a shot at answering the question.

Question -

I'm in Southern Missouri you have any idea how deep a hole it would take to have a temp of 65 degrees, our summers are brutal into the 90 degree temps. Im debating using aeriation kit at 10' depth when temps drop down at night. I have no idea where the thermocline depth is here.
















Re: MO question via email
ewest #531523 03/05/21 12:33 PM
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I'll jump in...

I'm thinking quarry size and depth...ambiguous, I know. Then again, so is the amount of details in the question. Not that it would help me much. It's been way too long since I studied thermodynamics!

My 10 foot deep 1/4 acre pond, with aerations, has no chance of staying cool. In fact, It gets borderline bath tub temps.

Considering that the 20 to 30 foot deep sub soils tend to stay at a constant 50 to 60° F...you would need a large volume of 50 to 60° water to pull to the top in order to keep the swimming depths near 65° F around the clock or even close.

The pond would have to be that 20 to 30 foot deep just to get to that constant sub soil temp. Then, there is the volume of cool water needed to not be overtaken by the mixing of the upper warmer water. The upper 10 foot would be the swimming volume and I am guessing that the volume of cool water would need to be at least 10:1 (meaning 100 foot deep, maybe 5:1 - that's still stupid deep if you are digging). This guess neglects the heat transfer (or cooling effect) of the deep water to soil interface which might lessen the needed depth. The above mumbo-jumbo assumes that the aeration would turn the pond several times a day and the pond owner wants to maintain the 65° temp pretty much 24/7.

We can cheat a bit. An un-aerated 20 foot deep pond will develop a thermocline (where? - I don't really know - it depends), let's say at 8 foot deep. Set the aerators up near/around the swimming area just below the thermocline and just turn them on before going swimming. This will bring cool water up pretty quick and cool the swimming area down for the time of use. The swimming water probably won't get to 65° in the heat of the summer, but certainly much cooler.

If it's a small pond, I would not want fish in the pond as sporadic aeration would not be easy on them and not having fish would reduce the potential stink of pulling bad water up.

My 2¢ in 1000 words or less.

Last edited by Quarter Acre; 03/05/21 01:18 PM.

Fish on!,
Noel
Re: MO question via email
ewest #531524 03/05/21 12:44 PM
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ewest,

There are many good maps available of ground temperatures. Basically, in most places the heat flow flux from the center of the earth is so small that once you get deeper than 10', the ground temperature is a very close approximation of the integral of the air temperatures over the course of the "average" year.

Below is a link from the heat pump guys. Their calculations require a fairly accurate estimate of your groundwater temperature.

https://assets.supply.com/ul_pdfs/284997_GroundWaterTemperatureMap.pdf

The ground temperature sets the lower limit for his "average" water temperature below the pond's thermocline. (Obviously, he will start with colder bottom water coming out of winter.)

After that, the calculations get very complicated very quickly. However, circulating water is very efficient at equalizing its temperature. Once he starts aerating from below his thermocline, he will lose his pool of cool water pretty quickly.

I don't see any possible way he could have an entire pond with 65 degree water with prolonged exposure to 90 degree air during the day and 70 degree air at night. (If that is his desire.)

The only exception would be having a deep, still quarry right beside his 2 acre pond. You could then pull cold bottom water for the duration of the summer and make it work.

I have been trout fishing around Atlanta, Georgia. Some of the spillways at the huge reservoirs have bottom drains that provide the only release water during periods with low lake levels. There are trout farms that exist just below the spillway outlets and can pull in this 60 degree water. That is the only way to make the physics work if I am understanding the email question correctly.

Rod

Re: MO question via email
ewest #531526 03/05/21 12:48 PM
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Quarter Acre said it more succinctly and better than I did!

I wish his post had been up BEFORE I started typing. cry

Re: MO question via email
ewest #531529 03/05/21 01:15 PM
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If the pond is small enough, would he have a better chance with a high volume well and keep pumping cool water in so it would be like it was spring fed? Have the water from the well enter in the bottom of the pond so the hotter surface water would flow out?


Bob


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Re: MO question via email
ewest #531531 03/05/21 01:21 PM
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Originally Posted by FishinRod
Quarter Acre said it more succinctly and better than I did!

I wish his post had been up BEFORE I started typing. cry

I don't know about better and I have to go look up "succinctly" now! Thanks Rod (I think laugh )

Last edited by Quarter Acre; 03/05/21 01:21 PM.

Fish on!,
Noel
Re: MO question via email
ewest #531538 03/05/21 04:52 PM
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In my central Missouri pond... summer water temps show little variation down to 10' with diffuser set at 12' in 16' of water.
My current thermometer has a 10' cord on the probe, so I can't measure deeper than that,
but I know there is some very cool water down there because we can feel it when we're swimming.

If anecdotal information was worth anything I would be a rich millionaire.

Re: MO question via email
ewest #531544 03/05/21 06:38 PM
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Augie,

If the actual facts on the ground don't match the engineering, then the engineering is wrong! (Or at least one of the assumptions used in your engineering calculations was wrong.)

What is your typical water temp in late July or early August? What is your elevation? What is your surface acreage?

Instead of a flat "no way", let's at least try to send the guy some useful information. Your personal pond experience is probably some of the best data that ewest could forward.

Re: MO question via email
ewest #531546 03/05/21 06:49 PM
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Bob's response is the most feasible solution.

With enough well water at X temperature, you could reach any temperature down to X for a small enough pond and a high enough flow-thru rate of well water.

Lifting water does get expensive on the electricity bill.

If it is a rich guy, then I suppose it would be possible to use a heat pump with a ground loop and a loop in the pond. Of course, it would be least efficient in the summer as you are dumping heat into the ground around your loop at the time when the ground is the warmest.

We are quickly getting beyond any of my areas of even tiny expertise. (Although that hasn't always stopped me from running my mouth in the past!)

Re: MO question via email
ewest #531548 03/05/21 09:11 PM
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I have a better question.. How do we know if the Hypolimnion is even present???
If it is, why do we want to blow air through it and dump surface heated water into the mix?
Why is it important to have 65 deg water? What is the reason behind that?
No, that doesn't answer the question but there are WAY too many variables to suggest we can achieve "X" temp at "X" depth.
Maybe I missed the target ewest is looking for.

Re: MO question via email
ewest #531554 03/05/21 11:04 PM
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Guys great job . Hope the emailer is reading . That is all the info I received so I can’t add any more. Looks to me that they are mixing questions and concepts.
















Re: MO question via email
ewest #531559 03/06/21 09:59 AM
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Originally Posted by ewest
Guys great job . Hope the emailer is reading . That is all the info I received so I can’t add any more. Looks to me that they are mixing questions and concepts.

All the while not providing enough information to get an answer that will do them any good.


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Re: MO question via email
esshup #531666 03/08/21 12:04 PM
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This is an example of "I don't know what I don't know" situation.

Lacking knowledge and experience, it is impossible to ask good questions, so you just have to start where you are and go from there.

I was the poster boy for topical ignorance when I started my pond renovation. If I hadn't found the PB forum I would have a leaky hole
full of boring fish in my front yard.

Best advice I could give to the person who posed the original question would be to read every single post in the archive. I did that very
thing after I discovered this place. By doing so, I cured my ignorance. Many of the questions that I didn't know I had were answered
before I ever had to ask them, and I had put myself in a position where I knew *just enough* to ask a sensible question, and make sense
of the answers that were given.

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