I am trying to calulate the amount of buried tubing I will need to cool water in a stored 2500 gallon cistern. Soil clay/sandy average temp. 48" below grade I beieve to be 52 degrees the water temp will be appox. 74 degrees. I am trying to cool the water to the soil temperature overnight, 12 hours. The water will be circulated through a fan coil during the day to provide cooling.

Hi chuck,

Isn't this redundant in some way ?
You need to cool 2500 Gallon via Geothermal for day cooling: So you would be getting a water water GHP anyway for the process
so why not just feed the cooled water from the GHP to the fan coil directly and simply forget the 2500 Gallon of water ?

Francis

Water at 52 degrees does not have a lot of cooling capacity. Also you would not get there without an infinite length of pipe as the earth does not cool down outsiide the pipe easily. You are totally off base from any energy transfer perspective. If you can pull water from the earth at a continuous 52 F you can get a little "free" cooling. But most places require a second well to return the water.
Don't waste your money. Better to cool your attic spaces with an attic fan so heat build up is reduced. Best bang for the buck in cooling and very often overlooked. Even better although noisy is pushing house air into attic from the house to displace hot attic air although you don't do this while running air conditioning

David - the amound of cooling or energy removal you need is approximately=

Btuh = 500 x gallons x (temp drop).

the amount of tubing you need, you will have to pay someone to tell you that (liability)  ; )