Well Question

I have an irrigation well, 4" caseing, submersible pump, about 25 gallon pressure tank. Following a hurricane I did not use the well for several months. Then, the next spring, when I did try to use it, I could only get the pressure up to about 20-25 psi. When I turn the pump off, the pressure quickly drops to zero.
No trees fell near the line except very close to the well casing. I have dug down to the pitless adapter and there is no leak there. I do not believe the water is leaking at inside the casing and running back down into the well because I do not hear it and because there is a diaphram on the drop pipe that fits tightly against the inside of the casing (to create a vacuum below it when the pump is running and increase water flow ) which would cause the casing to fill up. That is not happening.
Since I already has a spare pump, I pulled up the old one in case its check valve was bad, and replaced it. The drop pipe looked fine. Still the same problems (low pressure, pressure drops to zero when pump is off).
I do not think that the well is just no longer able to produce the necessary flow because that would not explain the drop in pressure when it is switched off.
I can think of no reason the underground pipe would be broken and it would be a huge job to dig along its length and check for a leak.
QUESTIONS: What is/are the most likely cause(s) of this problem other than a leak in the underground pipe.
The pipe is 18"-24" below ground (below our freeze line). Is there any to locate a large leak other than digging along its length?
MANT THANKS for any help.
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snipped-for-privacy@aol.com wrote:

Blank off the underground pipe at the wellhead. Then pressurize the line from the other end (either water or air). You won't need a whole lot of pressure.
Jim
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Thanks. I am planning on doing that as soon as I find my extra pitless adapter (I can blank off what is normally its input from the pump and plug it into the side of the well casing where the underground line connects. That way I don't have to dig a huge hole, cut a pipe and put fittings on pipe that I cannot move. (Close to the well it is schedule 80 PVC, elsewhere Sch. 40)
That should prove if there is a leak in that line. However I don't see how it will help me find the leak. I can run the pump for an hour or so and I don't see any evidence of a leak at the surface. The soil is sandy clay and percs well.

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snipped-for-privacy@aol.com wrote:

If you determine that the pipe is leaking, find a plumber with ultrasonic leak detection equipment to isolate the leak location for you.
Pete C.
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water companies use a big sthescope and listen to the ground to find leaks.
but chances are if one spot is bad the entire pipe is questionable.
if you dont use the well in cold weather you could replace the line with a shallow one but you will have to blow it out each year before winter
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Thanks for all of the replies. The problem actually turned out to be on the drop pipe. I did not see any problem when I pulled it up to change the pump, made a rig to do some testing at the pitless adapter and found I could not get more than 22 psi.
On the drop pipe was a steel and heavy rubber disk that seals between the drop pipe and the well casing so that air cannot go down the well. This causes a vacuum above the pump thus pulling more water into the well. The drop pipe screws into the top and bottom of the disk. The fittings had corroded nearly away and water could just gush out.
Does anyone know how effective these seals are? What are they called.
On Mon, 13 Mar 2006 11:50:17 -0500, snipped-for-privacy@aol.com wrote:

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