Weird septic tank refill

Here's one for the twilight zone..
Saturday, July 23, 2005. I discover water percolating from the ground where my 20-year-old septic tank is. I call a local service company, and they come out Sunday morrning (24th) and pump it out, It's easy to tell that the outlet is blocked, I pay $300 to have 1000 gallons of fluid and a relatively small amount of solids pumped out (Single person household)
I kept water usage to a minimum over the week, averaging 24 gallons per day, much of that not going down the drain. (Measured by taking water meter readings)
Today, 8 days later, I dig up the cleanout hatch to run a snake down the outlet pipe to see how close the blockage is (5 ft).
The really strange thing is that the 100 gallon tank is full again, just up to the bottom of the outlet. By my calculator, that's about 115 gallons extra per day.
Theories: 1. Rainwater from the downspout slightly uphill from the pipe that runs from the house to the tank is filtering into the fill around the house and into a hole in the pipe. If this were true, I'd expect to see a hole in the ground were the water has been entering, but I don't see any. Plus, we've only had scattered thunderstorms during the week, no steady rain.
2. The leaching field is completely saturated, and has been draining back into the tank all week,
3. My neighbor's house is hooked to my tank (100 feet away?)
4. Waterlogged ghosts using my toilets when I'm not home?
Pick a number, or propose a new theory...
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[...]
Your tank is cracked. You now have an artesian well.
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check to make sure you do not have a toliet running after a flush or other leaking faucets, etc.
wrote:

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The water meter would have revealed that. I was definitely comsuming less than 25 gallons per day.
I found the reason tonight. I dragged out the garden hose, and set the end in the downspout pan and turned it on full. The water flowed along a shallow grassy channel along the wall until it got to a point just above where the sewer line exits the basement, then it disappeared A closer look revealed a 3" diameter hole in the ground. The pipe was only a few inches below the ground, and the water had tunneled alongside the pipe toward the tank, where it was probably leaking in through where the pipe goes in, or, through a hole in the pipe. (more digging required)
There must have been some heavy rains during the week, The hole absorbed the full output of the hose with a 90PSI supply. For now, I've added an aboveground hose from the downspout to a point away from the house. If they bring digging machinery to repair the drain line/field, I'll have them make a trench for a new drain line for the downspout as well,
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---snip------

I grew up in an old farmhouse and we had a similar problem. In the end we found that the old concrete tank had cracked and groundwater was pushing it's way into the tank, filling it pretty rapidly and causing all sorts of trouble in the house.
In our case the only solution was to dig up the old tank and put a new one in place. I'm hoping that you don't have to do the same as it isn't cheap.
Drifter "I've been here, I've been there..."
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