You describe the symptons of a clogged drainfield.  The water level in the drainfield was higher that the level in the tank.  When you opened the tank, you allowed the pressure to be released and the water filled the hole.  The high water level probably indicates that some of the solids from the tank have passed from the tank to the drainfield.  You want to ensure that your tank is pumped, the drainfield pumped to remove water and I would suggest a drainfield treatment with Septic-Scrub as the most cost effective treatment to try to restore the drainfield.  

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