Leaky Sewage Basin

After building 8 years ago, I am now refinishing our basement. I removed the lid on our sewage basin and discovered that it was full of water. Nothing has been hooked up to it yet and the pump has been disabled. The seal/gasket around the 4” line-in is faulty. The level of water, at equilibrium, is even with the top of the 4” PVC pipe. Hoping that this is a symptom of trapped water under the basement slab, I have been pumping it over to the sump crock. After two weeks, the rate of inflow does not seem to have diminished. Maybe there is a spring under the house?
We have a leach field that is already distressed, having surface water that will run over it during heavy rainfall. We also have a lift tank between septic tank and field.. So, I do not want to pay to pump more than is required (twice) and especially do not want to add to the load of the leach field more than is necessary. I’d like to use the existing ruffed-in plumbing if possible and avoid the added expense and noise of an up-flush toilet.
Questions:
How can I repair the seal between the 4” sewage pipe and the basin? I would think this is possible because the head pressure is minor, but don’t know how best to approach.
Could I insert a Sanigrind type (self contained tank/pump) into the sewage basin with a direct connection to the 4” line? This would keep the ground water separate from the sewage but would need to be water proof, etc.