Plumbing Questions? Ask a Plumber and Get Answers ASAP.
I sorry to be the bearer of bad news but your description indicates to be that you probably have a broken sewer pipe under the basement slab. There is on other likely explanation. The best approach would be to call a drain cleaning service and have them put a camera down the main drain line to see what's going on. You won't be able to resolve the effluent in your sump pit until you get your sewer main fixed.