Water with sewage smell in the basement of a small apartment building

There's a sewage smell coming from the basement. The tenants have been complaining for a few months. We looked around and found water in a room below the staircase. The water is contained in a structure that looks like a well. The water don't overflow it. However, when we try to remove the water, it keeps coming back. The sewage smell comes from there.

I hired a plumber to come to take a look, he didn't know what to do neither, he advised us to remove the water 1st, but as I said, the water keep coming back to similar level.

We suspect that the sewage line is broken somewhere and it leaks into somewhere in the basement. Part of that leakage is shown in the "well".

Please take a look at the following video and picture and give me advice.

https://photos.google.com/share/AF1QipPs16XV-9if_CfPMEWr4Gk1jXxzUaXQxXW5gRBfATVoqcZLaqt-MuFX5213jZAc5Q/photo/AF1QipN7iH6ZFLNko_PkRCME5aFyBQKMY4KnPGICVLjM?key=S3FMX1R1UnRxMGlvY0xnOTgtSXZXY0lXN2R4ajdR