Wet Septic bed area can't be mowed?

This is our first experience owning a septic system. One area of the grass is very wet combined with all the rain recently but I think due to septic bed. The septic is fine as it was pumped last year so that isn't an issue (ie. not overfull) My daughter was mowing the very long grass (after several days of rain) and the tractor mower got stuck in the mud. This is the 2nd time. Now it is a sludgy looking area with tractor marks. I am wondering about converting that wettest part into a natural area or doing something else because how can we mow it if it is so wet? I used a push mower today in as close as possible but there is a lot of mud there. Or should I add topsoil to raise it up a bit. It is obviously a low spot.