I have a 40 year old house on a septic system. Yesterday I noticed water backing up into the shower when the toilet is flushed. The toilet is taking an unusual amount of time to complete flush. I have also notice that the gound is very wet around my septic tank and there is a strong smell of sewer in the back of the house.

Sounds like there is a clog in the pipes. Likely, a "T" intersection where the toilet and shower drain meet.

XenoSapien