I have black sludge? leaking out at septic tank. The grass over drain field is green. I had drain field de-rooted last year that helped the problem temporarly. I'm not sure how old system is, but it looks like work was done on it before I bought house 2 years ago. New PVC drain pipe is noticed from house to tank. House built in 1967. Septic guy says I need a new drain field. Do I need a new drain field or is there another problem-sludge level?