I have a low point in the bilge where the bilge pump is. I always have a gallon or so of water from the dripping shaft glands. Lately the water has been discolored black!! It has a distinct odor, not exactly like it came from the holding tank, but a kind of "sewer" smell (if that makes sense). I have no apparent oil leaks. I have a Vacuflush system which maintains vacuum well, (cycles every half hour or so for a few seconds). Could it be from the holding tank if it is under vacuum? If so where should I start to look?