I want to build a "bridge" approximately 8 feet long by 18 inches wide out of pressure treated pine removed from a 20 year old deck. The wood has been stained on all sides with some Benjamin Moore deck stain. The wood looks ok, but I don't know how to tell if it's not ok.

The bridge would go across two soil slopes paralleling a drain field and sit about one foot above intermittent water. The supports would be lengthwise 8 foot 4x4s with crosswise 2x6s as the floor. The 4x4 would lie on the ground (not in standing water) and the 2x6s not touch water or ground.

Should I use the old lumber? I hate throwing away so much wood, but I understand if I have to.