can't really tell from the pictures, but a thought came to me, since you call it a "troff" and I make the assumption that there may be some thought that it was water tight at one time?
Back in the old days, (not really that long ago, since I remember them) Milk was put into steel 5 gal or so cans, the cans were usually stored in a "troff" filled with water and if possible, spring fed. It was an early form of refrigeration to keep the milk 'cool' until picked up by a wagon or flat bed truck. (I remember the flat bed truck version, not the horse drawn one) I believe there used to be such troughs at rail depots too for loading on to trains. If this is a fairly Large farm site, and there is any sign of a Dairy operation, then your trough may be a milk can cooler. Just a guess from left field. My Father-in-law began picking up Milk stored that way after
WW-II, loaded it on a flatbed truck and unloaded the cans at the Dairy. He later switched to the more familiar present day tanker trucks, I still have a few of the old Milk cans.