I once saw a study that had been done on this very subject. At present, I don't recall the name of the publication (one of the hunting magazines). Their study showed that after approximately 30-45 minutes on regular earth (not ice or snow), that the smell of human urine appeared not to be detectible by deer as human urine. In one of the areas I hunt heavily on regular hunting trips, alfalfa is a big feed source. So, I buy alfalfa pills at wal-mart and overload on them, as many as 6-8 a day. Ever notice after a few big cups of coffee when you urinate you can smell the coffee? Same for the alfalfa, literally, you can smell it. I've often relieved myself in other deers scrapes, and they continue to be used with no problems. I'm sure that within the short term of an hour or so, the trace minerals might be distinguishable to deer, but after such time period, I think the point is moot.