But mercury is everywhere. The rivers of the west run with it but it's generally at the lowest gravel layers on bedrock where it can't get any deeper. I'm working a virgin area away from the river and I haven't seen a single flake of gold contaminated with mercury but in the past, while dredging in the river, half the gold recovered was an 'amalgam' of gold/mercury. I've been in many holes where little drops of mercury are resting in the fold of bedrock and actually had one dredge hole where a flask of mercury must have been spilled. Literally pools of the metal on the rock.
I've spent decades studying old mining records and find common mention of miners 'topping off' their sluices by pouring in tens if not hundreds of pounds of raw mercury weekly to make up for the 'loss' in the sluices. That's probably not primarily loss out the end but make-up for the 'used' mercury in the sluice that is no longer reactive,........but where I'm working(and probably in most mining areas)it's well known that the best gold recovery is downstream of the old workings due to the amount of gold/mercury/amalgam they lost.
Still not as bad as it sounds. The elemental mercury is not the huge environmental disaster the greenies make it out to be. "Bad" mercury pollution is the highly reactive compounds and oxides discharged from industrial applications.