The shedding of gold is a process that takes millions of years of weathering and erosion....500 years is but a mere pittance!
I dont think the Romans cut through gold bearing reefs saying to each other, "Oh look, isnt that pretty".
Sure, maybe the odd nugget or so has found its way into the sewerage as this is highly likely, but i cant see it being a plausible mining operation since it has only been in existence since the dawn of man......a very short time in the scale of geological creation.
Im just saying, the shedding of gold from hard quartz stone is not something that takes place on a daily basis.
The sewers are hard lined, they are not eroding soils and rock as water travels, like a river would. Anything found in them would have been washed into them by rainfalls from surrounding slopes passing into the storm water over a very short timeframe of the big picture.
I dont think this process would give way to a long term viable mining operation as some suggest.
Its propoganda and tall story telling which has long surrounded gold.....a few nuggets found suddenly turns from just that, into riches beyond ones dreams as the story travels from pub to pub, newspaper to newspaper.
Just my take on it.