I can't properly address your question but can discuss a side facet of Roman plumbing from an environmental course I took at university. Now the prof noted that Vitruvius was already suspicious of lead piping and had recommended terracotta but unfortunately lead is easier to work with and cheaper. That was his only comment on it so I can't provide a proper reference or context (other than he was a 1st BCE engineer/architect).

However regarding lead pipes, they may not leach that much lead into the water supply. Calcium Carbonate in the water would build up inside pipes over time as sinter and this would act as insulation from the lead. Now there were transfer tanks what were made of lead and likely would have leached some lead into the water. Also intramural pipes which would be replaced more often (and thus introducing pipes without a protective sinter insulation) and be another avenue for lead.

The hardness and alkaline levels of the water combined with the constant flow would also limit the time it had to absorb lead and would help ameliorate the lead content.

Another thing is that the water could have elevated lead levels, it's been posited that (for example) the city of Pompeii used water from the springs of Summa-Vesuvius which had inherently high lead levels. There's also the chance that it came from lead aerosol generated by mining and smelting of lead that got washed into the water supply.

Ultimately we can't know. It's safe to say there was some lead contamination and that this would lead to serious health effects. However, with 90% of the population living in the countryside that would only have affected a relatively small percentage of the population.