The United States Environmental Protection Agency set the ‘action level’ for mercury in drinking water at 0.002 ppm (parts per million), or 2 ppb (parts per billion). This means municipal water suppliers must test for the presence of mercury in drinking water on a regular basis, report their findings to the EPA, and take corrective measures if the mercury levels in their water reach or exceed 0.002 ppm.
“The regulation for mercury became effective in 1992. Between 1993 and 1995, EPA required your water supplier to collect water samples once and analyze them to find out if mercury is present above 2 ppb. If it is present above this level, the system must continue to monitor this contaminant every 3 months.
If contaminant levels are found to be consistently above the MCL, your water supplier must take steps to reduce the amount of mercury so that it is consistently below that level.” (source)
As you can see, the Federal Government holds public water systems accountable for the water they provide. Personal well owners, must monitor their own water supply. Products such as Boris Mercury Check can detect the MCL for mercury in 3 minutes and makes it possible for well owners to perform periodic checks on their well water without having to send it off to a laboratory.