It might be in the plumbing too...

Mercury often collects in plumbing systems in healthcare facilities and laboratories. Sometimes this occurs if mercury is poured down the sink (no mercury-containing waste should ever be disposed of in drains). However, it also builds up in plumbing by slowly concentrating in biological films, sludges and other solids from the trace levels of mercury that are often present in wastewater. When these solids are dislodged, excessive levels of mercury may be discharged, violating local pollution control limits.

The Massachusetts Water Resources Authority/ Medical Academic and Scientific Community Organization, Inc. (MWRA/MASCO) Mercury Work Group provides extensive information about sources of mercury emissions in wastewater from health care facilities, prevention and control strategies. Information includes a mercury management guidebook, recommended procedures for dealing with mercury contamination in plumbing and a searchable database of analytical data on mercury concentrations in commercial products. There are links to the Mercury Work Group documents in the Hatter's Links and Hatter's Library sections of this site. http://www.masco.org/mercury/index.htm