Low-Level Mercury

How Much Mercury is Really Present in Our Waters?

All municipal wastewater contains some level of mercury. The concentration of mercury in influents to municipal wastewater treatment plants can vary widely depending on the size and types of discharges to the sewer system. Based on information collected several years ago, concentrations ranged from 100 to 800 ng/L (0.1 to 0.8 micrograms per liter). Sources include dental and medical waste, some consumer products (for example, broken thermometers), human wastes (e.g. urine), rainwater infiltration, flush-out of historical deposits in sewers, among others. The largest quantities of mercury are associated with the large plants simply because of their flow volumes. Since that influent data was collected, a number of communities have voluntarily initiated pollution minimization programs.

Due to its behavior, an extremely high percentage of mercury contained in wastewater becomes concentrated in treatment plant bio-solids or sludge. Thus, bio-solids concentration data are good predictors of what plants have elevated mercury loadings.

As detection limits have decreased, we have discovered just how prevalent mercury is in the environment. It is not uncommon to find concentrations of 3 to 5 ng/L in Wisconsin lakes or rivers, significantly above the 1.3 ng/L water quality criterion. We have even found mercury in some of the most pristine lakes in the state. In Lakes Michigan and Superior, mercury concentrations are less than 1 ng/L.

Where does it come from? Researchers attribute most of it to atmospheric deposition. In other words, mercury is not only in our lakes and rivers, but also in the air. This complicates the task of obtaining reliable sample results.