Environmental Health - Toxic Substances Hydrology Program
Field studies conducted in the United States have shown that mercury concentrations in groundwater affected by wastewater disposal can exceed the drinking water maximum contaminant level (MCL) established by the Environmental Protection Agency (2 micrograms per liter of water, µg/L). Two recently published reports by scientists from the Woods Hole Oceanographic Institution, the University of Maine, and the U.S. Geological Survey (USGS) help to explain what can lead to elevated mercury levels in groundwater.
Scientists conducted studies at a site where treated wastewater was discharged to infiltration beds for six decades, after which the disposal beds were abandoned for more than a decade. Over time, mercury from the wastewater accumulated in the sediments in and near the infiltration beds. Now, the abandoned disposal beds continue to be a source of mercury to the aquifer. Sediment–bound mercury concentrations 10 to 20 times the natural abundance of mercury on sediment were measured in the area of the disposal beds, and dissolved mercury concentrations were measured up to 0.15 µg/L in groundwater under the beds, over 700 times background concentrations in the aquifer. Mercury concentrations in groundwater decreased significantly within 200 meters of the source, which corresponds to about one year’s travel time. Mercury concentrations also decreased significantly with depth in the aquifer.
The scientists explain that treated wastewater can be a source of mercury and a source of organic carbon and other nutrients that can lead to mobilization of naturally occurring mercury. Changes in geochemistry from the addition of wastewater rich in organic carbon and other nutrients can drive microbial processes that lead to mobilization of natural sources of mercury. There is sufficient abundance of mercury in crustal rocks that concentrations in groundwater can exceed the drinking water limits if only a small fraction is present in a form that can be mobilized from the aquifer material to groundwater. These processes are similar to those that lead to mobilization of naturally occurring arsenic.
The studies collectively showed that wastewater disposal to infiltration beds results in changes in aquifer geochemistry over time, changes in the chemical form (species) of mercury over time, and the processes that affect the adhesion of mercury to aquifer sediments and control whether the mercury adheres to sediments or moves with the groundwater. The findings are important to managing and designing wastewater disposal and protecting drinking water quality.
The studies were funded by the USGS Toxic Substances Hydrology Program, National Water–Quality Assessment Program, and Hydrologic Research and Development Program; the Woods Hole Sea Grant Program; and the Woods Hole Oceanographic Coastal Ocean Institute.
Kinetics of homogeneous and surface–catalyzed mercury(II) reduction by iron(II), Environmental Science & Technology, 2013, v. 47, p. 7402-7213.
Mercury speciation and mobilization in a wastewater–contaminated groundwater plume, Environmental Science and Technology, 2013, v.47, p. 13239-13249.