As the freshwater mussel Elliptio complanata grows, its carbonate shell captures long-term variations in the levels of trace elements in its aquatic environment. This kind of natural record-keeping could serve as an archive of road salt pollution in freshwater bodies, a new study suggests.Winnick et al.

Long-term variations in the levels of trace elements captured in freshwater mussels’ carbonate shells as they grow can serve as an archive of road salt pollution in streams, a new study suggests.

Road salt used in the winter to clear icy highways is tainting many waterways in the Northeast (SN: 9/24/05, p. 195). The vast majority of such streams don’t have instruments in place to monitor that pollution, says Matthew Winnick, a geophysicist at Vassar College in Poughkeepsie, N.Y. The cost of the monitoring equipment has been prohibitive. However, freshwater mussels such as Elliptio complanata might serve as biological sensors of a stream’s water chemistry, he and his colleagues reported Wednesday in Houston at the annual meeting of the Geological Society of America.

For their study, the researchers collected water samples and living E. complanata mussels from four streams that flow into the Hudson River near Poughkeepsie, about 100 kilometers north of Manhattan. Then, the team used a dental drill to collect a 1.5- to 4-milligram sample of each mussel’s shell. Although mussel shells are primarily made of calcium carbonate, the shells also include trace elements from the water where the mussels lived, says Winnick.

In water samples taken from the Sawkill, Fallkill and Crum Elbow creeks — all of which run through watersheds with relatively few roads — concentrations of sodium ions ranged between 15 and 20 milligrams per liter of water. However, in the sample taken from the Casperkill Creek, which runs through a relatively developed area with many streets and roads, sodium levels were about 120 milligrams per liter of water. Similarly, the concentrations of other trace elements found in road salt, such as manganese and barium, were much higher in water from Casperkill Creek than they were in the other streams.

Those differences showed up in the mussel shells, the researchers found. In general, the higher an element’s concentration in the water was, the higher the element’s concentration in the mussel shells. Because freshwater mussels can live as long as a decade, their shells — which sport growth rings that record changes over time just as trees do — could serve as long-term monitors of road salt pollution, the researchers contend.

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