Excess nitrogen in wastewater may be a culprit in causing higher than normal levels of mercury in groundwater that feeds into coastal waters.
Carl H. Lamborg, associate scientist in the marine chemistry and geochemistry department at Woods Hole Oceanographic Institution did a study, published in Environmental Science and Technology, measuring mercury levels at different points in a plume that originates at Joint Base Cape Cod and flows through the Ashumet Valley ending at Green Pond in East Falmouth.
“The mercury concentration in the groundwater is about five times higher, and that results in about two times more mercury making it into ponds and bays than if this phenomenon wasn’t happening,” Dr. Lamborg said.
The disposal of untreated wastewater has caused problems in the coastal ecosystems before. The extra nitrogen from human waste making its way into coastal waters results in more algal growth. Bacteria decomposing algae deplete oxygen levels in the water leading to fish kills, said Jared V. Goldstone, also a scientist at WHOI and the chairman of the board of health in Falmouth.
Dr. Lamborg has found that excess nitrogen is again causing an imbalance in nature, in a different way. The wastewater plant at the base ran from 1936 to 1995 and pumped wastewater into infiltration beds, essentially enormous leach fields where the wastewater soaks down into the groundwater.
High levels of nitrogen and carbon from human waste in the wastewater also reached the groundwater, which provided a smorgasbord of food for bacteria.
“These levels give the bacteria something to live on,” Dr. Lamborg said. “They use oxygen to breathe and burn food and they strip the oxygen out of the groundwater.”
This feeding frenzy results in an altered environment where new bacteria that don’t need oxygen to live come and thrive.
“It creates a niche,” said Dr. Lamborg, “where bacteria can do weird things to mercury, converting it to other forms.”
At different points along the plume’s path Dr. Lamborg found higher levels of certain forms of mercury that were more soluble in water. Mercury has different forms depending on the electron composition of the atom. The most common form of mercury Hg2+ tends to stick in dirt and soil.
His measurements found two other types, elemental mercury and monomethylmercury, the form that accumulates in fish. Usually a few percent of each type is found naturally; however, in some samples Dr. Lamborg took the elemental mercury was 30 percent of the total mercury measured and the monomethylmercury was 100 percent of the total amount measured.
“There was clear evidence that mercury was being transformed coming off the soil,” Dr. Lamborg said.
These high mercury levels in the groundwater, however, pose no risk to Falmouth’s drinking water. The levels are way below the US EPA drinking water standards, Dr. Lamborg said.
“Though the microbial action alters the form of mercury and makes it more mobile there are no drinking water sources or wells in that plume,” Dr. Goldstone said.
Also drinking water sources are regularly tested. People would be notified within 30 days if there were abnormal levels of any contaminant, Dr. Goldstone said.
However, other bodies of water could be affected.
“What’s more of a concern is the mercury going into the coastal ponds and increasing the level of mercury in seafood harvested in the area.” Dr. Goldstone said. Monomethylmercury accumulates in fish, especially large fish such as swordfish, shark and tuna, which are higher up on the food chain. Eating fish with high levels of mercury can make adults sick. Pregnant women and young children under 12 are especially vulnerable because it can interfere with central nervous system development, Dr. Goldstone said.
Michael H. Bothner, a scientist emeritus at the US Geological Survey on Cape Cod is looking to find out how this mercury inflow may be affecting fish in the area. During his career he studied mercury in sediment; now he is using his skills to measure mercury in fish. So far, it is a small study, he said, with local fishermen donating many of his samples.
Results are preliminary but they support data published in a paper by Maria N. Piraino and David L. Taylor at Roger Williams University in Rhode Island that found 75 percent of legal-sized striped bass sampled in Narragansett Bay had mercury levels beyond the US EPA regulatory threshold of 0.3mg Hg/Kg wet weight, he said.
“Carl’s research contributes to understanding where mercury in the food web is coming from,” Dr. Bothner said.