BERLIN – The results of a Dartmouth-led study of the Chlor-Alkali Super Fund site reveals mercury from the site is being transformed into a more toxic form of the metal by periphyton or algal communities living on rocks.
The finding adds a new way in which mercury from the old chemical plant on the Androscoggin River can enter the food chain.
The assumption had been that the mercury leaking into the river from the site was converted into the more toxic methylmercury by the action of anaerobic organisms or bacteria that work in low oxygen environments such as
"The assumption with mercury is that methylation, changing mercury from its elemental, silver form, to a type of molecule that easily enters living tissue, occurs only in anaerobic sediments – oxygen-free muck. Therefore, we should only see mercury enter the food chain in quiet waters further downstream. The area near the site has no anaerobic muck – it's just rock and fast moving water, and therefore no transfer of mercury should occur," said Darryl Luce, project manager for federal Environmental Protection Agency.
But the Dartmouth study noted surface sediment next to the site had methylmercury levels up to 40 times higher and total mercury levels up to 30 times higher than other reaches of the river. Mercury concentrations in the water
next to the Superfund site were up to five times higher than other sampling areas.
In a paper published in the Environmental Toxicology and Chemistry, the Dartmouth researchers and their US Geological Survey colleagues attribute the elevated levels to inputs from the Superfund site, but also noted that to the periphyton – a complex mixture of algae, bacteria, fungio, and detritus attached to submerged rocks, is converting the mercury into methylmercury. This transformation by periphyton means that methylmercury can be available to enter the food chain in areas where there is not a lot of sediment.
"The novel idea that this paper put forth was that the mercury could be similarly transformed by rock slime – the stuff we slip on when we wade, said Luce.
Dartmouth Professor Celia Chen, a research professor of biological sciences and principal investigator in Dartmouth's Toxic Metals Superfund Research Program, said the periphyton is important component of the aquatic ecosystem, providing food to small fish and invertebrates. But at the same time, the methylmercury travels up the food chains as larger predator fish eat the smaller fish.
"This paper explains one means of how the mercury may transform into something that bugs and fish can concentrate in their bodies," said Luce.
Interestingly, methylmercury concentrations in crayfish, mayflies and shiners didn't increase downstream from the site, which had been expected since previous studies had shown that happening in large adult fish. Total mercury and
methylmercury levels in small fish showed no clear pattern and the study said it could not completely explain why the patterns were different between the predator fish and the smaller invertebrates. One theory is the impact of the
leaking mercury may be more localized than expected.
"While our study clearly demonstrates that the chlor-alkali Superfund site is impacting this section of the Androscoggin River, future studies could investigate whether other factors such as dams, river grade, wetlands or upland drainage influence the patterns of bioaccumulation," the authors note in the manuscript.
In a phone interview, Chen said the college got involved in the Superfund study because of its longstanding Superfund Research Program and contact with Darryl Luce. Later, they collaborated with the U.S. Geological Survey, which
had been contacted by EPA to look at sediment and the distribution of mercury downstream of the site. Together with the USGS, the Dartmouth group decided to look at the periphyton or rock algae. The Dartmouth researchers came to Berlin to do field work in the summers of 2010 and 2011, funded by a grant from the National Institute of Environmental Health Sciences.
The data collected was analyzed and reviewed and lead to the findings outlined in the paper.
The 4.6-acre site on the east bank of the Androscoggin River was placed on the national Super Fund list in 2005. The contamination was actually discovered back in 1999 during the sale of the property by Crown Vantage to American Tissue. A slurry wall was installed around two sides of the site to inhibit groundwater flow into the area, and the property was capped with a synthetic membrane to keep the rain from percolating into the site. Over 135 pounds of elemental mercury have been removed from the site. The investigation looked at 24 miles of the Androscoggin River – from Pontook Dam north of the cell house site south to Shelburne Reservoir. Soil samples were taken as well as toxicity testing and sampling of insects, fish, birds, and bats by the EPA and other researchers.
In an e-mail, Luce said the EPA plans to do additional investigations on the site this summer and have a complete site remedy to release in 2016.