Posted by: Footler PJ2 APR 2014
Researchers in Cape Cod, Massachusetts, have studied the way microbial action transforms mercury in waste water into more mobile, more toxic forms. The water treatment site surveyed, which was operational from 1936 to 1995, created a 3km plume of contaminants that were found to travel at about 200m a year through the aquifer to accumulate in a coastal saltwater pond.
The findings, published in Environmental Science and Technology, showed that different forms and quantities of mercury were released along the plume. Near the point of entry the sandy soil enables waste water to disappear from sight quickly. Here, microbes break down the carbon and nitrogen present while consuming all the oxygen. In the resulting anaerobic environment microbes use iron to break down the waste. During this process, known as iron reduction, the predominant form of mercury, ionic mercury, which “sticks” to the sediment, converts into “less sticky” elemental mercury which seeps more easily into the groundwater to be transported downstream.
Further downstream researchers noted another chemical process, denitrification, where microbes use carbon combined with nitrates to break down organic matter. Here they found a high concentration of another type of mercury, monomethylmercury (MMHg), which is the form that accumulates in fish at levels that can have a detrimental impact on human health. Curiously, whereas previous studies showed low levels of MMHg where denitrification occurs, this site exhibited high levels of MMHg coinciding with denitrification.
The researchers were also surprised to find even more mercury in the plume than was originally thought to have been deposited from the waste water and released by the action of microbes. It is thought that mercury already in the aquifer or sand was drawn out as the groundwater became anoxic and was now on the move.
It is hoped that these findings will help guide future studies evaluating potential groundwater contamination with mercury.