As towns across Cape Cod struggle with problems stemming from septic systems, a recent study by a Woods Hole Oceanographic Institution (WHOI) scientist focuses on one specific toxic by-product: mercury. In a study of local groundwater, biogeochemist Carl Lamborg found microbial action on wastewater transforms it into more mobile, more toxic forms of the element.
His findings were published in Environmental Science and Technology in November 2013.
Mercury (Hg) is a toxic trace metal. Wastewater contains small amounts of it, but Lamborg found the chemical processes that break down waste increase mercury levels in the ground and water.
Between 2010 and 2012 Lamborg measured the concentrations and various forms of mercury at sampling wells installed by U.S. Geological Survey surrounding a wastewater treatment site operated by the Massachusetts Military Reservation in western Cape Cod. Wastewater was disposed into the ground at this site from 1936 to 1995, creating a 3-kilometer-long plume of contaminants spreading downstream from this site. The plume travels about 200 meters per year through the aquifer and ends in a coastal saltwater pond.
"The amount of mercury flowing out of the watershed and into the ocean and these ponds is something like twice as much as it would be if wastewater was not being put into the ground," said Lamborg, who analyzed the chemistry of the samples in his lab at WHOI.
To better understand why, he looked at two sites along the plume where microbes have broken down the carbon and nitrogen from the waste and have consumed all of the oxygen in the sediment and groundwater, making the sites anoxic.
Near the upstream point of entry, he found the microbes were using iron to break down the waste, a process called iron reduction. In this process, he observed the most common form of mercury, (Hg2+), which, he said, normally sticks to the sediment, was reduced into "less sticky" elemental
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Woods Hole Oceanographic Institution