In-ground disposal of wastewater triggering an increase in this toxic element
By ecoRI News staff
As communities 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 byproduct: 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.
Mercury 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 soil and water.
Between 2010 and 2012, Lamborg measured the concentrations and various forms of mercury at sampling wells installed by the 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 1936-1995, creating a 3-kilometer-long plume of contaminants spreading downstream from this site. The plume travels about 200 meters a 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,” Lamborg said.
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, Lamborg observed the most common form of mercury, (Hg2+), which, he said, normally sticks to the sediment, was reduced into “less sticky” elemental mercury (Hg0) that more easily seeps into groundwater and is then transported downstream.
At a second sample site further downstream, he found a high concentration of another type of mercury, monomethylmercury, (MMHg), which accumulates in fish at toxic levels and can be detrimental to human health.
“In some cases, 100 percent of the mercury present at that location was MMHg,” he said. This form of mercury also easily moves through water and can accumulate in ponds and the ocean, where fish and other animals absorb it.
He noted that the level of MMHg isn’t high enough to make the water dangerous to drink. However, the concentration would be even lower if wastewater hadn’t been disposed into the ground in the first place.
“This should make us all think twice about what we dump into the ground,” Lamborg said. “Adding more nitrogen into the ground through wastewater, and even fertilizers for our agricultural fields and golf courses, offers a potential for mercury to accumulate and move through the aquifer to our ponds, lakes and the ocean. That’s something I don’t think people are really thinking about.”
Lamborg looked further into the chemical process occurring at the second site and found something even more surprising. He observed a chemical process where microbes use organic carbon with nitrate to break down organic matter, a process called denitrification. While previous studies have shown low levels of MMHg where denitrification occurs, this site exhibits high levels of MMHg occurring from denitrification.
“This kind of thing where you see denitrification resulting in the methylation of mercury has never been observed before,” he said.
“What it looks like is, the mercury that was already there in the aquifer or sand is being mined out when the groundwater goes anoxic,” Lamborg said. He said mercury that has been stored in the ground for several thousands of years is being drawn out and is now on the move.
Along the Ashumet Valley, a 3-kilometer depression between the towns of Falmouth, Sandwich and Mashpee, mercury continues to accumulate and move downstream in a plume ending in Green Pond in East Falmouth.
According to Lamborg, it’s a community-wide problem. Because the soil on Cape Cod is sandy, waste dumped into the ground disappears quickly. Although it may seem out of sight and out of mind, contaminants such as mercury accumulate and resurface in local waters.
“This is just one really big example, but it’s happening in a small way through everybody’s backyard septic system, which leaches a little bit of mercury out of the aquifer and accumulates,” he said. “You don’t need a really big industrial scale thing for this to happen.”
These findings should help guide future studies evaluating potential groundwater contamination with mercury, said Doug Kent, a U.S. Geological Survey hydrologist, who collaborated on this research.