By Catherine Clabby
When a new well is dug in North Carolina, a permit, inspection and water test are all required to make sure it is located in a place that is unlikely to be contaminated, that it is well built, and that nothing of concern contaminates its drinking water.
But insights into risks can emerge later, including potential dangers from the presence of naturally produced substances such as the metal manganese.
Just two years ago medical scientists linked higher levels of manganese detected in drinking wells in North Carolina’s Piedmont to increased incidence of heart defects among newborn babies.
Now North Carolina State University are estimating that more than 380,000 out of 1.5 million people using wells in the central swath of the state could be drinking water with too much manganese, with shallow wells posing the highest risk. That number could grow as more wells get drilled in a growing part of this state.
But there is good news too. The scientists have dug a more precise picture of the natural forces that put the higher amounts of the element into the groundwater that feeds drinking wells.
Drilling deeper wells or installing filters on home plumbing systems can shield people from unwanted exposure, they say.
In the Piedmont, about 3 feet of thick soil sits atop anywhere from 3 to 75 feet of a substance that once was bedrock called saprolite. After atmospheric gases, water and organic material from plants penetrate the ground, they fuel a series of chemical changes known as weathering that very slowly make the rock softer and more porous.
Weathering is extensive underground in the Piedmont, the swath of land between the coastal plain and the Appalachian Mountains. Within the weathered rock, particularly where the bedrock below is slate, chemical interactions have converted manganese into a form that moves easily with water.
As water migrates down through the porous saprolite, the weathered rock beneath the surface, it carries the metal with it, again very slowly increasing concentrations at depths where wells draw in groundwater. “When all these things are just right— as in much of the Piedmont—manganese in well water can build up,” Polizzotto said.
Manganese concentrations can reach 4 parts per million in some North Carolina wells, according to Polizzotto. That far exceeds that 0.05 parts per million North Carolina drinking-water standard and U.S. Environmental Protection Agency secondary maximum contaminant level. Those secondary levels are guidelines developed to guide public water systems on how to reduce risks of changes in taste, color and odor from a contaminant that EPA has not classified as a human health risk.
In order to produce a more detailed picture of manganese concentrations from the surface down to the bedrock in the Piedmont, Elizabeth Gillispie, a doctoral student in Polizzotto’s lab, and other researchers examined lots of material and data collected by state and federal agencies there over decades.
Among the cores were surface-to-bedrock samples extracted during the installation of wells at North Carolina Department of Environmental Quality research stations between 2001 and 2007. They sampled soil and all that is below up to 180 feet deep. Gillespie also analyzed water test results taken from more than 14,000 wells in the Piedmont collected by DEQ and the U.S. Geological Survey, paying special attention to how manganese concentration differed among wells of different depths.
Combining that data with some of Gillispie’s own water and soil sampling, the team assembled building blocks of a map that predicts where wells are more vulnerable to higher levels of manganese. They did so because concerns about potential ill health effects from exposure to higher levels of manganese in water are on the rise.
In the study released two years ago regarding heart defects in newborns, Rebecca Fry at UNC-Chapel Hill compared environmental exposures experienced by women who gave birth to children with birth defects to exposures experienced by women who did not. Newborns had a higher chance of being born with heart defects if their mothers drank well water where magnesium exceeded .05 parts per billion.
Evidence also suggests that exposure to excessive manganese in drinking water may produce neurological damage and respiratory problems.
Gillispie, Polizzotto and collaborators recommend that people dependent on wells in Piedmont not look at county averages to assess to assess if a well is at risk. The geologic qualities beneath, not geographic boundaries on top, are most important.
If metals are detected in well water, they suggest installing filters to shield anyone drinking the water from the metals, including manganese.
The researchers also recommend that new wells in the Piedmont be drilled deep into bedrock, despite the higher cost. The wall of wells, called casings, should extend deep too, the researchers said.
“The well depths of highest risks vary based on location, but in general, wells that are shallower than about 130 feet have higher risks of high levels of manganese,” Polizzotto said.
Gillispie has shared the project’s findings with both environmental health experts in county health departments, who are involved in testing of local wells, and with people who drill new wells to spread the word about this research as it developed.
Rick Bolis, a DEQ Division of Water Resources hydrogeologist and a co-author on the well paper, said he recommends that people have their well water tested every five years “unless you have reason to think there might be a problem.”
If that’s the case, the DEQ assistant regional supervisor in Raleigh counsels, consider acting sooner.