Half of U.S. states have a high or very high prevalence of well water that's corrosive enough to leach lead from pipes.
That's the consensus of a new report from the U.S. Geological Survey that assessed groundwater of more than 20,000 private wells nationwide. About 44 million people in the U.S. get their drinking water from private wells.
"The corrosivity of untreated groundwater is only one of several factors that may affect the quality of household drinking water at the tap," Stephen Moulton, chief of the USGS National Water-Quality Program, said in a press release. "Nevertheless, it is an essential factor that should be carefully considered in testing for water quality in both public and private supplies nationwide."
Eleven states and the District of Columbia have a "very high prevalence" of potentially corrosive groundwater, a finding that could effect as many as 8 million people who depend upon self-supplied groundwater. To be designated "very high prevalence" the states scored high on both measurement indexes used in the study.
Fourteen states have a "high prevalence," meaning they scored high on one of the two gauges used in the study. About 16 million people live in states in this category and rely on self-supplied groundwater.
Nineteen states were seen as having a "moderate prevalence," and six states were classified as "low prevalence," according to the report.
The states with the largest percentage of wells with potentially corrosive groundwater are located primarily in the Northeast, the Southeast, and the Northwest. Virginia and Pennsylvania are states where private water sources, such as wells, springs, or cisterns, are especially common.
Kenneth Belitz, one of the researchers involved in the study, told CNBC they're only beginning to look at the factors that cause corrosivity. But geography and proximity to sources of chloride may be important factors.
"The rock and minerals affect chemical composition," he said. "The coastal plains on the East Coast tend to have more issues... The rainfall contributes chloride. Rain enters the ground and then it begins to move toward wells. As it moves through the system its chemical composition changes. The amount of time and the amount of rock that it has encountered will also affect the composition."
Maintenance, testing and treatment of private water supplies are the responsibility of the homeowner. Self-supplied water is not generally regulated and often is not treated. "It requires self awareness and spending one's owns money," Belitz said.
In contrast, public water supplies are regulated by the U.S. Environmental Protection Agency and are required by law to test.
"Fortunately, in most areas of the country and with appropriate safeguards, the majority of homeowners can get good quality drinking water from private wells," Moulton said in the release. "But this study is a good reminder that prudent, routine testing of the water, including its interaction with the water supply system, is an essential first step so homeowners and their families can confidently drink water from their faucets."
Belitz said additional steps could include identification and evaluation of additional indices of corrosivity, and evaluation of the relation between the indices and the factors that may affect groundwater corrosivity. These factors include, but are not limited to, aquifer type, mineralogy of the aquifer materials, distance of the well from recharge areas, depth of the well, groundwater age, climate, and proximity to sources of salinity, according to the report.
In the study, two indicators were used to identify which areas in the U.S. might be more susceptible to elevated concentrations of metals in household drinking water and which areas might be less susceptible.
One was the Langelier Saturation Index, used to indicate the degree of saturation of calcium carbonate in water. This mineral is naturally present in water and acts as protection against minerals dissolving from pipes. In the absence of a protective, lead, if present, may dissolve into the water.
"[Calcium] can either precipitate and form a lining in scale in a pipe or not," Belitz said. "It depends if it is undersaturated, saturated or oversaturated."
On the basis of the LSI, about one-third of the samples collected from about 21,000 groundwater sites are classified as potentially corrosive, according to the report.
And then there was the Potential to Promote Galvanic Corrosion. Galvanic corrosion of lead is an electrochemical process that can occur when lead pipe or lead solder is in contact with a dissimilar metal such as copper.
Lead and copper are rarely detected in most drinking water supplies. But some household plumbing fixtures may contain lead or copper, and corrosive waters may pick up lead and copper from household plumbing pipes after entering a home.
"Like a car battery. There is a conductor fluid and one moves from post to the other," he said.
On the basis of the PPGC, about two-thirds of the samples collected from about 27,000 groundwater sites are classified as moderate PPGC, and about one-tenth as high PPGC.
Potential sources of lead include lead pipes or fittings in homes built prior to 1930, lead solder used in copper fittings in homes, "lead-free" brass components, which, in all states, except California, may have contained up to 8 percent lead, prior to 2014 and galvanized steel that contained 0.5 to 1.4 percent lead, prior to 2014, according to the study.
The report points out that this is not to be confused with the water problems faced in Flint, Michigan, that exposed 6,000 to 12,000 children to drinking water with high levels of lead. The problems in Flint were related to treated surface-water from the Flint River, whereas this report focuses on untreated groundwater nationwide, the agency said.