Drinking water is tightly regulated in the United States and, for the most part, is remarkably safe. Recent contamination episodes in Flint, Michigan, and elsewhere, however, have highlighted the fragility of this public health success story and the serious health risks lead poses in significant portions of the U.S. drinking water supply. Exposure to lead, even at low levels, has adverse health effects for people – especially children, pregnant women, and their developing fetuses. While these risks are widely known, lead continues to pervade the tap water of many American communities. This is due largely to the extreme difficulty and high cost of identifying, locating, removing, and preventing the many potential sources of lead across thousands of U.S. water systems, which vary widely in size, type, age, source supply, ownership, and maintenance.
The lead problem
In the United States, lead enters drinking water primarily through leaching from corroded pipes and plumbing fixtures that contain lead. Physical disturbances to these materials can also sometimes release lead particles into drinking water.
Lead pipes were widely used in water-supply systems until the early 20th century, when many communities began to recognize the health effects of lead exposure. Use of lead-containing solder, service lines, and plumbing components, however, continued through the mid-1980’s.
In 1986, these were banned from use in new U.S. plumbing systems, but they remain throughout much of the country’s drinking water infrastructure, which largely pre-dates the ban.
Until 2014, plumbing fixtures with as much as 8% lead-by-weight could be legally labelled “lead free”. While the allowable percent is now reduced to 0.25%, fixtures installed prior to the revision remain in use throughout the country. Because of the way lead-by-weight averages are calculated, even some newer fixtures can release significant concentrations of lead.
Roughly 87% of the U.S. population receives water for household use from “public water systems” – defined as those serving on average at least 25 people for at least 60 days per year. More than 150,000 of these systems exist in the United States in a range of sizes. They can be owned publicly or privately (despite their name), and typically draw from either groundwater sources such as wells or surface water sources such as rivers, lakes, and reservoirs.
The diversity of systems and their varying degrees of upkeep means there can be no one-size-fits-all approach to identifying and removing lead from all drinking water systems.
In recent surveys by the American Water Works Association, nearly a third of U.S. water systems reported service lines containing lead. The total number of lead service lines in the United States is estimated to be upwards of six million.
A single plumbing fixture containing lead can contribute to contamination of tap water, even when associated larger water mains and service lines are fully lead-free.
Because it is virtually impossible to monitor all fixture types and uses at the individual building and household level, the full extent of lead sources across U.S. water systems is unknown.
Consumers generally cannot see, taste, or smell lead in their drinking water. The only way to know whether tap water contains lead is to test it.
While individual filtration systems can be used to remove lead from drinking water at the tap, there are several barriers to their widespread use, including a lack of consumer awareness about whether their drinking water is indeed contaminated, the need to install and replace filters correctly, and the cost of purchasing and regularly replacing filters at each point of use.
Drinking water is just one pathway for people to become exposed to lead. Others include deteriorating paint and household dust containing lead; older painted toys, furniture, and jewelry; lead-glazed pottery; and lead-contaminated soil or air, among others.
There are several well-established adverse health effects of lead exposure, even at low levels. Young children, pregnant women, and their developing fetuses are especially vulnerable to these risks.
Among the known health effects for children are lower IQ, slowed growth, behavior and learning problems, anemia, and hearing problems.
Studies have also suggested that children who do not display overt symptoms may still experience lead’s effects later in life, though it can be difficult to distinguish between the effects of early lead exposure and other correlates, such as poverty. Multiple studies have identified a relationship between early childhood lead exposure and future violent behavior or criminal activity, for example, but such observational studies are not designed to prove direct causality.
Lead can also accumulate in human bones over time. During pregnancy, lead is released from the mother’s bones, along with calcium, and used in fetal bone formation. A fetus can also be exposed to lead from across the placental barrier. These exposure pathways can result in premature birth, gestational hypertension, reduced fetal growth, and fetal death.
Breast milk is another pathway of lead exposure for infants. Lead released from a lactating mother's bones can exit the body via breast milk.
In adults, chronic low-level lead exposure has been linked to cognitive decline, hypertension, and adverse reproductive outcomes, among other health effects.
Rules and regulations
The 1974 Safe Drinking Water Act (SDWA) requires the U.S. Environmental Protection Agency (EPA) to establish and enforce health-based safety standards for all public water systems.
Enforcement of these standards is primarily carried out by individual states through various methods of water-system monitoring and reporting.
When monitoring identifies the presence of a contaminant at a level above the SDWA safety standard, states and the EPA must work with utilities to reduce or remove the contamination and notify consumers.
For most contaminants regulated under the SDWA, the EPA sets a safety goal called the Maximum Contaminant Level (MCL) – a level below which there are no known or expected health risks to people over a lifetime of exposure.
For lead and copper – which can contaminate a water supply based on plumbing fixtures in individual homes and private plumbing systems (and are thus impossible to fully control at the water-system level) – the EPA instead established an enforceable monitoring and treatment strategy for water systems to help control corrosion and reduce leaching from plumbing.
When testing shows that more than 10% of customer taps in high-risk homes in a given water system (such as those known to be connected to lead-containing service lines) exceed lead concentrations of 15 parts per billion (ppb), the system must install or improve corrosion control treatment.
Corrosion control most often involves introducing chemical treatments to make the water supply less corrosive or to facilitate the buildup of protective scales inside water pipes, which reduces leaching of lead. These requirements are laid out in the 1991 Lead and Copper Rule, which is currently under consideration for revisions.
While the Lead and Copper Rule establishes a public health goal of zero lead in drinking water, this standard is unattainable and unenforceable because:
There are many household-level or privately-owned sources of lead that cannot be controlled by public water systems.
Lead releases can vary significantly between taps in a single household and may not be reflected by sampling of associated water mains.
There is no complete chemical fix that utilities can use to fully stop the corrosion and lead-leaching process.
Lead continues to pervade old plumbing systems and be added (albeit in declining quantities) to new plumbing systems.