EWG finds widespread contamination of drinking water with chromium-6, nitrate and arsenic across the U.S.

More than 100 million Americans are drinking tap water contaminated with a mix of cancer-causing metals, according to a new Environmental Working Group analysis. The study examined data from more than 17,700 community and groundwater systems across the United States and found that roughly 82 percent of the population is exposed to at least one of three chemicals — chromium-6, nitrate and arsenic — with a substantial share receiving all three.

Arsenic, a naturally occurring metalloid; hexavalent chromium (chromium-6), largely derived from industrial activity; and nitrate, largely from agricultural runoff, are each linked to cancer and other health concerns. Arsenic is associated with cancers of the bladder, lungs and skin; nitrate exposure has been tied to colorectal cancer and thyroid disease and poses a risk of blue baby syndrome in infants; chromium-6 is linked to cancers of the lung, stomach and liver. Federal limits for these toxins are uneven: arsenic in drinking water is capped at 10 parts per billion (ppb), nitrate at 10 parts per million (ppm), while there is no universal federal limit for chromium-6. The California OEHHA (Office of Environmental Health Hazard Assessment) sets far stricter guidance, with proposed safety levels far below federal standards.

For this study, the researchers combined national testing data from the Environmental Protection Agency with state-level records spanning 2011 to 2023 and calculated cancer risk using OEHHA safety levels, which are hundreds of times stricter than federal limits. OEHHA’s maximum recommended levels are 0.004 micrograms per liter for arsenic, 0.02 micrograms per liter for chromium-6 and 0.14 milligrams per liter (140 micrograms per liter) for nitrate. The EPA’s current federal limits remain higher for arsenic and nitrate, and there is no universal federal limit for chromium-6. In some cases, the lifetime cancer risk at the legal arsenic limit (10 ppb) is estimated at about 1 in 300. The study notes that the nitrate standard may also be insufficient to guard against stomach cancer, based on UK and Norwegian research, which found that a 10 mg/L increase in nitrate nearly doubles stomach cancer risk. The California public health goal for chromium-6 of 0.02 ppb is framed by OEHHA as not a “safe” level but one corresponding to a one-in-a-million lifetime cancer risk.

The analysis integrated data across more than a decade of testing and emphasizes how multi-contaminant exposure can drive risk higher than assessments that consider each toxin individually. Dr. Tasha Stoiber, a senior scientist at EWG and the study’s lead author, said: “Drinking water is contaminated mostly in mixtures, but our regulatory system still acts like they appear one at a time. This research shows that treating multiple contaminants together could prevent tens of thousands of cancer cases.” For its part, the EWG’s team argues that regulating groups of toxic chemicals in tap water could deliver greater health benefits and that water utilities could pursue shared treatment solutions to curb multiple contaminants simultaneously.

Geography helps explain why these patterns persist. Arsenic is largely tied to natural geology, especially in the West, where communities rely more on groundwater and groundwater systems have longer contact with arsenic-containing rocks. The region also tends to report contamination with all three toxins less uniformly than some other areas, though pockets exist. Chromium-6 and nitrate, conversely, show a stronger presence in the Southeast, which has a long history of industries that used chromium in metal plating, textile manufacturing and cooling tower construction, along with intensive agricultural activity that can drive nitrate into groundwater. States in the Southeast — including Kentucky, Tennessee, Alabama and Georgia — see a higher burden of chromium-6 and nitrate exposure. Rural states with fewer contaminated systems generally report lower cancer rates than the national average, while urban and peri-urban areas in the West and Southeast show more pronounced multi-contaminant exposure.

The EWG noted that the burden falls most heavily on communities that rely on groundwater and on low-income regions where upgrading treatment infrastructure can be cost-prohibitive. The group argues that addressing contaminants in concert could be more cost-effective for utilities and customers than installing separate treatment lines for each chemical. Sydney Evans, a senior EWG scientist and co-author of the study, said: “Ensuring clean drinking water for all communities is about fairness and equity. Communities in the US that rely on groundwater are often affected by these contaminants. New water treatment technologies offer a chance to improve water quality overall. This strengthens the case for action and investment.” By addressing multiple contaminants with shared treatment solutions, utilities could reduce long-term capital and operational costs while improving public health and resilience.

If regulators were to pursue a multi-contaminant standard—such as a five ppb limit for chromium-6 while concurrently reducing arsenic exposure by about 28 percent—the study estimates roughly 2,647 lifetime cancer cases could be prevented, a figure that would double the benefit of addressing chromium-6 alone. The analysis also emphasizes that public investment in modern water treatment technologies would likely deliver broader societal benefits beyond health, including improved productivity and well-being linked to access to safe drinking water. Utilities bear primary regulatory responsibility, but the financial burden is shared with residents through bills and with taxpayers via assistance programs. The authors say that a forward-looking, integrated approach to water treatment could strengthen the safety net and promote healthier communities across the country.