Most of the nation's water systems contain harmful contaminants although they meet federal standards, according to a new study.

More than 40,000 water systems nationwide contain contaminants linked to cancer, brain and nervous system damage, developmental defects, fertility problems or hormone disruption, a new study from the Environmental Working Group found.

The environmental advocacy group collected and analyzed data over the past two years from almost 50,000 public water systems in all 50 states and the District of Columbia. Five hundred contaminants were tested for, and 267 were found, including 93 linked to an increased risk of cancer, 78 associated with brain and nervous system damage, 63 connected to developmental harm to children or fetuses, 38 that may cause fertility problems, and 45 linked to hormonal disruption.

More than 40,000 water systems had detections of likely carcinogens at levels that slightly exceeded local and federal guidelines, posing only negligible health risks and are not legally enforceable.

One of the pollutants was the industrial chemical made infamous by the film "Erin Brockovich," chromium-6 or hexavalent chromium, which was found in drinking water supplies for more than 250 million Americans in all 50 states. The chemical remains unregulated.

More than 1,800 water systems that were tested, serving 7 million Americans, showed chemicals from animal waste or agricultural fertilizers at levels the National Cancer Institute says increases the risk of cancer.

Rural communities, where agriculture plays a significant role in daily life, were shown to have a higher level of toxins in their drinking water.

That was the case in Topeka, Kan., where at least four pesticides used on corn fields were found. One pesticide, atrazine, the second most widely used weedkiller in the U.S. was found. Some studies have found it can turn male frogs into females after exposure to levels of the pesticide commonly found in sources of drinking water throughout the Corn Belt.

The East Los Angeles Water District had the most pollutants in its water system, with 14 discovered in its 2015 water tests that were well above established health guidelines for acceptable levels of contaminants.

Among the cleanest water systems in the nation is Merrick, N.Y., on Long Island, where the Environmental Working Group found only one contaminant in its system that was above health standards.

The study also found that Flint, Mich., still has not recovered from a water crisis that exposed an estimated 12,000 children to lead-tainted water. It identified levels of nine other contaminants that are above health guidelines, including chloroform, which is known to cause cancer and problems with pregnancy, along with manganese, which has been found to harm the brain and nervous system, the group said. An additional 14 contaminants were also found in the city's drinking water.

"Just because your tap water gets a passing grade from the government doesn't always mean it's safe," group President Ken Cook said. "It's time to stop basing environmental regulations on political or economic compromises, and instead listen to what scientists say about the long-term effects of toxic chemicals and empower Americans to protect themselves from pollutants even as they demand the protective action they deserve from government."

That's why his group has launched an online Tap Water Database, where users can find all contaminants detected in tests by the water utilities and reported to federal or state authorities. The group wants users to compare current data against legal limits they say are outdated.

The Safe Water Drinking Act was enacted in 1974 to regulate all of the nation's more than 170,000 public water systems. It originally focused on treatment as the means of providing safe drinking water, but a 1996 amendment shifted the focus to source water protection, operator training and funding for water system improvements.

However, the environmental group argues the EPA has not added a new contaminant to the list of regulated drinking water pollutants in more than 20 years, calling it an "inexcusable failure of the federal government's responsibility to protect public health."

The group says there are no legal limits for the more than 160 unregulated contaminants their tests detected in the nation's tap water and are calling on state governments to enact their own safe-drink water measures.