April 11 (Reuters) - The U.S. Environmental Protection

Agency has finalized the first federal regulations on toxic

"forever chemicals" in drinking water, setting tight limits that

essentially require public water systems to all but eliminate

their presence in American tap water.

The highly anticipated rules target six per- and

polyfluoroalkyl substances, or PFAS, that are commonly used to

make thousands of commercial and consumer products like

semiconductors, firefighting foams and stain resistant fabrics.

PFAS are known as forever chemicals because they do not

easily break down in the environment or in the human body, and

they have been linked to cancers and other health concerns.

Here is what you need to know.

WHAT DOES THE RULE DO?

The EPA's rule issued on Wednesday under the Safe Drinking

Water Act sets strict limits ranging from 4 to 10 parts per

trillion for five individual kinds of PFAS, and includes limits

for several other PFAS if they are present in combination in

water.

All public water systems have three years to complete their

monitoring for these chemicals and must inform the public of the

level of PFAS measured in their drinking water.

In cases where PFAS is found at levels that exceed the

standards, the water systems are required to implement measures

to reduce PFAS in their drinking water within five years.

HOW DOES THIS RULE FIT IN WITH OTHER PFAS REGULATIONS?

The rule is the most aggressive regulation yet under the

EPA's so-called "PFAS roadmap."

The agency previously bolstered requirements that

manufacturing facilities report their use and disposal of PFAS,

and issued a rule preventing companies from using PFAS in new

manufacturing processes without EPA approval, among other

things.

Going forward, the agency is expected to finalize rules

designating at least two PFAS as hazardous substances under the

U.S. Superfund law, which could expose many industries to

potential cleanup liability.

ARE LEGAL CHALLENGES TO THE RULE LIKELY?

Legal experts say court challenges seeking to block the rule

are likely to be filed by manufacturers, business groups and

potentially water systems themselves.

The challengers are likely to argue that the EPA's rules

were crafted without adequately considering the cost of

compliance or without adequate evidence showing the need for the

rules, in violation of requirements under the Safe Drinking

Water Act and federal administrative law.

The National Association of Manufacturers, the American

Chemistry Council and the U.S. Chamber of Commerce said in

comments last year to a draft version of the rule that it

overstated the benefits of imposing the limits while

underestimating costs. Water utility industry groups said in

comments that compliance could cost water systems billions of

dollars.

Texas, which has frequently challenged Biden administration

rules in court, has called elements of the rule "oversimplistic"

and said it would be difficult for small water systems to comply

given the costs.

HOW WILL THE RULE BE ENFORCED?

Under the Safe Drinking Water Act, most states are given

primary enforcement authority to ensure drinking water standards

are met. The EPA typically gets involved when states are not

enforcing the requirements or are otherwise unsuccessful in

enforcement efforts.

State and federal regulators can issue administrative orders

that lay out steps for water systems to come into compliance,

file legal actions or fine water systems that are not meeting

the standards. Experts say any future fines are likely years

away, and would come after numerous warnings.

Citizens can also sue the government or water systems under

the law to force compliance.

COULD THE STANDARDS BOLSTER EXISTING LAWSUITS?

Lawsuits brought by hundreds of water systems against

chemical manufacturers accusing them of negligence and of

creating a nuisance by contaminating water with PFAS have

already yielded major settlements.

Last year, 3M ( MMM ) reached a settlement worth $10.3

billion with water systems across the U.S. that will help pay to

clean up drinking water contaminated with PFAS, while DuPont de

Nemours Inc ( DD ), Chemours ( CC ) and Corteva ( CTVA ) reached

a similar deal worth $1.19 billion.

The settlements were a part of sprawling multidistrict

litigation (MDL) in South Carolina federal court, where other

pending lawsuits against manufacturers may ultimately yield more

money for the water systems.

The new rule could make it easier for water systems that are

suing or considering suing companies that produce or use PFAS

near them for polluting waterways with the chemicals. Legal

experts say that is because the rules create an unambiguous

standard for what levels of PFAS in drinking water are

acceptable, and so could make it easier for water systems to

prove they have been harmed by the pollution.

(Reporting by Clark Mindock, Editing by Alexia Garamfalvi)