State and local water officials insist that tap water is safe to drink on Long Island, though many residents still have their doubts. 

Long Island's drinking water is held in like a sponge in deep layers of underground sand. Experts say the aquifers are threatened by decades of unchecked industrial development.

"The contamination that we see in our groundwater now  [is]  really due to activities that occurred 40, 50, 60 years ago," says Dennis Kelleher, president of H2M Water. 

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Those toxic chemicals that seeped into aquifers created what are called toxic plumes.

In Nassau County, years of aerospace and military development at the former Grumman plant is believed to be responsible for one such toxic plume. It now threatens the water supplies of Bethpage, South Farmingdale and Massapequa. 

"The public is understandably concerned when they hear about a contamination problem this big," says Basil Seggos, commissioner of the state Department of Environmental Conservation. 

In Suffolk, the Water Authority uses seven labs to test for a group of contaminants. The county requires that they test for 149 contaminants in total. Officials admit they can't test for every contaminant and that new ones are discovered every year – many of which are unregulated. 

One such contaminant is the chemical solvent 1,4-Dioxane, commonly found in cosmetics and household products. It was first discovered in the groundwater in 2013. Tests have shown that it is present in 70 percent of the supply wells on Long Island.

Since no standards have been set yet to regulate 1,4-Dioxane, the state has put together a study to determine how dangerous it is to humans. While a study is being performed, the state has allocated funds to remove 1,4-Dioxane through an advanced oxidation process, or AOP.

Joe Rocarro, a water quality engineer with the Suffolk County Water Authority, says they treat water with hydrogen peroxide and UV light, which creates "a very powerful oxidant" that destroys contaminants. The process can treat almost 2 million gallons a day. The problem is the cost, which is about $1.5 million to treat a single well. Installing filters at all the affected wells would take years.

Many residents, like Gina Farrelly of Long Beach, say they are taking matters into their own hands by installing whole house filters – a short-term fix to what officials say is a long-term issue. 

"We're not doing enough yet, but we're at the beginning of the remedial program," says Dick Amper, of the Pine Barrens Society. "As long as there's stuff we can do and make progress on, that's where the energy should be focused."

Earlier this month, the state announced $27 million in grant funding to support 13 essential Long Island drinking water and wastewater infrastructure projects.