Hari Sreenivasan:

Between 2013 and 2015, the EPA required every large water provider in the US to test for 1,4-dioxane. That's the first step in whether or not a contaminant will be regulated under the Safe Drinking Water Act. Results showed that Long Island was a hot spot.

More than 70 percent of water authorities here had levels of 1,4-dioxane above .35 parts per billion. That's the level that the EPA calculates poses a lifetime, one-in-a-million increased chance of developing cancer.

That may not sound like that high a risk level, but many sites here on Long Island had levels way above the ones associated with that long-term cancer risk. In fact, officials here in Hicksville had to shut down this well in 2015 because it was found to have levels of 1,4-dioxane that were the highest in the country, amounts that were nearly a 100 times higher than that one-in-a-million risk level.

But 1,4-dioxane's presence in drinking water doesn't mean it'll necessarily be regulated at the federal level. the EPA says it will not make any final determination until at least 2021.