IF THE United States is going to curb its greenhouse gas emissions, it desperately needs a replacement for the high-carbon coal that fuels almost half the nation’s electricity. Unfortunately, there are downsides to all the alternatives, from nuclear power, which carries a high cost and emits toxic waste with no place to store it, to wind turbines, which also have a high cost and require extensive transmission lines to link windy areas with cities.

Now new deposits of natural gas previously locked in shale formations are making that fuel look like a possible transition to a low-carbon future. Federal and state regulators have to ensure, however, that the rush to exploit this new source of gas does not cause severe environmental damage. The US Environmental Protection Agency could have been an effective referee over this process. Yet the gas industry managed to slip into the 2005 energy bill an exemption from EPA review of the special drilling that shale formations require. Congress should repeal that provision.

Natural gas emits half the carbon dioxide of coal, per unit of energy. Until recently, limited supplies of gas kept its price relatively high. But in the past decade gas drillers have learned to tap into deposits of shale gas that are abundant in areas ranging from New York to Texas.

Back in 1990, gas from “unconventional’’ sources like shale made up just 10 percent of total US production. Today it’s 40 percent and rising rapidly, with shale gas leading the way. Proven reserves have expanded so dramatically that the United States is now estimated to have at least a 90-year supply of the fuel.

The catch is to ensure that producing the gas from shale does not pollute water supplies, either at the surface or underground. To free the gas from the rock, drillers blast it with a mixture of water, sand, and toxic chemicals. The industry says this hydraulic fracturing or “fracking’’ occurs at levels too far underground to affect aquifers, but critics say that the drilling process itself and surface discharges have contaminated many water supplies. There is also concern that drilling equipment can introduce harmful invasive species to new bodies of water.

Increasingly, gas producers will be fracking in populated states like Pennsylvania and New York. Residents of those states should be able to rely on the EPA and not just state regulators to ensure that their wells and reservoirs are not poisoned by the new drilling operations - however beneficial they are to the nation’s effort to reduce its greenhouse gas emissions.

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