The environmentally perfect fuel choice for our cars is absurdly impractical for most of us: it's the choice to stop driving a car. Most cars run on gasoline made from crude oil, the booty from serial wars in the Middle East. For all the costs, hidden and not, it's hard to feel good about gassing up an auto these days.

While somebody out there is thinking up a fuel solution superior to everything known—something smarter, cheaper, friendlier and more sustainable than fossil fuels—we have a few imperfect options to consider as we wait for that somebody's phone call.

I've nicknamed these equal-of-two-evils Sophie's Choices because all of them do some harm. (Until investing two hours and a box of Kleenex as the rest of us have, those who missed Meryl Streep and Kevin Kline in the 1982 film Sophie's Choice will not understand the concept of a truly difficult choice.) One option is fueling cars with compressed natural gas.

Natural gas is touted as a transition fuel, which means it's better than gasoline for a number of reasons. For example, it's already here in this country. It's plentiful. It's cheaper. And it produces 25 percent less carbon than what's produced by a car running on gasoline. A school district in Oklahoma allegedly saved so much money by using compressed natural gas in school buses that it was able to retain 14 teaching positions. Imagine that—use compressed natural gas, save a teacher. It sounds so win-win, who wouldn't want to save education, cut down on fuel costs and reduce carbon? But wait. Here comes a Sophie: choosing this manner of carbon reduction and cash savings can also mean that we are choosing to contaminate drinking water supplies across the country.

Natural gas may occur naturally, but getting it out of the ground is a fracking ordeal—"fracking" being the industry slang for "hydraulic fracturing." A fossil fuel, natural gas is deposited in shale. Using a mining technique known as horizontal drilling, gas companies drill vertically into the ground for a mile and then make a 90-degree turn and start going at it horizontally. To release the gas from the shale in this sideways direction, they inject chemicals in a high-power blast that fractures the rocks. Et voilà. Gas is gotten. But in the process, the groundwater passing adjacent to the drilling can be contaminated by the chemicals used to fracture the shale.

Among those saying "not so fast" to compressed natural gas mining are concerned citizens, including those living closest to the drilling operations. In a letter to the Environmental Protection Agency this month, groups including the Natural Resources Defense Council and the Sierra Club expressed concern over allegedly illegal fracking practices and asked the EPA to enforce the Safe Drinking Water Act to protect underground water sources.

Under the Bush administration, hydraulic mining was exempted from the Safe Drinking Water Act unless diesel fuel was involved. Diesel contains benzene, toluene, ethylbenzene and xylene, which poison water even in very low levels; the amount used in mining releases these toxins at levels exceeding what has been established as safe.

The 31 organizations that signed the letter—citizens groups spanning the continent from California to Delaware—point out that "fracturing companies B.J. Services Company and Halliburton injected diesel in hydraulic fracturing operations in 2005, 2006 and 2007 in as many as 15 different states." Since gas is mined in 30 states, the groups suggest that drilling companies be required to disclose to citizens exactly what kinds and amounts of chemicals are being injected in and around drinking water supplies. Their goal is to stop use of diesel injections and any other chemical contaminating drinking water.

Natural gas may in fact be a good transition fuel in cars and as a power-plant replacement for coal. Last year, NPR reported that enough natural gas lies buried between New York and West Virginia to equal 80 billion barrels of oil. And that's just one deposit of many. If the EPA jumps into action on this, maybe getting natural gas on line won't be a choice between poisoning our air and poisoning our drinking water.

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