With oil prices rising, interest in ethanol, a plant-based gasoline additive, has skyrocketed. But the rising demand for ethanol has a downside: it takes about 300 million gallons of water to make 100 million gallons of ethanol each year, says the Renewable Fuels Association.

In Illinois, the second largest ethanol producer in the country after Iowa, proposed ethanol plants would use about 2 million gallons of water per day, which has city officials worried.

Illinois is already home to seven ethanol plants, and plans for at least 30 more exist. Iowa has 24 operating ethanol plants. 

Not a Concern on a Statewide Scale

While scientists in Illinois and Iowa have expressed concern about the plants' water demands, others say the amount is not that big when you look at it on a statewide scale. Mid-size municipalities use about 23 million gallons of water a day, and larger cities, like Chicago, use 500 million gallons daily.

For comparison, the Mahomet Aquifer in Illinois, along which several ethanol plants have been proposed, contains 13 trillion gallons of water, which would take over 100 years to run dry.