Longmont, Colo.

IMAGINE you are a farmer in the center of the country, where it seldom rains enough. Now imagine that a well driller came to your farm and told you that he could bore a hole deep into the ground, and that forever after you could pump out as much water as you needed to grow your crops. That is exactly what happened on the Great Plains in the mid-20th century. The wondrous resource containing all that water was the Ogallala Aquifer.

The Ogallala underlies portions of eight large states — 174,000 square miles of crop and range land all the way from South Dakota to Texas. Over the last several months, it became about as famous as a geologic formation can get. With the nation’s environmentalists at their side, Nebraska landowners battled ferociously against the Keystone XL pipeline, which would have carried oil extracted from Canadian tar sands through the environmentally sensitive Nebraska Sand Hills. If the pipeline leaked, they argued, chemicals and oil would seep down into the aquifer, contaminating a precious resource responsible for 27 percent of the nation’s irrigated agriculture.

They won — for now. President Obama agreed last month to reconsider the pipeline’s route. So is the Ogallala now safe? Not quite. Regardless of whether the pipeline was a good or bad idea, it was never the real danger. The true threat is posed by agriculture as it’s currently practiced on the Great Plains by the farmers themselves, many of whom opposed the pipeline vehemently. The aquifer is being wasted and polluted. Wasted, that is, on corn, a thirsty crop that requires over 20 inches of irrigation water in parts of the Plains. And polluted with pesticides and nitrogen fertilizers.

It’s not that I don’t empathize with the Nebraska farmers. I grew up on a Kansas farm. Like them, we called our Ogallala water “precious” and bragged that it was the best in the world. But the aquifer’s only natural recharge comes from rain and snow. In our Kansas district, less than half an inch of that reached the aquifer in a given year. We were allowed to pump out over 30 times that amount.