From the Sacramento Bee:

On a crisp winter day, look east from Orosi for a world-class view of the snow-capped Sierra Nevada and California’s purest water on ice.

The snow melts, rushes down through granite canyons to reservoirs and eventually turns farmlands green in the southern San Joaquin Valley.

But somewhere between the glistening snowpack and the verdant countryside, a dangerous change takes place. The underground water becomes tainted with chemicals called nitrates. And the contamination winds up in tap water.

The county’s $4 billion farming industry is the prime suspect. And, since Tulare County is the biggest dairy county in the nation, cows and their prodigious waste often get most of the blame.

The animals create more waste than all the people in Los Angeles, says Elanor Starmer, San Francisco-based regional director of the nonprofit advocacy group Food & Water Watch. None of this nitrate-laden dairy waste is treated.

“I don’t believe there is any way to manage that much waste,” she said “It’s pretty obvious.”

But scientists haven’t conclusively shown that it’s the main source of the problem. A leading ground-water scientist, Thomas Harter of the University of California at Davis, suspects farm fertilizers, which have been applied for more than six decades. He is studying the sources of nitrates in the Valley.

He has heard the theories, like the one that says nitrates from decomposing trees and brush come streaming out of the Sierra Nevada. Another one says nitrates are carried in an upwelling of water deep beneath the Valley floor.

Harter says he doesn’t expect to find any mystery sources. The nitrates appear to come from farm fields, he said.

Although they also come from human sewage, “The largest source of nitrate in groundwater in this region is fertilizer and animal manure,” he said.

Not so fast, say farmers. Septic systems around rural towns are much closer to the drinking-water wells than most agriculture, they say.