OROSI -- 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's 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, who was once a researcher at UC Kearney Agriculture Center in Parlier, says he doesn't expect to find any mystery sources. The nitrates appear to come from farm fields, he said.

"The largest source of nitrate in groundwater in this region is fertilizer and animal manure," he said. "And, scattered across the Valley, there is septic leachate and municipal sources, such as sewage treatment, which locally affect groundwater."

Farmers may attribute the nitrate levels to the latter, even to a greater degree than fertilizer and manure. They urge scientists to take a closer look at those septic systems and sewage treatment plants filled with nitrates from human waste. Septic systems around rural towns are much closer to the drinking-water wells than most agriculture, they say.

Tulare County Supervisor Allen Ishida, a long-time citrus grower near Lindsay, says the farming industry is blamed without any proof. His citrus is along the east side of the county, and he welcomes a full investigation of the nitrate sources from all of agriculture.

"If it's my fertilizers that are causing this problem, then we'll adapt," he said. "We can put spray fertilizers on the trees, rather than putting it on the ground. But let's find out what the source is, instead of blaming us farmers or dairies."

The layer cake below

Beneath the Valley floor, there's a layer cake of sediments, spread one atop the other over time as rivers and streams have carried gravel, sand and mud from the mountains.

Geologically, the Valley is like a monstrous bathtub where these layers have been building for tens of millions of years.

The soil and sediments beneath your feet might be several miles deep, depending on where you're standing.

Water soaks into the layers over centuries. One well might be pumping water that seeped into the ground during the Civil War. Deeper in the ground, another well might be tapping water from the time of Christopher Columbus.