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.
The top 300 feet of the layer cake is where the biggest nitrate problems are found -- the strata most likely to harbor irrigation drainage, runoff from cities and leakage from septic and sewage treatment.
On the Valley floor itself, there are more clues to the nitrate sources in Tulare County. The county has three distinct sections: a citrus belt to the east; row crops, such as alfalfa or corn, in the center; and large dairies to the west and south.
Nitrates could come from fertilizers used on the citrus and the crops. Large dairies also could be a source.
The dairies get a lot of attention from environmental activists. With more than 1 million head, Tulare County has 20% of the dairy cows in California.
Farmers use dairy waste water to grow crops that cattle eat. With animals that void up to 20 times more than a human, the waste quickly adds up.
The state has rules intended to prevent farmers from overloading the soil and crops with the waste.
In addition, Tulare County began regulating the placement of dairies years ahead of most counties in California. Any new dairy in the county must obtain an expensive environmental impact report, and they are not allowed within a half mile of schools, water wells or homes.
But there are only nine state inspectors for 1,400 dairies in the San Joaquin and Sacramento valleys -- more than 30,000 square miles. The inspectors are spread too thin to inspect every dairy each year.
So dairy owners make their own inspections and report to the board annually.
"They're responsible for sampling the water, soil, crops and the wells on their property," said Doug Patteson, a state supervising engineer based in Fresno.
That's not a good idea, says Starmer of Food & Water Watch. The state should be policing dairies, she says.
Starmer this year wrote a report criticizing the Central Valley Regional Water Quality Control Board, saying there are high levels of nitrates around too many dairies, too few state inspectors and almost no penalties for violations.
Other activists say state rules are not enforced well enough. In 2008, they sued the regional water board, hoping to force more monitoring of all dairies. The court rejected the claim, but the activists are appealing the decision.
Activists involved in the case include Asociación de Gente Unida por el Agua, a nonprofit association based in Visalia. They suspect many communities such as Earlimart, Pixley and Poplar have nitrate problems because of the dairy industry.
But experts say dairies are in the wrong place to be harming water quality along the foothills where there are highly publicized problems.
Like rivers, the water beneath the soil moves downhill -- generally southwest through the county, away from the foothills.
"The groundwater around the highest concentration of dairies moves away from those communities along the eastern side of the Valley," said water engineer Dennis Keller, who has worked on these issues for four decades.
So what's the problem along the foothills? According to one theory, fertilizers used decades ago along the county's citrus belt could be inching into water wells in East Orosi, Seville, Tooleville, Tonyville, Lemon Cove, Ivanhoe and many other small towns.
Another theory: Nitrate is a natural element in the soil. It comes from decaying plants. Maybe that is the problem.
Some speculate the problem is a combination of old fertilizers, natural nitrate and septic systems in the area.
These days, farmers do not apply fertilizer as liberally as they did in the past, mostly to reduce costs and boost efficiency. The practice also reduces nitrates in the groundwater.
But with nearly 1 million acres of farming in Tulare County, fertilizers still are considered a prime suspect.
A group of UC Davis professors, led by hydrologist Harter, will try to figure out the extent of nitrate contamination over the next year.
Among the bigger questions: Are the Valley's nitrate problems going to get worse?
Because groundwater moves very slowly, nitrates from fertilizers applied decades ago may increase over the next half century. The leading edge of the plumes only now may be invading underground wells.
Tests of Tulare County water have shown everything from a banned fumigant called DBCP to perchlorate, a thyroid-damaging rocket fuel ingredient that was used in a Chilean fertilizer before 1950.
Tests also reveal bacteria, arsenic, chromium, radium, lead, uranium and alpha particles, a form of radiation from mining and natural sources.
Federal officials are considering adding 123-TCP, another long-banned pesticide, to the list of 80 chemicals that must be monitored nationally. It also is believed to be in Tulare County's water.
Nitrate, however, is the dominant problem.
A state-sponsored study in 2006 showed 70% of the private wells tested in the county have too much nitrate.
Monitoring wells can be placed upstream and downstream of a town's well to figure out where contamination comes from, scientists say. If a nearby septic system is directly upstream, it would become the prime suspect.
Scientist Harter says water scientists use highly specialized chemical analyses to look for fingerprints -- called stable isotopes -- in the nitrates. Such forensic evidence can help them identify the nitrate sources, such as human septic systems, fertilizers or animal waste.
Unfortunately, the stable isotope fingerprints for various sources of nitrates can sometimes be quite similar, making it difficult to be certain of the source without other evidence, Harter says.
"We are looking into fingerprinting," Harter said. "It's an important tool, but it's never going to be an exact science."
Engineer Keller says it's a good idea to figure out the nitrate sources and control them. It's the best way to take care of future problems.
But, he adds, nitrate contamination will persist, no matter what sources are found.
"You could stop all the sources right now and you wouldn't stop the nitrate problem for many years," he said. "The nitrates already down there will continue to slowly move into wells. Controlling the sources is something that will help your grandchildren."