Becky Quintana walks along the gravel shoulder of a rural two-lane road through the sprawling orange groves of California's Central Valley, the snow-white jags of the Sierra Nevada at her back. "On a clear day, it's like you can almost touch the mountains," says the 57-year-old school bus driver, who has lived all her life in Seville, 35 miles south of Fresno. The vast majority of the town's 500 residents are Latinos, and most toil for meager wages in Tulare County's vast nut, olive and citrus orchards.

The nearby Kaweah River, which flows from headwaters in the high peaks of the Sierra, is cool and clean. But most of its flow is diverted into irrigation canals and delivered to a faraway mosaic of farms and cities. In spite of Seville's proximity to the Kaweah, the tiny town's drinking water doesn't come from the river but from wells punched into the intensively cultivated land around town. Quintana points out the array of white tanks and a U-shaped pipe plunging earthward: This, she explains, is where the town's water comes from. As a groundwater activist and founder of a local group called the Committee for a Better Seville, Quintana has worked for several years to improve Seville's primitive water system.

A white PVC pipe runs down the middle of an irrigation canal, which carries three or four inches of water. The pipe -- actually many pipes, loosely connected by plastic couplings -- is the town's water main. Quintana pushes on the rickety assemblage, which creaks and dips below the surface of the canal. She explains that when the canal is full, the pipe is submerged, and when pressure is low (usually in the summer, when people use lots of water), canal water can seep in through loose connections, carrying sand and other debris. A neighbor says a small tadpole once wriggled out of her kitchen tap. In the canal's shallow water, beside the main, the carcass of a dog slumps in a grisly state of putrefaction. "Lots of tourists come through here on their way to Sequoia National Park," Quintana laughs. "They stop to eat in the café. I bet they wouldn't, if they knew what was in the water."

The most harmful ingredients can't be seen. The groundwater underlying Seville, like that beneath dozens of small towns throughout the Central Valley -- the 50-by-400-mile agricultural basin, home to 4 million people, that effectively separates coastal California from the Sierra Nevada -- has long borne the brunt of the region's industrial-scale agriculture and the industrial-scale pollution that comes with it.

While dozens of contaminants, both manmade and natural, have been detected in the region's groundwater, nitrates are the pollutant of greatest concern. Derived from hundreds of thousands of tons of synthetic fertilizer and animal wastes applied to crops each year, nitrates pose an especially acute risk to infants; long-term exposure has also been implicated in various forms of cancer, including gastric, esophageal, ovarian and non-Hodgkins lymphoma. According to a recent University of California, Berkeley report, nitrate exposure's health impacts fall disproportionately on the poor Latino communities of the Central Valley -- the same people who make up most of the low-wage workforce of the agriculture industry itself.

Nitrates and other contaminants are less of an issue in larger, wealthier communities, since treatment or blending with cleaner water can often reduce concentrations to meet health standards. By contrast, the small Latino communities of the Central Valley -- where median household incomes are under $15,000 per year -- simply do not have the tax base to support the construction and operation of treatment plants, or to secure alternative sources of water.

The struggles of these poor communities hint at much larger problems. Unlike every other state in the Western U.S., California does not regulate the quantity of groundwater pumped, although more than eight in 10 of the state's residents rely on groundwater for at least a portion of their water supply. A report released in February by the State Water Resources Control Board identified 31 principal contaminants, including arsenic, uranium, perchlorate and pesticide residues, in the groundwater serving 21 million Californians.

As the state's population grows and its complex water systems are further racked by climate change -- with Sierra snowpack expected to dwindle by as much as a quarter by mid-century -- residents across all income levels will become more and more dependent on increasingly scarce and polluted groundwater. And many already drink water that's less than clean. "As many as 8.5 million Californians rely on supplies that experienced more than five incidences of excessive levels of contaminants in the drinking water in a single year," former Assemblyman Mike Eng, from Los Angeles, testified before the California Senate Committee on Natural Resources and Water.

In response, last October, Gov. Jerry Brown signed into law AB 685, the "Human Right to Water" bill. The 250-plus-word addendum to the state water code is ambitiously phrased, declaring, "Every human being has the right to safe, clean, affordable, and accessible water adequate for human consumption, cooking, and sanitary purposes."

The bill, which reaffirms the larger goals of the 1974 Safe Drinking Water Act, is one of the first clear victories in acknowledging the unequal burden of water contamination in California. It's the product of an aggressive seven-year-long grassroots-inspired legal campaign focused on the Central Valley. But successfully turning the bill's fine words into reality won't be easy. The effort to secure clean drinking water in the Central Valley requires reversing a century's worth of pollution, and it will be a slow, expensive process -- entailing reform of one of California's most powerful industries, which has transformed the valley into one of the planet's most heavily engineered and industrialized landscapes. In the meantime, says Laurel Firestone of the Community Water Center, the effort to bring clean water to places like Seville will require determination, creativity -- and a recognition of the problem's multiple facets. "On one hand, the solution is complicated, and on the other it's not," Firestone says. "We need to look at what our priorities are as a state and what we are using our resources on -- it's pretty obvious it hasn't been on bringing safe drinking water to places like Seville. … All of us have to play a part in creating that solution."