In the Central Valley, however, what is "reasonable" often clashes starkly with what is. According to the Community Water Center, one in five Tulare County communities is unable to provide clean drinking water on a daily basis. To see the conditions facing those communities and their tens of thousands of inhabitants, I traveled to several small agricultural towns in the county, outside Visalia. In East Orosi, a tiny hamlet of 500 people, residents live in small wood-frame and stucco bungalows, many painted in bright pastel colors reminiscent of a rural Mexican village. My guide, 19-year-old Jessica Sanchez, shows me a recent warning from the East Orosi water district, citing nitrate levels that exceed the state maximum of 45 milligrams per liter. The notices that frequently come in the mail are often obvious facsimiles of previous warnings. "A lot of times, you can see tape marks around the date," says Sanchez.

Sanchez has been active in local water issues since high school, but these days she has a new reason to be concerned -- her 11-month-old son, Jordan, whose stroller she pushes along a trash-strewn gravel shoulder. Sanchez points out an abandoned-looking trailer tagged with graffiti -- the main office of the East Orosi Community Services District. "There's no one there," she says with a laugh. "They hardly ever are."

As in Seville, the East Orosi's Community Services District delivers water to homes with "no method of treatment such as coagulation and flocculation, sedimentation, filtration or disinfection," according to a 2011 Tulare County report on the area's small community water systems. Moreover, its groundwater pumps sit a few yards from an orange orchard -- meaning whatever is applied at the surface can potentially percolate into the shallow groundwater below and into drinking supplies.

Local municipal groundwater pumps are often located beside orchards, alongside agricultural canals and beside sprawling dairies and their huge sewage lagoons. "The Third World conditions of these systems are truly shocking, particularly for a state that is a leader in so many areas of environmental governance," says Richard Frank, an environmental law professor at the University of California, Davis. "It's a striking anomaly."

Farms and dairies are responsible for 96 percent of the nitrates entering groundwater in the Central and Salinas valleys, according to a 2012 UC Davis study. Some 220,000 tons leach into that groundwater every year -- more than four times the "benchmark" level at which nitrogen will not further degrade the region's groundwater. However, since the bulk of it comes not from single point sources, but from application of fertilizers over vast areas, farms are not required to have discharge permits for the large quantities of nitrogen pollution they generate. California's dairies are now required to submit waste and nutrient management plans if they are located in "high risk" areas, over shallow groundwater, say, or near municipal water supplies. But much of the manure and sewage sludge generated by these dairies is destined for fields, potentially jeopardizing the groundwater beneath.

There is mounting evidence that the nitrogen in the groundwater today originated decades ago. Which is to say, the Central Valley's problems stand to get significantly worse. "Even if we got rid of all of the sources tomorrow, it's going to be decades before this mess is cleaned up," says Thomas Harter, a co-author of the UC Davis nitrate report. "To think that this is a problem that we're simply going to be able to remediate away is the wrong path."

In the meantime, Latinos living in the Central Valley are suffering disproportionately from nitrogen contamination, according to a study published in 2011 in the journal Environmental Health Perspectives. That study's lead author, Carolina Balazs, a UC Berkeley researcher, says that previous research on water contamination overlooked socioeconomic and ethnic disparities, assuming that all communities served by small water systems faced similar risk of nitrate contamination. "We found that, yes, small systems do tend to have higher nitrate levels. But it's small systems (serving) high percentages of Latinos that have the highest levels of nitrates," says Balazs. Both economic and social factors may play a role in exposure risk. Data from the 2000 Census show that more than one in four Spanish-speaking families in the Central Valley are "linguistically isolated," meaning that all adults in a household speak a language other than English, and none speaks English very well. Because of this, these families are less able to advocate for themselves and successfully use civic channels available to effect change.

For mothers like Sanchez, nitrates are particularly worrisome since they can cause methemoglobinemia, or "blue baby syndrome," a sometimes-fatal condition in which an infant's red blood cells cannot carry enough oxygen. "I definitely won't use this water to make formula," says Sanchez, looking down at Jordan in his stroller. "But should I even give it to my dogs?"