But the debate in Sacramento feels far away in East Orosi, a farmworker community of about 500 nestled along the foot of the Sierra Nevada that is surrounded by fields of oranges. There, residents complain of conditions that resemble the developing world, not the richest state in the nation. Fears of nitrate exposure in the tap water — which numerous studies have linked to an increased risk of infant death, and at high levels, an elevated risk of cancer in adults — compound other difficult realities like faraway grocery stores and doctors, grueling work conditions, and a lack of political clout.

Veronica Corrales, the president of the East Orosi water board, wonders why more people are not outraged that, in 2019, people living in a state as wealthy as California lack such a fundamental necessity.

“Everyone is saying ‘America First,’ but what about us?” she said.

Many factors have led to the groundwater contamination reflected in the state’s data, but public health experts say the region’s agriculture industry has played an outsize role. Chemical fertilizers and dairy manure seep into the ground and cause nitrate contamination, like the kind plaguing East Orosi. Such contamination, which is common throughout the valley, takes years to materialize and even longer to clear up.

Arsenic is naturally occurring in some areas but can become worse with exhaustive groundwater pumping, which has been a longstanding problem in the valley and accelerated during the drought between 2012 and 2016.

It is exceedingly difficult to say with certainty whether any illness is directly tied to specific environmental factors, including contaminated water. But an article published last month in Environmental Health, an academic journal, estimated that 15,500 cases of cancer in California could occur within 70 years because of unsafe drinking water.

For years, Martha Sanchez and her husband, Jose — who live in East Orosi and make their living filling crates with oranges or picking cherries — have received notices from the local water system that their taps are unsafe to drink from because of contamination. The family spends at least $60 a month for tap water they can’t use, Ms. Sanchez estimates, which is factored into the rent. To cook and wash dishes, Ms. Sanchez ladles bottled water into pots and pans from heavy blue jugs kept in the kitchen. She and her children shower using the water from the pipes, but she says it makes their skin itch.

“Some people around here drink it,” Ms. Sanchez said. “Here at home, I don’t use it at all for cooking, not even for beans.”