This is part two of a three-part series supported by the USC Annenberg Center for Health Journalism’s 2025 California Health Equity Fellowship. Find part one here.
The Salinas Valley produces more than half of the country’s lettuce. In one of the many fields, Huntington Farms pest control and crop advisor Mark Mason points to a large hose attached to metal cylinders.
“We have a filter station set up here where we can track irrigation volume, pressures, flow rate,” he says.
Huntington farms covers about 5,000 acres in the Salinas Valley. It’s all conventional rather than organic, meaning they use synthetic pesticides and fertilizers.
Fertilizers add nutrients to the soil that plants need. Nitrogen is one of the most important.
“Plants use nitrogen, just like we do, to build proteins,” says University of California Cooperative Extension water advisor Michael Cahn.
The leafy greens and other produce grown in the Salinas Valley need lots of fertilizer, but that demand plus the fact that most of these crops have shallow roots, means it’s easy for extra nitrogen to get into the groundwater here. It dissolves in water and sinks below the roots, eventually reaching the aquifer.
And once it’s there, nitrate—which is the form of nitrogen most fertilizers take—is hard to remove.
“It's just like a salt,” says Cahn. “It is not easy to get salts out of the water once it's in. So you'd have to use something like desalinization, reverse osmosis…”
That’s part of the challenge for the Central Coast, where over 14,000 people rely on water with dangerous levels of nitrates that can elevate risks of cancers, thyroid problems and blue baby syndrome.
Cahn helps growers like Huntington Farms manage their water and reduce the amount of nitrates leaving their fields.
“One of areas we've done a lot of research in is understanding exactly how much do these crops need in terms of nitrogen fertilizer and their uptake rates,” Cahn says.
Limiting the amount of fertilizer to only what plants need, and irrigating more efficiently, helps. Cover crops - which are grown in the off season and meant to improve soil health, can also help.
Hannah Waterhouse is an assistant professor of agroecology at the University of California Santa Cruz. At a test field on campus, she’s researching how cover crops benefit soil moisture and water quality.
“They can potentially reduce percolation to groundwater. We're looking at that,” she says. “They can reduce runoff—so, keeping that water in the field and helping that soil really soak up those winter rains.”
Waterhouse says cover cropping is more common in other places.
“About 5% of the region is cover cropped, which is pretty low,” she says. “Maryland, for example, has like 30% cover crop. So there's room for improvement there.”
Waterhouse is now on a California State Water Resources Control Board expert panel that’s trying to determine whether there’s enough data to regulate the amount of fertilizer applied to fields.
The Central Coast Regional Water Quality Control Board tried to set nitrate limits in 2021, but after pushback from growers, the state board blocked the regulation, citing a need for more information.
This is the second expert panel to review the issue. At the most recent meeting in August, Santa Clara University professor of water resources Iris Stewart-Frey told the panel there are decades of scientific studies that support the need for change.
“Arguing that there is not enough evidence to develop limits on nitrate applications and discharge rates is akin to the tobacco industry actively resisting regulations for decades,” she said.
Daniel Rath, a soil scientist with the Natural Resources Defense Council, pointed to nitrogen fertilizer limits set by the European Union in the 1990s and New Zealand in 2023.
“We see that N (nitrogen) limits generally are associated with improved EU surface and groundwater quality, particularly in the regions with stronger regulatory approaches,” he said.
In some areas, Rath told the panel regulations have decreased nitrates in groundwater by as much as 60%. But it’s taken 30 years.
Nitrates often take decades to sink into aquifers, so even if limits in the Salinas Valley were set now, communities would still have contaminated water for years.
But people are feeling the health effects now. In response, the central coast water board is making plans to provide alternative water to more than 14,000 people for the next 10 years.
More on that in part three.
