There is concentrated bittern left over from the salt-making process, which they argue could present a threat to nearby wildlife, in the case of a release.
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"The agencies need to go out and inspect these ponds and make sure we're not on the verge of a catastrophic toxic spill into San Francisco Bay," said David Lewis, executive director of Save the Bay.
The group recently co-authored a letter to the San Francisco Bay Conservation and Development Commission, questioning whether the agency is effectively monitoring the ponds and surrounding levees that Cargill maintains. Among their concerns: threats to the levees from sea level rise, king tides, or a strong earthquake -- and the potential for a spill or seepage to reach the nearby Don.
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"There are toxic sites all around the edge of the bay that are much more heavily regulated. It's baffling that the agencies haven't taken a stronger stand about protecting the bay from this toxic material," Lewis said.
Those concerns come as Cargill is rolling out a major proposal that could transform the way the company stores and process the bittern, also known as mixed sea salts. It is a plan that would ultimately dispose of them back into the bay.
Don Brown is Cargill's land resources manager. He says the company wants to build a multimillion-dollar pipeline project designed to be a long-term solution.
"We understand that sea level rise and climate change is coming. And we realize the time to act is now. So we're taking a proactive approach to protect the bay by doing the enhanced processing of these mixed sea salts," Brown said.
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The pipeline would run some 16 miles connecting Cargill's ponds near Newark with the wastewater treatment system run by the East Bay Dischargers Authority. Once the brine is mixed with treated wastewater and diluted, it would be piped to this facility in San Leandro and released back into the bay.
General Manager Jackie Zipkin says the project would be one of the largest additions to the East Bay wastewater system in years. And she believes the dilution process is a safe way to dispose of the bittern.
"We consider it our mission to protect the bay. That's our primary focus. And so we wouldn't take in anything that we think would compromise that mission. So we've done a lot of water quality testing to demonstrate that the combined effluent and brine from Cargill would be safe to the bay, it meets all of our permit limits," says Zipkin.
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The environmental impact report is still under consideration. If approved, the pipeline would be owned and built by Cargill. Eileen White is an executive director with the San Francisco Regional Water Quality Board. While acknowledging her agency hasn't inspected the two ponds specifically, she says Cargill has provided a recent inspection report including aerial photography of the site. She also supports the pipeline plan.
"So it's kind of returning it back. It came from the bay, you're returning it back to the bay, but just not at such a high concentration, that it could be toxic to the environment," says White.
Still, groups like Save the Bay argue it could be years before construction is completed. And they point to recent levee failures like the breach in Monterey County as an example of the heightened risk. And in the meantime, they're pushing for increased oversite by agencies like the Bay Conservation and Development Commission and the Regional Water Quality Control Board.
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"There's a lot of letters going back and forth between agencies and the company, that's not adequate. They need to go out and inspect this site and see how close the material is to the top of the ponds," Lewis maintains.
An environmental balancing act in a corner of San Francisco Bay that's both the center of a thriving industry and home to a fragile and majestic wildlife refuge.