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Pond 7 of the old Cargill Salt Co. plant between Napa and Sonoma is an improbable sight: a lifeless salt flat spreading across more than 300 acres along the edge of the San Pablo Bay.

The surface is a twisted mass of filthy salt crystals, devoid of plants and avoided by the migratory birds that inhabit nearby marshes.

As long as the pond remains in this state, officials say, it poses a threat to the ecologically sensitive bay: should a rain storm flood the pond and breach the dirt banks, it could wash salt into the open water in concentrations high enough to kill fish and other wildlife.

But finally, two decades after the salt pond was abandoned, there is hope for safely returning it to productive use.

The Sonoma County Water Agency is in the final days of building a $10 million, 3.4-mile pipeline to bring recycled water from the nearby Sonoma Valley sewage treatment plant to the former salt plant, now owned by the state and known as the Napa-Sonoma Salt Marsh.

The agency plans to hold a ceremony Friday to mark the completion of the pipeline, including elected officials and representatives from the local, state, and federal agencies involved in the long-term salt marsh restoration effort.

By this fall, the agency hopes to use the water to help dilute the excessively salty water in a nearby pond, known as Pond 7a. By sometime next year, depending on when the Army Corps of Engineers can complete a separate portion of the project, the agency plans to use the water to dilute the salt pan in Pond 7, releasing it little by little into the bay in safe doses.

The restoration project may take as long as 10 years because of the need to dilute and discharge the salt residue, known as "bittern," slowly and carefully.

The project to restore Pond 7 and several other parts of the old salt plant has been in the making more than 15 years. It has involved a variety of agencies, including the state Department of Fish and Wildlife, which owns the land; the Army Corps of Engineers, which is doing much of the infrastructure work; and the state Coastal Conservancy and federal Bureau of Reclamation, which are providing money and technical expertise.

"We underestimated how long it would take to do this and how many agencies, how many partners" it would require, said Grant Davis, general manager of the water agency, as he stood last week at the end of the recycled water pipeline, next to Pond 7.

About $7.5 million of the pipeline cost was paid by the Sonoma Valley County Sanitation District, which is operated by the water agency. The rest was paid by various state and federal grants, including money from the Coastal Conservancy and Bureau of Reclamation.

If this pipeline "didn't bring fresh water down," Davis said, gesturing at the moonscape of Pond 7, "this would have been left here as a liability for a long time."

Pond 7 is one of a series of ponds along the edge of the bay that have been restored over the past decade, perhaps the largest wetland restoration effort in the state. The ponds were created by farmers in the 1850s for cattle and to irrigate crops.

The Cargill company began using the area for salt production in the 1950s, sucking in sea water and cycling it through the ponds, allowing evaporation to concentrate the salt in the water before the company extracted sodium chloride, common table salt. The leftovers, various other unusable salts and minerals, were dumped in Pond 7, where it remains as a sludgy brine during the rainy season and dries to a hard salt crust in the summer.

The state bought the salt plant in 1994. Together with some neighboring parcels, that created an area of almost 10,000 acres along the shore of the bay at the mouth of the Napa River.

The next step will for the Corps of Engineers to reconfigure a small neighboring pond where the fresh water from the sewer plant can mix with salty sludge from Pond 7 so it can be safely released into the lower Napa River and on to the bay.

When this final step is completed, about half of the 10,000 acres will have returned to something like the natural tidal marshes that used to line the bay in the area. The other half, including ponds 7 and 7a, will be maintained as standing ponds to simulate habitat lost to development in other parts of the San Pablo and San Francisco bays, said Amy Hutzel, program manager for the state Coastal Conservancy.

The entire project, including buying the land, performing extensive environmental and engineering studies, rehabbing the ponds and building the new pipeline, will end up costing about $60 million.

Conservation officials say that cost is justified by the pressing need to restore the wetlands around the bay, up to 90 percent of which have been damaged or destroyed by development since the Gold Rush. Those wetlands provide habitat for birds, fish and other creatures, many of them on the state and federal lists of endangered species. They also provide protection against storms and sea-level rise, which is a danger in the face of climate change.

Regional governments have set a goal of restoring about 100,000 acres of wetland habitat, making the Napa-Sonoma Salt Marsh an unusual opportunity to make a giant stride toward the goal in one location.

For the water agency, however, there is an additional benefit that justifies the cost of the pipeline, Davis said.

Studies over recent decades have shown that the water table in the Sonoma area is under enormous pressure from both development and agriculture. Not only does that mean potential shortages of drinking water, but a sinking water table could suck salt water from the bay into wells, ruining the quality of what fresh water remains.

The new pipeline will help take some pressure off the groundwater, he said, since it extends the sanitation's district network for distributing recycled water to vineyards and other agricultural operations throughout the region, businesses that now use well water.

That water comes from the processing of sewage produced in the Sonoma area, but it is fully treated and decontaminated. It is safe for a wide variety of uses, including watering lawns and crops, but without a pipeline to deliver it to users, the sanitation district ends up discharging much of it into the San Pablo Bay, in effect wasting good water.

Users of the recycled water will pay 91 cents for every 1,000 gallons during the dry summer season, and 23 cents during the rainy season.

The first priority for the new pipeline will be the salt pond restoration, Davis said, but after that is complete, all the recycled water will be available for agricultural users in the area. That will help the sanitation district meet its goal of recycling all of its water, discharging none at all directly into the bay.

And for farmers, he said, it will help wean them off their groundwater wells.

"They need a reliable supply of clean water in perpetuity," he said.

You can reach Staff Writer Sean Scully at 521-5313 or sean.scully@pressdemocrat.com.