HAYWARD — When the state and federal government paid Cargill Salt $100 million for more than 15,000 acres of industrial salt ponds ringing San Francisco Bay in 2003, the purchase was heralded as a way to bring back ducks, fish and other wildlife by restoring long-lost natural marshes.

Now, another species is also coming back: people.

On Friday, officials are scheduled to open 3.8 miles of new hiking and bicycling trails, a kayak launch ramp and interpretive signs at Eden Landing Ecological Reserve along the Hayward shoreline. The milestone, in an area that had been in industrial salt production since 1872, represents the largest public access project anywhere in the former salt pond areas since the purchase 13 years ago.

“We’re not just trying to bring the bay back to the people. We’re trying to bring the people back to the bay,” said John Bourgeois, ﻿project manager with the California Coastal Conservancy, a state agency overseeing the South Bay Salt Restoration Project.

Work on the area, located half a mile south of the San Mateo Bridge, began three years ago. Two large areas known as ponds E12 and E13 — once white, dry, salt-encrusted moonscapes for much of the year — have been restored as habitat for a wide variety of shorebirds, including avocets, stilts and endangered snowy plovers.

This week, stiff breezes sent ripples through pickleweed, gum plant and yellow mustard flanking the levee trails. Herons and egrets poked in the newly restored ponds under a blue sky filled with puffy clouds. At the levees’ farthest edges, where the formerly closed area had been opened to San Francisco Bay a few years ago, a harbor seal poked its head out of the water in a habitat now rich with bat rays, leopard sharks and cormorants.

“It was a bathtub before — walled off,” Bourgeois said. “Now it’s an extension of the bay. We’re restoring the natural processes, and the species are responding.”

The $8 million project, funded with state park and water bond money, also highlights history.

The old Oliver Saltworks operated from 1872 to 1931. Now visitors will be able to walk along a boardwalk to see the pilings and brick foundations of old wharves, warehouses and other buildings where Japanese laborers 100 years ago produced salt with wheelbarrows and shovels. They then placed the salt on short-draft schooners, which headed to and from San Francisco.

“Until now, people have been able to walk along the shoreline, but not get this far in toward the bay,” said John Krause, a biologist with the state Department of Fish and Wildlife, which manages the Eden Landing site.

The Cargill purchase, negotiated by U.S. Sen. Dianne Feinstein, set in motion one of the largest wetland restoration projects ever attempted in the United States — a 50-year journey to undo much of the disruption to the bay’s shoreline from Hayward to Alviso to Redwood City over many generations.

Since the Gold Rush of 1849, San Francisco Bay has shrunk by a third, as people diked, dredged and filled its waters to create hay fields, industrial salt evaporation ponds, housing subdivisions like Foster City, even airport runways. From 1800 to 1988, the bay lost 79 percent of its tidal wetlands, going from 190,000 acres to about 40,000 acres.

The rampant filling largely stopped in the 1970s, with the advent of modern environmental laws such as the federal Clean Water Act. Over the past 25 years, environmental groups and government agencies have been restoring wetlands around the bay, slowly pushing it back into its historic footprint. The largest such effort is the South Bay salt ponds work.

When the newly restored Eden Landing area opens to the public at noon Friday after a dedication with elected officials, biologists and others, it will represent a new chapter in the salt ponds saga. The project’s completion heralds the end of phase one of the work. In that phase, 3,750 acres of salt ponds were restored for wildlife in Alviso, southern Alameda County and parts of San Mateo County, like the Ravenswood area near Menlo Park.

Total cost: $95 million.

A study released in October by researchers with the U.S. Geological Survey and other agencies found that populations of ducks and shorebirds have roughly doubled in the salt ponds area, going from 100,000 in 2002 to 200,000 today.

“The ponds that have been restored have succeeded beyond people’s hopes,” said David Lewis, executive director of Save the Bay, an Oakland-based environmental group. “The wildlife and vegetation have come back faster and in bigger numbers than anyone expected. The challenge is to accelerate the pace.”

The second phase of the work will involve another 3,000 acres, much of it around Alviso, Mountain View, Menlo Park and Hayward. Those areas, where work is expected to begin after 2018, are more difficult to restore, largely because flood control issues must be addressed.

It will cost an estimated $1 billion to finish the entire salt ponds restoration job over the next 40 years. Environmentalists and business groups have put a $12 annual parcel tax, Measure AA, on the June 7 ballot in all nine Bay Area counties. The measure, which will require an overall two-thirds majority to pass, would raise $500 million toward wetlands restoration and flood control in the bay over the next 20 years.

“We need a lot more of this in a busy urban area,” Lewis said of the Eden Landing site. “It’s premier open space. The public needs to be able to access parts of these areas to appreciate the wildlife and open space in our midst.”

Paul Rogers covers resources and environmental issues. Contact him at 408-920-5045. Follow him at Twitter.com/PaulRogersSJMN