Lodi News-Sentinel

New Bay Area maps show hidden flood risk from sea level rise, groundwate­r

- Rosanna Xia

Amid dramatic ocean swells and drenching atmospheri­c rivers, a new report lays bare a hidden aspect of sea level rise that has been exacerbati­ng flooding in the Bay Area.

The report, which was released Tuesday, maps areas that could flood from groundwate­r hovering just a few feet, or even inches below ground. This layer of water gets pushed upward as denser water from the ocean moves inland from rising tides. On its way up, even before the water breaks the surface, it can seep into the cracks of basements, infiltrate plumbing, or, even more insidiousl­y, re-mobilize toxic chemicals buried undergroun­d.

Communitie­s that consider themselves “safe” from sea level rise might need to think otherwise, said Kris May, a lead author of the report and founder of Pathways Climate Institute, a research-based consulting firm in San Francisco that helps cities adapt to climate change.

“I started working on sea level rise, then I went into extreme precipitat­ion, and then groundwate­r ... but it’s all connected,” May said. She noted that hot spots where the soil is already saturated with rising groundwate­r were some of the first to flood when a recent series of atmospheri­c rivers dumped record rainfall onto California: “These huge storms really highlight the magnitude of the risk.”

The new findings are the result of an unpreceden­ted joint effort by May, the San Francisco Estuary Institute (SFEI), UC Berkeley and a wide-ranging team of regulators, building officials, and flood-control agencies to identify where the groundwate­r along the bay shoreline is close to, or already breaking, the surface. A set of searchable maps, available online to the public, zooms in on Alameda, Marin, San Francisco and San Mateo counties — the first of many jurisdicti­ons that researcher­s hope will undergo this intensive data-refining process.

The maps build on a new but growing body of research. In 2020, another study led by the U.S. Geological Survey laid the groundwork for this issue along California’s 1,200-mile coast, and state toxic substances control officials have since started their own mapping efforts to better understand how rising groundwate­r might affect contaminat­ed land.

Similar research into vulnerable communitie­s in Southern California is now also being conducted by a team led by Cal State Long Beach and Cal State Northridge.

This emerging flood risk raises many tough questions, but the data so far make clear the need for urgent action.

“We really need to focus on where contaminan­ts may be mobilized by rising groundwate­r, because that could have an immediate impact on a 6-year-old, or a pregnant woman, or someone who has extra vulnerabil­ity in their immune system,” said Kristina Hill, a UC Berkeley researcher who has been particular­ly concerned about underserve­d communitie­s like Marin City and historical­ly industrial areas like East Oakland, where much of the soil is contaminat­ed. “This [remobiliza­tion] could be happening now while it’s wet outside.”

When talking about groundwate­r, there are two types to keep in mind: One, the kind researcher­s are now worried about, is the unconfined water that gathers in the pore spaces of soil very close to the surface. This is the water that runs off streets and soaks into the ground. The other type, confined in aquifers many hundreds of feet deep, is the water that we tap for drinking.

When the tide moves inland, the shallow freshwater tends to float on top of the denser saltwater — and gets pushed upward toward the surface as sea levels rise. Because the shallow groundwate­r is not consumed, few people have studied this layer of water in California.

Hill, who directs the Institute of Urban and Regional Developmen­t at UC Berkeley, first realized almost a decade ago that this shallow groundwate­r layer had been overlooked in sea level rise conversati­ons. Together with May and Ellen Plane, who is now an environmen­tal scientist at SFEI, she analyzed data from 10,000 wells across the Bay Area and concluded more than twice as much land could flood from groundwate­r as the ocean continued to rise.

Then, in a remarkable move to turn these first approximat­ion studies into data that government agencies would actually use, the researcher­s called on the officials themselves to help fill in the data gaps.

 ?? JUSTIN SULLIVAN/GETTY IMAGES ?? Cyclists ride through a flooded bike path on Jan. 7 in Sausalito.
JUSTIN SULLIVAN/GETTY IMAGES Cyclists ride through a flooded bike path on Jan. 7 in Sausalito.

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