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