Groundwater, rising seas, and toxic coastal sites

Groundwater rise will create a toxic soup in coastal cities and ecosystems far sooner than what salt water will do on its own — it could already be happening today.

As sea levels rise, high tides will reach farther and farther inland. We’ve all seen those maps by now — like this one for the Richmond, California shoreline. It’s from the Our Coast, Our Future (OCOF) flood viewer, which uses light blue to show the areas that will flood with about 3 feet of sea level rise and light green to show areas that will flood more often during storms:

Sea level rise of about 3 feet shown in light blue, season flooding shown in green. White circles mark dangerous contaminated sites, in pink if radioactive, and in red if they have federal Superfund status. But the other sites have equally or even more dangerous chemicals, including “forever” chemicals that accumulate in our bodies and the environment, and cancer-causing chemicals like benzene and vinyl chloride.

We’re seeing a lot of maps like this, and headlines about the year 2100. But the really difficult toxicity problem is one that we already face today, and that will worsen significantly in the near future. It’s invisible on these maps of rising sea levels, which is scary because cities and state agencies are using these future flood maps to justify approving new housing projects on top of hazardous waste sites. The problem is visible in this cross-section, drawn as if we’ve sliced through the land and can see what’s inside it:

A rise in sea level of a few inches could allow groundwater to mobilize extremely hazardous chemicals. While the land (and any future houses) on top of the wastes may not get wet, chemicals can now move out of the mound and into people’s homes and businesses, as well as into shallow coastal waters where they could be carried along a much larger stretch of shore.

There’s no easy solution to this immediate and hidden component of the sea level rise problem. It will be invisible to us as we walk around on the surface, but groundwater will rise up from below and carry hazardous chemicals out to the sides of a polluted site, as well as out into the shallow ocean. Caps designed to prevent rain from flowing through the chemicals, keeping them high and dry, will no longer prevent water from reaching these toxins because it is coming from the sides and from below, not from above.

Most discussions of sea level rise focus on the fact that the ocean could be several feet higher thirty, fifty or eighty years from now. But the deadly risk of hazardous wastes escaping capped sites is a game of inches, not just of feet. Which means the timing could be next week or next month, or it could have already begun. Some hazardous sites are not monitored well enough to pick up these changes as they begin to occur. Many are checked twice a year, not necessarily at high tide or after a few rainy days, which is when groundwater will be at its highest level. Some are not even checked at all anymore, because the files have been closed in anticipation that the wastes will eventually decompose.

This map from OCOF uses a US Geological Survey dataset of modeled depth-to-groundwater with about 3 feet of sea level rise, using the assumption that the ground is moderately permeable and the groundwater is sometimes more than 6 feet below the surface. No one has checked yet to see if this assumption is correct.

This OCOF map shows the possible depth to groundwater with about three feet of sea level rise if the soil is more permeable, which would produce higher groundwater. It shows most of the contaminated sites with groundwater sometimes at the surface (red) or just below it, at 0–3 feet deep (orange). Until someone checks this, a prudent planner would assume this is true to avoid creating health hazards by allowing new housing.

If houses are built on top, there will be no opportunity to remove the remaining hazardous chemicals. People and ecosystems surrounding this sites will be at serious risk of harm, in many cases from so-called “forever chemicals” that degrade so slowly that they are essentially around forever, and that accumulate in our bodies and the bodies of other animals. Other chemicals may degrade into even more harmful things — as when the solvent PCE breaks down into vinyl chloride, which is known to cause cancer. The Richmond shoreline has just about every pollutant you can imagine in this groundwater flood zone — including benzene, PCE, toluene, arsenic, hexavalent chromium, radioactivity, and the list goes on.

Rising groundwater will cause a complex set of other problems, including increasing the risk of violent shaking in an earthquake. Along the Richmond shoreline, the US Geological Survey has mapped a suspected fault that is a branch of the big, dangerous Hayward fault — one that is expected to generate a major earthquake soon. Urban fill and old wetland soils, which the Richmond hazardous waste sites sit on top of, can behave like a liquid as the energy of an earthquake wave passes through them. As groundwater rises, the risk of this “liquefaction” impact can increase significantly. Any underground trenches put in place to try to keep the wastes dry could easily fail in an earthquake if the soil liquefies. Buildings built on top would also be likely to fail, tilting or even falling over as they did in the San Francisco marina district during the last big Loma Prieta earthquake in 1989.

Next time you see an important study of how rising seas may affect toxic coastal sites, like this one from the Union of Concerned Scientists (which only considered federal Superfund sites when it came out in 2020), remember — the leaks are happening already, and no one is watching. State and federal agencies need to change their policies to monitor what’s already going wrong, and remove the contaminants before these poisons are swimming in even deeper water, where they can travel farther and faster.