23 Groundwater
23.1 Groundwater rise
Pierre-Louise
Unlike rising seas, where the dangers are obvious, groundwater rise has remained under the radar. Hydrologists are aware of the problem and it’s all over the scholarly research, but it has yet to surface in a significant way outside of those bubbles. Groundwater rise is only briefly mentioned in the most recent edition of the National Climate Assessment, released in 2018; it’s absent from many state and regional climate adaptation plans, and even from flood maps.
Any coastal area where “the land is really flat, and the geology is [the kind of] loose material that water moves through really easily,” says Hill, is “where this is really going to be a problem.” This includes places like Miami, but also Oakland, California, and Brooklyn, New York. Silicon Valley communities like Mountain View are susceptible to groundwater rise, as is Washington, DC. Worldwide, the area at risk includes portions of northwestern Europe and coastal areas of the United Kingdom, Africa, South America, and Southeast Asia.
And because of how groundwater moves, people who are at risk may not know it until it’s too late. “One of the most important things about the groundwater is that the rising groundwater level precedes any inundation of the surface,” says Rozell. Put another way, we will experience groundwater flooding long before the ocean comes lapping at our front door.
It might seem puzzling that rising seas could cause groundwater to rise. At first blush the two seem unrelated, but the connection is actually simple. That it has long been ignored reflects our bias toward addressing problems we can easily see.
To understand the link, it first helps to understand a bit about groundwater. The water nestled in sediments underground started as surface water, like rain or snow, and eventually seeped down. A layer of saturated soil rests below a layer of unsaturated soil; the boundary between the two is what’s known as the water table. And in many coastal areas this layer of saturated soil, which can be meters thick, rests atop salt water from the ocean. As sea levels rise, the groundwater gets pushed up because salt water is denser than fresh water.