As subterranean water inches higher, so do threats to air and water.

Oceans do not stop where the sea meets the shore. Along the coasts, saltwater creeps through porous soil and rock, creating an underground saltwater table that can extend miles inland.
Many Americans are familiar with sea-level rise. As we crank up the planet’s thermostat, the melting of glaciers and ice sheets and the thermal expansion of seawater mean the oceans are rising and intruding farther and farther inland — both on top of the land and underneath it.
Few regions expect an inundation from below, explained Kristina Hill, a professor at the University of California, Berkeley, who studies rising groundwater in urban coastal areas. “They think that building a levee is going to protect them from rising seawater. But, of course, a levee doesn’t affect much about the way that groundwater rises behind it.”
One of many concerning possibilities is that rising groundwater will mobilize contaminants that have been lurking in the soil for years, left behind by industrial and military sites, and allow them to spread, unnoticed, beneath our feet. 
This slow-moving crisis is popping up in communities across the U.S., but there are some common steps that can be implemented anywhere to help stem the spread of contaminants through climate-driven groundwater rise. Hill said one of the most important for government agencies and municipalities to take is simply more monitoring — in particular, at “maximum groundwater moments,” such as a few days after a heavy rain or at a high tide. Currently, sampling tends to be so infrequent that it doesn’t catch the movement of the contamination. 
“There are ways that we could be sampling and trying to catch the maximum risk, instead of kind of smoothing it all over with sampling that isn’t related to rain events or tide events,” Hill said. “Ideally, we’d help local people be involved in that sampling so that they know what’s happening in their own neighborhoods.”
Understanding, though, has to be paired with action. Along with taking broader steps to address climate change and its impacts, agencies need to ensure polluters clean up toxic sites, rather than just capping them and hoping for the best. 
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