Sea level rise is typically mentioned in the context of erosion, coastal flooding, storm damage at the coasts. All of those are issues, to be sure. But rising ocean levels can have other effects – on septic systems and drinking water supplies farther inland. Potential impacts on groundwater are the subject of a new reportfrom US Geological Survey’s New England Water Science Center.
Melting glaciers and warming oceans are causing sea levels to rise around the globe. Tides already reach nearly a foot higher than they did a century ago. A century from now, they’ll be at least another one to three feet higher. Many say two to six feet is a more realistic projection. And, as the ocean rises, so, too, does freshwater farther inland.
That's because the ocean doesn't stop at the high tide line. There is a layer of salt water that intrudes well inland, but below ground. Fresh water - both groundwater, and surface waters, like ponds and streams - sit on top of that salt water layer like a floating bowl.
As sea level rises, it pushes both inland and upward on the freshwater aquifer, and the water table rises. That poses a threat to basements and septic systems that sit close to the water table, and could end up flooded by the rising water level. In addition, drinking water wells could become tainted with salt water.
But, it turns out, it's not necessarily a one-to-one relationship. A foot of sea level rise doesn't always translate into the water table rising by a foot. If there is enough stream flow, the rise in freshwater levels can be counteracted. That appears to be the case for most of Cape Cod where, on average, the water table is expected to rise by
That's still nothing to scoff at. There are currently about five thousand septic systems on Cape Cod that sit in areas less than five feet from the top of the water table. If sea level rises by two just feet, that would raise the water table an average of eight inches, and the number of septic systems with less than five feet of clearance would double.