Ocean science questions ANSWERED!: post 4 - Submarine groundwater discharge

      

      Q: I have often wondered: if there is "groundwater" beneath the earth's surface, there must also be groundwater beneath the ocean floor too. Is it true? How deep does it go? Is it all salt water? How salty? (Marshall)

A:  Yes, its true!  Well, to a certain point.  When you say, “ocean floor”, think of the shoreline.  That’s where the groundwater is.  There’s a very new, very exciting line of study of groundwater discharge.  Its fascinating, and its possible that there is a significant amount of water exchange between the fresh and saltwater bodies that we really didn’t know about before.  It doesn’t go all that deep – maybe 100 m or so.  And it is certainly not all saltwater – using radon as a tracer, people have shown that there are significant amounts of freshwater seeping out.  However, saltwater can also seep into the freshwater, and there are freshwater aquifers that are often an important source of drinking water for coastal communities, that are becoming contaminated by saltwater intrusions.  This is exacerbated by draining the aquifers:  if you’re pulling water out, laws of physics indicate that new water will fill in, and saltwater may be more readily available, especially at the ocean’s shoreline.  So the water may be saline, fresh, or brackish (usually salt water is defined as having above 20 parts per thousand of salt, whereas freshwater has zero).  Also, in the deep ocean at the hydrothermal vents and spreading centers (the Mid-Ocean Ridges) there is significant water exchange into the rocks and sediment.  This is all saltwater, though.