Yesterday I visited three sites on the south shore here on Oʻahu in search of submarine groundwater discharge (SGD) seeps. As you can see from the video, my search was successful! These seeps occur at the interface between subterranean estuaries and coastal marine waters. The basaltic islands of Hawaiʻi have three very important traits when it comes to groundwater: 1). most basalt is porous and able to hold water, however, 2). the different lava flows are very heterogeneous in their composition and porosity (their compositions and ability to hold water differ widely), and, in most places, 3). a less permeable caprock prevents water from flowing out from the island surface. The freshwater lens that occurs in volcanic islands extends below sea level, due to the buoyancy of the fresh water. The fresh water floats on top of the sea water, while gravity pushes the fresh water downwards, causing the fresh water to push the sea water down as it extends below sea level. Where permeable layers of rock meet the shoreline we see submarine groundwater discharge pushed out by this pressure. This is especially obvious at low tide, when the weight of the overlying seawater is lessened or removed.
This SGD seep occurs below the low tide line, but has enough pressure pushing fresh water out of it that a boil occurs on the seawater surface!
At very low tides SGD seeps can be seen above the tide line, free flowing from the beach.
A SGD seep above the low tide line. You often feel this fresh water by its characteristically cooler temperature signature.
Veronica Gibson, PhD Candidate 