Hydrologists Ann Mulligan of WHOI and Holly Michael and Charles Harvey of MIT have cleared up a mystery of why so much salty water emerges from aquifers into the coastal ocean. The researchers discovered a counterintuitive seasonal pumping system at work.
Groundwater discharge appears to be an important factor for determining the chemistry of the coastal ocean. As fresh groundwater flows toward the sea, it rises up over denser, salty water. The fresh and salty water mix along the interface, and the resulting fluid discharges at the shoreline. This interface between underground water masses has recently been described as a “subterranean estuary,” a mixing zone between fresh and salty water analogous to the region where a river meets the ocean.