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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.

A variety of reactions and transformations both inland and near the coast can influence the amount of dissolved chemicals passing through this underground estuary. Nitrogen, phosphorous, and other contaminants may be introduced into groundwater through a variety of mechanisms. Wastes and nutrients leach from septic systems. Rainfall carries atmospheric pollutants and ground spills down through the soil. Eventually, these chemicals flow underground toward the ocean just as they do in surface tributaries.

Once these dissolved chemicals reach coastal waters, they can influence the abundance of plants and other living species. It is not unusual, for instance, for groundwater to contain dissolved nitrogen in concentrations 100 to 1,000 times greater than in seawater. Nitrogen from human sources, for instance, has led to the over-enrichment (eutrophication) of many coastal bays and waterways.


 GroundWater diagram

Subterranean Estuary Geochemistry

Recent studies indicate that submarine groundwater discharge (SGD) may contribute significant fluxes of dissolved chemical species to the oceans.

View of a beach
Biogeochemical Cycling of Nitrogen in Permeable Sediments

Investigating nitrogen removal pathways in a major land-ocean interface


 Radium Isotopes as Tracers of Groundwater Discharge

A multi-tracer approach for estimating submarine groundwater discharge in salt marsh estuaries

Nighttime thermal infrared image

Aerial Infrared Imagery

Submarine groundwater discharge: identification and quantification via remote sensing, hydrologic sampling, and geochemical tracers

Glacier in Greenland
Meltwater Tracing in the Greenland Ice Sheet

Naturally occurring radioisotopes used to trace the routing of water beneath the ice sheet

Actinium - 227 +Magma Transit Times

Melt generation and transport during basalt petrogenesis: Constraints from measurements of 227Ac-231Pa activity ratios