An intertidal saltwater-freshwater mixing zone forms beneath the beach as a result of seawater infiltration from waves and tides. The mixing zone is largest when the inland water table is lowest, and smallest when the inland water table is highest. Credit: Heiss and Michael

Nutrients and pollutants each flow out to sea in similar ways. Scientists commonly think of surface water as feeding into the ocean. However, studies have shown that groundwater can carry large amounts of nutrients into coastal waters. These nutrients and other groundwater chemicals in freshwater and saltwater mix and react in underground beach aquifers before reaching the sea.

Heiss and Michael modeled how salinity in a sandy beach aquifer changes with the tides and seasons. The researchers measured this coastal saltwater-freshwater mixing zone over the course of a year, focusing their research at Cape Henlopen, a well-studied sandy spit in northern Delaware. They collected samples 45 times under various conditions, with an additional sample taken 1 day after Hurricane Sandy made landfall to the north. The samples were then paired with hydrological and meteorological data, such as dune water table elevation and rainfall recordings.

Based on these measurements and modeling, the authors were able to show for the first time that salinity varied most strongly with seasonal inland water table changes, whereas changing tides caused only minimal variations in salinity. The biggest mixing zone area was created as the water table at the dune dropped to its lowest point, which strengthened the amount of saltwater circulation.

The study has implications for the fate of contaminants discharging into coastal waters because such mixing zones host chemical reactions and can spread pollution. A better understanding of these sandy beach aquifers will help scientists predict chemical fluxes to marine environments. The researchers say the changes they observed over the year should also give pause to other scientists trying to extrapolate data from beach measurements made at just a single point in time. (Water Resources Research, doi:10.1002/2014WR015574, 2014)

—Eric O. Betz, Freelance Writer

Citation: Betz, E. O. (2015), A new level of understanding for coastal aquifers, Eos, 96, doi:10.1029/2015EO023175. Published on 4 February 2015.

Text © 2015. The authors. CC BY-NC 3.0
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