Northern Guam geospatial information server

Mixing zone

Mixing zone is a layer within the phreatic zonewhere salinity of water rapidly increases with depth. It is a layer of brackish water that separates the fresh groundwater of the aquifer above from salty groundwater below. When the salinity gradient is abrupt, the layer is relatively thin and is called halocline.

Though the mixing zone exists within fully saturated pores in the bedrock, parts of it can be directly observed and explored by people inside caves. Guam has several sufficiently large caves with rooms that are fully filled with groundwater. SCUBA divers descending into freshwater in cave pools encounter the mixing zone, easily identifiable by "blurriness" caused by differential refraction of light within media of different densities. Clear visibility is regained as soon as the person passes through the halocline and enters fully saline groundwater.


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