Seasonal variability of saltwater intrusion at a point‐source submarine groundwater discharge

Submarine groundwater discharge provides freshwater and nutrients to coastal environments. In some places throughout the world, this direct connection between aquifers and oceans may also allow saltwater intrusion. Saltwater intrusion was studied at a submarine spring within a fringing reef lagoon on the eastern Yucatán Peninsula by observing its intratidal and synoptic‐scale variations during wet and dry periods. Saltwater intrusion was linked to wave‐driven setup, no rain, high tides, and sea‐level rise caused by remote forcing from Yucatán Current variability. Jet discharge velocities were inversely related to tidal oscillations, with maximum velocities at low tides. The wet period produced saltwater intrusion at high tides associated with three different conditions: syzygy tides, wave setup, and Yucatán Current weakening. During the dry period, saltwater intrusion occurred throughout most high tides and was aided by Yucatán Current weakening and wind‐driven setup within the lagoon. These results suggested that seasonal precipitation was most important in modulating spring discharge, followed by syzygy tides, Yucatán Current variability, and wave events. The spring discharge was modeled with a modified Bernoulli energy equation that included a friction term. Aquifer elevation and a friction factor were used as free parameters. The dry period produced the best model results because of infrequent rainfall that yielded a relatively steady aquifer level. Precipitation during the wet period most likely led to a more variable aquifer level, reducing the variance explained by the model that assumes a constant aquifer elevation. Nevertheless, the model predicted saltwater intrusion events reasonably well using simplified physics.