Influence of ocean tides and atmospheric pressure on a coastal karstic aquifer

In coastal aquifers, one of the most common processes is seawater intrusion, which is the incursion of seawater towards the continent, displacing and mixing with the fresh groundwater, causing multiple difficulties in drinking and agricultural coastal water supply systems.

In the peninsula of Yucatán, a karstic platform in the southeast of Mexico, seawater intrusion is frequently found along the coast. However, the extension of the saline water mass is still undetermined as well as its temporal variations. Particularly, the lack of deep (> 100 m) sampling sites in the central and western part of the peninsula, hinders a general understanding of the groundwater system dynamics.

Sea level is one of the main drivers of seawater intrusion, therefore, by analyzing the effect of tidal oscillations on groundwater levels a better understanding of the aquifer connectivity and seawater intrusion can be achieved.

In previous studies, tidal oscillation analysis in lakes and sinkholes water levels have suggested that the central region of Yucatán is heavily connected to the sea. This has been explained by the high porosity and fracturing of the local lithology. Nevertheless, the geological settings in the peninsula are exceptionally variable, which is the result of many karstic processes involved and the existence of the Chicxulub crater.

In the present contribution, groundwater levels were measured in different wells and sinkholes from the coast across the west Yucatán through the Ring of Cenotes (a highly hydrogeological connected area) and its surroundings. We compare the groundwater signals to atmospheric pressure and sea level time series, by performing cross spectral analysis and coherence tests. Sea level measurements were collected from the National Mareograph Service, and the atmospheric pressure was registered by the National Meteorological Service.

The data show a similar response in all the sites: a very clear tidal effect on groundwater levels near the coast (10 km from the beach at Celestún), which is strongly attenuated inland and absent inside the Ring of Cenotes. On the other hand, the cross spectra between groundwater level and barometric pressure, suggests a strong influence at diurnal, semidiurnal (similar frequencies to tidal oscillations) and low frequencies at all sites. In other words, there is a major influence on the groundwater levels in the aquifer produced by the atmospheric variations. These results suggest that the link between the ocean and the groundwater in this area is relevant close to shore, but not as relevant inland as was previously suggested. This opens more inquiries about the complexity of the geological settings, the extension and temporal variability of the seawater intrusion in the zone, and the implications of the atmospheric variations in the groundwater flow pattern.