Coastal aquifers are characterized by a mixing zone with freshwater-saltwater interactions, which have a strong relationship with hydrological forcings such as astronomical and storm tides, aquifer recharge and pumping effects. These forcings govern the aquifer hydraulic head, the spatial distribution of groundwater salinity and the saline interface position. This work is an empirical evaluation through time-series analysis between aquifer head and groundwater salinity associated with the sea-level dynamics and the aquifer recharge. Groundwater pressure, temperature and salinity were measured in a confined aquifer in the northwest coast of Yucatan (México) during May 2017-May 2018, along with precipitation. Cross-correlation and linear Pearson correlation (r) analyses were performed with the data time series, separating astronomical and meteorological tides and vertical recharge effects. The results show that the astronomical and meteorological tides are directly correlated with the aquifer head response (0.71 < r < 0.99). Salinity has a direct and strong relationship with the astronomical tide (0.76 < r < 0.98), while the meteorological tide does not (r < 0.5). The vertical recharge showed a moderate correlation with the aquifer head (0.5 < r < 0.7) and a nonsignificant correlation with the groundwater salinity (r < 0.5). In this study, the sea level (r > 0.7) is a more important forcing than the vertical recharge (with 0.5 < r < 0.7). Empirical relationships through time-series analysis and the separation of individual hydrological forcings in the analysis are powerful tools to study, define and validate the conceptual model of the aquifer.

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The Ring of Cenotes (RC) extends along the edge of the Chicxulub crater, in the limestone platform of the Yucatan Peninsula (YP), where groundwater shows two preferential flow paths toward the coast near Celestun and Dzilam Bravo towns. The objectives of this study were to describe the regional hydrogeochemical evolution of the groundwater in the RC, and its association with the dissolution/precipitation of the minerals present along its pathway to the ocean. These objectives results were obtained by: a) characterizing groundwater hydrogeochemistry; b) calculating calcite, dolomite, and gypsum saturation indexes in the study area; and c) developing a hydrogeochemical model using PHREEQC (U. S. Geological Survey) inverse modelling approach. The model predictions confirmed that there are two evolution pathways of the groundwater consistent with the preferential flow paths suggested in a previous regionalization of the RC. On the western path, where groundwater flows towards Celestun, marine intrusion influences the hydrogeochemical processes and represents a risk for the freshwater. On the eastern path, where groundwater flows toward Dzilam Bravo, rainfall has an important effect on the hydrogeochemical processes, evidenced by a higher concentration in sulfates during droughts than during rainy periods. Then, monitoring of marine intrusion and phases dissolution in the RC is highly recommended

Springs are common features on the Yucatán coast. They can discharge either under the sea (submarine) or inland in coastal lagoons and wetlands. Previous observations of a coastal lagoon located on the northern Yucatán Peninsula (La Carbonera) reported sea water intrusion on a spring that discharge on a coastal lagoon (lagoon tidal spring). The saltwater intrusion occurs when the tide is at its lower level, which is the opposite to what has been reported for submarine springs in the Yucatán Peninsula. In this study, the hydrodynamics of the spring is analyzed and the driving forces controlling the seawater intrusion are identified and discussed. Time series of water levels, salinity, and velocity measurements in the lagoon, the aquifer, and the spring are analyzed by means of tide component decomposition and cross-correlations analysis of the tide signals. Results show that the main driving forces causing the intrusion are the density differences and pressure head gradients, and the mechanisms influencing the driving forces driving those differences are the tides, the friction in the lagoon, and the confinement of the aquifer; other mechanisms are discussed to present a complete idea of the complexity of the interactions between the coastal aquifer, the coastal lagoons, and the sea.

The study of karstic aquifers is a challenge because of their heterogeneous and anisotropic nature; the groundwater flow in these systems often cannot be explained using Darcian theory. Many techniques (direct and indirect) have been used for the characterization of karstic aquifers. Dissolved oxygen alteration method (DOAM) consists in injecting air into wells and tracing the evolution of this parameter over time; this practice offers advantages because it is cheap and harmless. Previous experiences show acceptable results in the identification of fractures and preferential flow in heterogeneous aquifers. The air injection used during the drilling process (rotary perforation with air cleaning) could be used as a tracer. This research monitored the physical parameters in groundwater after drilling wells, especially the dissolved oxygen. Besides, karstic zones have been identified using video surveys in drilling wells. Finally, the results using the DOAM method were contrasted with the physical evidence of karstic zones to compare the technique's accuracy. Results show that DOAM offers a qualitative approach to the evidence of karstic zones and preferential groundwater flow. DOAM can be considered as a useful choice to study the karstification influence on the aquifer groundwater flow and transport in karstic aquifers. The identification of these zones areis important for the design of supply and wastewater projects; therefore, this approach represents an improvement in the wells' design process in aquifers with similar conditions.