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Numerical modelling is increasingly used as a tool for improving management strategies in aquifers and to support the design of comprehensive projects considering natural and anthropogenic processes. Overall, numerical simulation in karstic aquifers poses a major scientific challenge due to the non-Darcian groundwater flow dynamics. In specific cases, the equivalent porous medium approach has shown acceptable results, particularly in poorly karstified aquifers with regional/subregional scales such as this case. The Yucatan coastal karstic aquifer (Mexico) has been defined as a complex regional heterogeneous system, partially confined, thus allowing the discussion of multiple conceptual models. In this research, a two-dimensional numerical model of flow and transport was implemented using SEAWAT for the NW Yucatan aquifer. Four likely conceptual models were audited, calibrated and verified using hydrogeological field data, to select the best one, considering their fit and complexity. The numerical model accuracy was evaluated using the root-mean-square error, Nash Sutcliffe efficiency and the Pearson coefficient. The Akaike information criterion and Bayesian information criterion were included for evaluating the complexity of the numerical models. In addition, the signal of tide propagation into the aquifer was assessed as a proxy to improve the numerical calibration process. Results show that the most complex numerical model has a better calibration than the simpler models, but the model accuracy is worse when compared to less complex numerical models in the verification exercise. This research offers enhancement in the knowledge of numerical modelling in heterogeneous coastal aquifers within a conceptual-model uncertainty setting.

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 r r r r r > 0.7) is a more important forcing than the vertical recharge (with 0.5 r

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.

In the process of solute transport, dispersivity and hydrodynamic dispersion coefficients are important factors that define the distribution and migration of dissolved salts in the aquifer. Tracer tests were carried out in wells located in the northeast coast of Yucatan, Mexico, whereby only the freshwater thickness of the aquifer was analyzed. Sodium chloride (NaCl) was used as a tracer. Groundwater flow velocity was estimated with the Drost dilution point method. To analyse the solute transport, the analytical model for instantaneous injection was chosen, which is derived from the advection–dispersion equation, and the dispersivity and longitudinal dispersion coefficients were determined. The results indicate that the longitudinal dispersivity and hydrodynamic dispersion of the aquifer vary from 2.56 to 2,500 m and between 1 × 10−3 and 1 × 10−2 m2/s, respectively. In the areas of Telchac Puerto, Dzilam Bravo and San Felipe, advection is the most important solute transport mechanism, while El Cuyo has lower velocity ranges than other locations in the study area and salts tend to disperse. Groundwater flow velocity varies between 3.1 × 10−7 and 8.65 × 10−5 m/s, associated with the flow in the karstic aquifer matrix, with maximum values ​​of 1.5 × 10−4 m/s, associated with the karstification of the aquifer. Groundwater flow velocity increases towards the coast, which is consistent with the fact that the aquifer discharges to the sea. It is observed that in the first 10 m of aquifer, the velocities are higher than those determined in deeper layers, up to an order of magnitude.

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

Karst aquifers show dissolution/precipitation processes of the minerals present in the carbonate rocks. 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 preferential flow paths toward the coast near Celestun and Dzilam Bravo towns. This study aimed to describe the regional hydrogeochemical evolution of groundwater of the RC, and its association with the dissolution/precipitation of the minerals present along its path to the ocean. To achieve this aim, we: a) characterized groundwater's hydrogeochemistry; b) determined the calcite, dolomite, and gypsum saturation indexes (reaction phases with the groundwater) in the study area; c) proposed a hydrogeochemical model developed through PHREEQC using an 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, an important marine intrusion influences the hydrogeochemical processes and represents a risk for the prevalence of freshwater. On the eastern path, where groundwater flows toward Dzilam Bravo, the hydrogeochemistry in the sinkholes correlates well with rainfall, suggesting a higher vulnerability during droughts than during rainy periods.

The Yucatan state, Mexico, encloses a large karstic aquifer, which is confined near the coast. There are places where this confinement is fractured creating springs that discharge to wetlands or shallow lagoons, which are important because of the freshwater ecosystems that develop around them (locally known as Petenes), and because they provide a thermohaline equilibrium in these systems. Previous studies reported that during spring low tide, the spring becomes a sink, reversing the flow in the Peten channel connecting the lagoon to the spring. Potentially, this inversion of the flow can cause the intrusion of brackish – and sometimes hypersaline - water from the lagoon into the aquifer, which can also have ecological implications. The flow reversal was observed in two sampling campaigns and it occurred a total of 16 times. In the last sampling campaign, it was confirmed that the spring becomes a sink, with measured water velocities of about 0.37 m/s at the channel end close to the spring. Preliminary results suggest that the flow reversal is controlled by the discharge from the aquifer, the confinement of the aquifer, the geometry of the lagoon and the inlet channel, as well as the sea tides. A simple 1D analytical solution was used to explain this phenomenon, which describes the hydrodynamic interaction between the lagoon and the aquifer by solving the heat equation, which can be used to simulate flow in both systems, a confined aquifer and a highly frictional lagoon. Therefore, the same solution was used to simulate the gradients observed between the confined aquifer and the lagoon. The solution was implemented as a Python library using Fourier series, and has the advantage that it can be used with more general boundary conditions and finite length systems. Finally, we present coefficients that can be used to determine under which conditions the flow reverses at the lagoon.

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.