Search

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

Barrier islands in tropical regions are prone to coastal flooding and erosion during hurricane events. The Yucatan coast, characterized by karstic geology and the presence of barrier islands, was impacted by Hurricanes Gamma and Delta in October 2020. Inner shelf, coastal, and inland observations were acquired simultaneously near a coastal community (Sisal, Yucatan) located within 150 km of the hurricanes’ tracks. In the study area, Gamma moved at a slow speed and induced heavy rain, mixing in the shelf sea, and northern winds exceeding 20 m s-1. Similar wind and wave conditions were observed at this location during the passage of Hurricane Delta. However, a higher storm surge (0.5 m) was measured due to wind setup and the drop (< 1000 mbar) in atmospheric pressure. Beach morphology changes, based on GPS measurements conducted before and after the passage of the storms, show alongshore gradients ascribed to the presence of coastal structures and macrophyte wracks on the beach face. Moreover, net onshore sediment transport during the storm contributes to the increase in beach elevation. Urban flooding occurred mainly on the back-barrier associated with heavy rain and the confinement of the coastal aquifer which prevented rapid infiltration. Two different modeling systems, aimed at providing coastal flooding early warning and coastal hazards assessment, presented difficulties in forecasting the coastal hydrodynamic response during these seaward translating events, regardless of the grid resolution and wind forcing employed. Compound flooding plays an important role in this region and hence must be incorporated in future modeling efforts.

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.

A sandbox device was developed to test MODFLOW CFP1 under various conditions. The experiment represents a three-dimensional confined karst aquifer. MODFLOW CFP1 is a public-domain software developed by the US Geological Survey that is becoming popular for the simulation of karst hybrid models. Since geometry and hydraulic parameters of the matrix and conduit related to the experiments can be measured accurately, this study is focused on the flow exchange coefficient used by MODFLOW CFP1 to simulate the flow exchange between matrix and conduits of karst aquifers. The flow exchange coefficient is commonly calibrated given our limited knowledge of the underground system. In this work, we discuss the issues encountered during the calibration of this coefficient. It was found that the calibrated parameter values depend on the direction of the flow exchange something that is not considered in the current definition of this parameter. The calibration also revealed the structural inadequacy of the linear model used in MODFLOW CFP1 for simulating the flow exchange. These results are useful for further evaluation of MODFLOW CFP1 at laboratory and field scales.

This study develops an approach based on hierarchical cluster analysis for investigating the spatial and temporal variation of water quality governing processes. The water quality data used in this study were collected in the karst aquifer of Yucatan, Mexico, the only source of drinking water for a population of nearly two million people. Hierarchical cluster analysis was applied to the quality data of all the sampling periods lumped together. This was motivated by the observation that, if water quality does not vary significantly in time, two samples from the same sampling site will belong to the same cluster. The resulting distribution maps of clusters and box-plots of the major chemical components reveal the spatial and temporal variability of groundwater quality. Principal component analysis was used to verify the results of cluster analysis and to derive the variables that explained most of the variation of the groundwater quality data. Results of this work increase the knowledge about how precipitation and human contamination impact groundwater quality in Yucatan. Spatial variability of groundwater quality in the study area is caused by: a) seawater intrusion and groundwater rich in sulfates at the west and in the coast, b) water rock interactions and the average annual precipitation at the middle and east zones respectively, and c) human contamination present in two localized zones. Changes in the amount and distribution of precipitation cause temporal variation by diluting groundwater in the aquifer. This approach allows to analyze the variation of groundwater quality controlling processes efficiently and simultaneously.

The mechanisms of sinkhole formation, development, and collapse are investigated in this study using experimental and numerical methods. Sandbox experiments are conducted to understand how excessive groundwater pumping triggers sinkholes formation. The experimental results indicate that the change of hydrologic conditions is critical to sinkhole development. When seepage force increases due to increase of hydraulic gradient, clay and sand particles start moving downward to form a cavity. The confining unit is of particular importance because the cavity is first formed in this layer. Based on the conceptual model developed from the sandbox experiments, the Fast Lagrangian Analysis of Continua (FLAC) code and Particle Flow Code (PFC) are coupled to simulate the sandbox experiments. PFC was used to simulate particle movement in the sinkhole area, and FLAC is used for other areas. While the current numerical simulation can simulate the experiment results such as the sizes of the cavity and the sinkhole, the simulation capability is limited by the computing cost of PFC. More effort of model development is necessary in the future study. Introduction Sinkholes are a common geological feature of karst landscape in Florida, southeastern United States, and worldwide. In particular, cover-collapse sinkholes occur abruptly and can cause catastrophic damages such as death, injury, and property damage. In Florida, a Tampa resident vanished into a sinkhole that opened under his bedroom on a night in March, 2013. In the last several years, sinkholes have become Florida’s insurance disaster due to sinkhole collapse in urban areas. Covercollapse sinkholes also do severely damage buildings, drain farm ponds, damage roads, and wreck farming equipment, and lead to engineering and environmental problems (Beck, 1988). There is an urgent need to understand the mechanisms of sinkhole development and catastrophic collapse. Cover-collapse sinkholes occur in the soil or other loose material overlying soluble bedrock. The thickness and cohesiveness of the soil cover determine the size of a cover-collapse sinkhole. Figure 1 shows a typical process of cover-collapse sinkholes formation caused by excessive groundwater pumping. A karst aquifer is the Xiaohu Tao Geophysical Fluid Dynamics Institute, Florida State University, Tallahassee, FL, 32306 College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing, Jiangsu, 210098, P.R. China, taoxiaohu_hhu@126.com Ming Ye Department of Scientific Computing and Geophysical Fluid Dynamics Institute, Florida State University, Tallahassee, FL 32306, U.S.A, mye@fsu.edu Dangliang Wang Department of Scientific Computing and Geophysical Fluid Dynamics Institute, Florida State University, Tallahassee, FL 32306, U.S.A, dlw9800@163.com Roger Pacheco Castro Department of Scientific Computing and Geophysical Fluid Dynamics Institute, Florida State University, Tallahassee, FL 32306, U.S.A, matbnt@gmail.com Xiaoming Wang Department of Mathematics and Geophysical Fluid Dynamics Institute, Florida State University, Tallahassee, FL 32306, U.S.A, xwang@fsu.edu Jian Zhao College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing, Jiangsu, 210098, P.R. China, zhaojian@hhu.edu.cn