DETERMINATION OF FLOW CHARACTERISTICS IN THE AQUIFER OF THE NORTHWESTERN PENINSULA OF YUCATAN, MEXICO

Flow characteristics were studied in the Northwestern Peninsula of Yuc atan, Mexico. The Yucatan aquifer is a mature karst system, influenced by the Ring of Cenotes (sinkholes). This zone of aligned sinkholes is a high-permeability zone with respect to its surroundings. The aquife r is unconfined within the study area. Water-level measurements at 48 locations were made in June and October 1994; they were measured up to four times in October. Water levels in a limited group of wells showe d variations up to 60% during October 1994 whereas others had variatio ns of the order of 5% in the same period. By studying the resulting eq uipotential regimes, a highly variable zone (HVZ) was identified. Drai n characteristics are significantly different from the rest of the stu dy area, which leads to the hypothesis that the HVZ represents an inde pendent subsystem of interconnected fractures. To determine directions of high permeability within the study area. azimuthal resistivity sur veys were conducted at 22 locations. Resistivity curves particularly i n the HVZ show two or more peaks, each indicating a direction of high permeability. Directions of high permeability are interpreted to be pr eferential directions of aligned fractures. Comparison with the two di fferent equipotential regimes in October 1994 shows that in the majori ty of the cases the direction of the hydraulic gradient coincides with the direction of one fracture system. The hydraulic gradient can be c onsidered to be the major control in the dissolution process at least in the HVZ, resulting in the selective opening of the fractures. Peaks in the resistivity curves are of the same order of magnitude. This sh ows that both regimes of equipotential lines and the corresponding cha nges of the hydraulic gradients are sufficiently frequent to be able t o create different fracture systems with a comparable fracture density . The variations of the equipotential regime in time may have severe c onsequences in terms of the risk of aquifer contamination in the highl y variable zone. Contaminants, generated mainly in the city of Merida, are generally believed to flow towards the north coast of the Peninsu la. However, reversals in the hydraulic gradient may allow contaminant s to flow towards the southeast of the city, at least for limited peri ods in the year.