Determination of flow characteristics in the aquifer of the Northwestern Peninsula of Yucatan, Mexico

Plow characteristics were studied in the Northwestern Peninsula of Yucatan, Mexico. The Yucatan aquifer is a mature karst system, influenced by the Ring of Cenotes (sinkholes). This zone of aligned rinkboles is a high-permeability zone with respect to its surroundings. The aquifer 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 showed variations up to 60(percen)during October 1994 whereas others had variations of the order of 5(percent)n the same period. By studying the resulting equipotential regimes, a highly variable zone (HVZ)was identified. Drain characteristics are significantly different from the rest of the study area, which eads to the hypothesis that the HVZ represents an independent subsystem of interconnected fracures. To determine directions of high permeability within the study area, azimuthal resistivity surveys were conducted at 22 locations. Resistivity curves particularly in the HVZ show two or nore peaks, each indicating a direction of high permeability. Directions of high permeability are nterpreted to be preferential directions of aligned fractures. Comparison with the two different quipotential regimes in October 1994 shows that in the majority of the cases the direction of the tydraulic gradient coincides with the direction of one fracture system. The hydraulic gradient can be onsidered to be the major control in the dissolution process at least in the HVZ, resulting in the elective opening of the fractures. Peaks in the resistivity curves are of the same order of magnitude. 'his shows that both regimes of equipotential lines and the corresponding changes of the hydraulic radients are sufficiently frequent to be able to create different fracture systems with a comparable racture density. The variations of the equipotential regime in time may have severe consequences in :rms of the risk of aquifer contamination in the highly variable zone. Contaminants, generated minly in the city of Merida, are generally believed to flow towards the north coast of the Peninsula. However, reversals in the hydraulic gradient may allow contaminants to flow towards the southeast f the city, at least for limited periods in the year.