Birgit Steinich

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

Abstract Flow 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 sinkholes 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% during October 1994 whereas others had variations of the order of 5% in 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 leads to the hypothesis that the HVZ represents an independent subsystem of interconnected fractures. 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 more peaks, each indicating a direction of high permeability. Directions of high permeability are interpreted to be preferential directions of aligned fractures. Comparison with the two different equipotential regimes in October 1994 shows that in the majority of the cases the direction of the hydraulic gradient coincides with the direction of one fracture system. The hydraulic gradient can be considered 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 shows that both regimes of equipotential lines and the corresponding changes of the hydraulic gradients are sufficiently frequent to be able to create different fracture systems with a comparable fracture density. The variations of the equipotential regime in time may have severe consequences in terms of the risk of aquifer contamination in the highly variable zone. Contaminants, generated mainly 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 of the city, at least for limited periods in the year.

Hydrogeologic Site Characterization Using Azimuthal Resistivity Surveys

The purpose of this work is to evaluate whether azimuthal resistivity surveys (ARS) provide diagnostic information for mapping preferred hydraulic conductivity directions in unconsolidated sediments as they do in fractures/jointed media. Nineteen azimuthal resistivity surveys (ARS) were conducted in three different places within a shallow, unconsolidated, sand and gravel aquifer located at the Michigan Integrated Remediation Technology Laboratory (MIRTL). Two of the three places where soundings were conducted lie along the flow path where a controlled tracer release using bromide took place. The ARS were conducted using a Wenner array, with electrode separations (AB/3) that ranged from 4.0–11.0 meters. Electrical anisotropy ratios gave a range of 1.08–1.37 for all 19 soundings. Ground water direction of the induced plume was along the regional flow path as determined for the period from July 1996, using continuous water level recorders. Thus, the observed electrical anisotropy ratios were not significant ...