Fracture hydraulic conductivity in the Mexico City clayey aquitard: Field piezometer rising-head tests

A regional lacustrine aquitard covers the main aquifer of the metropolitan area of Mexico City. The aquitard’s hydraulic conductivity (K′) is fundamental for evaluating the natural protection of the aquifer against a variety of contaminants present on the surface and its hydraulic response. This study analyzes the distribution and variation of K′ in the plains of Chalco, Texcoco and Mexico City (three of the six former lakes that existed in the Basin of Mexico), on the basis of 225 field-permeability tests, in nests of existing piezometers located at depths of 2–85 m. Tests were interpreted using the Hvorslev method and some by the Bouwer-Rice method. Results indicate that the distribution of K’ fits log-Gaussian regression models. Dominant frequencies for K’ in the Chalco and Texcoco plains range between 1E−09 and 1E−08 m/s, with similar population means of 1.19E−09 and 1.7E−09 m/s, respectively, which are one to two orders of magnitude higher than the matrix conductivity. In the Mexico City Plain the population mean is near by one order of magnitude lower; K’=2.6E−10 m/s. The contrast between the measured K’ and that of the matrix is attributed to the presence of fractures in the upper 25–40 m, which is consistent with the findings of previous studies on solute migration in the aquitard.