Resultados del pozo San Lorenzo Tezonco y sus implicaciones en el entendimiento de la hidrogeología regional de la cuenca de México
Keywords: hydrogeology, deep formations, groundwater residence time, basin of Mexico, Mexico
AbstractResults from the 2008 m deep San Lorenzo Tezonco exploratory borehole allow investigating the hydrogeological characteristics of deep formations in the basin of Mexico. Based on the analysis of the lithology and well logs five hydrogeologic units are defined: a) from 0 to 70 m aquitard composed by clay-rich lacustrine sediments; b) from 70 to 500 m upper aquifer composed by volcanoclastic materials; c) from 500 to 750 m aquifer composed by lava and pyroclastic flows (with a lacustrine layer embedded from 590 to 604 m); d) from 750 to 1140 m aquitard composed by ignimbrites, clay and volcanoclastic materials, which exhibited electric resistivity ranging from 2 to 20 Ωm; e) from 1140 to 2008 m aquifer composed by volcanic rocks interbedded with aquitard layers of electric resistivity ranging from 2 to 10 Ωm and which exhibited almost no drilling mud invasion as shown by the electric log. These five hydrogeologic units are highly heterogeneous in their composition and their hydrogeologic parameters as inferred from the electric well log. Average hydraulic conductivity and specific storage were computed for the fifth hydrogeologic unit from a step drawdown test. Since only cuttings were available for analysis, the lithology record obtained is uncertain; to reduce this uncertainty it will be necessary to obtain cores in future explorations. In addition, the chemical and isotopic characteristics of water from 1176 to 2008 m (which is the screened length of the well) were analyzed. The isotopic composition (δ18O = -67.3 ‰ and δD = -9.44 ‰) suggests an origin by infiltration of precipitation. Carbon 14 dating (corrected by carbon 13) resulted in a residence time of 14,237±265 years. Microfractures filled with calcite and gypsum observed in cuttings from 1800 to 1920 m and values of δ13C = -5.8‰ suggest deep groundwater flow through carbonated rocks, which were not found in the San Lorenzo Tezonco well but are inferred at greater depths on the basis of results from the Mixhuca and Tulyehualco wells.