Origin of pore water and salinity in the lacustrine aquitard overlying the regional aquifer of Mexico City Adrian Ortega-Guerrero 1 , John A. Cherry, Ramon Aravena* Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Received 22 December 1994; revised 11 June 1996; accepted 12 September 1996 Abstract A series of six ancient connected lakes occupied the broad plain of the closed Basin of Mexico. This study pertains to two of these lakes. 18 O, 2 H and chemical patterns in the pore water of the broad aquitard formed from sediment in these lakes indicate that the origin of this paleo-lake water was local meteoric water, which entered the ancient lakes as runoff, and groundwater from springs. Evaporation of surface water in the former Chalco Lake caused concentration of dissolved salts and enrichment in the heavy isotopes 18 O and 2 H. During high water periods, the south easternmost ancient Lake Chalco fed partially evaporated water to the closed Lake Texcoco, the lowest of the lakes, where further evaporation occurred causing higher concentration of salts and enrichment of heavy isotopes. These major ions and stable isotopes follow the evaporation trends characteristic of closed or nearly closed evaporatic basins elsewhere in the world. Paleo-lake evaporation lines in Lakes Chalco and Texcoco originated from the same starting water and developed as a consequence of the topographic position of the former lakes and local climatological conditions. A modern evaporation line developed after the drainage of the lakes, reflecting evaporation of modern infiltrat- ing meteoric water. The presence of brackish pore water in most of the Chalco aquitard, which has a maximum thickness of 300 m, indicates that upward vertical groundwater flow has not been capable of displacing the paleo-lake water. This suggests that vertical advection was not significant through- out the hundreds of thousands of years of existence of the aquitard sediments. One-dimensional numerical simulations of the deeper part of the 18 O profile representing the thicker part of the aquitard sediments, 140 m, showed that this profile was developed by molecular diffusion with a small upward advective flux, where apparently the Chalco aquitard always had enriched brackish water overlying the fresh water aquifer.  1997 Elsevier Science B.V. 0022-1694/97/$17.00  1997– Elsevier Science B.V. All rights reserved PII S0022-1694(96)03280-5 Journal of Hydrology 197 (1997) 47–69 * Corresponding author. 1 Current address: Institute of Geology, National University Autonomous of Mexico, Ciudad Universitaria, Apartado Postal 70-324, Coyoacan 04510, Mexico City, Mexico.