Large‐Scale Aquitard Consolidation Near Mexico City

Abstract

AbstractThe extensive lacustrine Chalco Plain in the southeastern part of Mexico City is underlain by an aquitard up to 300 m thick composed of a layered sequence of very porous (80‐90%) fine‐grained, organic‐rich Quaternary deposits, with thin horizontal interbeds of volcanic sand (“Capas Duras”). The aquitard overlies a thick sequence of alluvial‐pyroclastic sediments which form a highly productive regional aquifer. The Chalco Plain was a shallow lake until the 1940s when it was drained for agricultural use and human habitation. Historic information indicates that the Chalco Plain was an area of ground‐water discharge prior to the onset of heavy ground‐water extraction from the semiconfined aquifer. Due to aquifer pumping, however, the hydraulic gradient has reversed throughout the full thickness of the aquitard in areas where the aquitard is thin (< 100m), and recharging conditions now prevail. Where the aquitard is thick, the hydraulic head data show a progressive decline with time even though the hydraulic gradient still indicates upward flow in at least the upper part of the lacustrine sequence. Between the early 1960s, when major ground‐water extraction began beyond the periphery of the aquitard, and the onset in 1982 of heavy pumping from aquifers beneath the aquitard, the land surface subsided approximately 3 m. An additional subsidence of 2 m occurred between 1984 and 1989, causing a shallow lake to form and gradually expand. If the present rate of ground‐water withdrawal from the Chalco Basin continues, total land subsidence in the middle of the plain will probably continue to a rate of about 0.4 m per year for many years, and could eventually reach a total subsidence of tens of meters in the thickest part of the Chalco Plain. Pore water in much of the aquitard is saline; however, release of salts and other chemical constituents to the underlying aquifer has not yet significantly impaired the aquifer water quality. A better understanding of the behavior of the aquitard under the influence of aquifer pumping is needed to assist in long‐term management of ground water and land use in the Chalco Plain.