Abstract
Semiconfined aquifers used as the principal water supply within the Basin of Mexico are overlain by lacustrine deposits that provide a degree of protection from contamination associated with metropolitan Mexico City. Solute transport behavior and the nature of chemical interactions with mineral components in these sediments is poorly understood. The objectives of this paper were to identify the clay mineral phases of the lacustrine sediments and to determine the significance of the exchange properties of the day minerals on contaminant transport processes. Samples obtained from two cores were separated into sand, silt, and clay-size fractions. The clay-size fraction was analyzed by X-ray diffraction and Fourier transform infrared spectroscopy and for specific surface area and pH-dependent cation and anion exchange capacity. The clay-sized fraction averaged 56% of the sediment. Analyses indicated that the clay was predominantly composed of a Si-rich allophane with approximately 30% montmorillonite. Halloysite was also present to a depth of about 1.5 m, but was absent deeper in the sediment. Kaolinite and illite, reported in previous studies, and imogolite were not found in the samples. Solute transport in the sediment was modeled to demonstrate the impact of exchange properties imparted by the allophane compared to other possible clay mineral assemblages. The predominance of allophane in the Basin of Mexico sediments is responsible for many of the fundamental characteristics of the material including: high porosity (0.8–0.9), high water content (200–400%), and an extremely high and pH-dependent cation exchange capacity. The pH of the pore water within the lacustrine sediments of the Basin of Mexico is typically between 6.5 and 12. Measured cation exchange values ranged from ≈ 450 meq kg −1 at pH 6.5 to ≈ 650 meq kg −1 at pH 12 which could produce variable cation mobility in the semiconfining aquitard. The simulations illustrated that allophane is very effective for retarding the leaching of cationic metals compared to other possible clay mineral suites, particularly at high pH values. Simulation of long-term infiltration indicated that trace metals will become concentrated in the clay matrix adjacent to fracture faces in the sediment near ground surface. As such, the aquitard behaves as an efficient barrier to the infiltration of cationic metals into the underlying aquifer.
Published Version
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