CLAY-MINERALS IN BASIN OF MEXICO LACUSTRINE SEDIMENTS AND THEIR INFLUENCE ON ION MOBILITY IN GROUNDWATER

Semiconfined aquifers used as the principal water supply within the Ba sin of Mexico are overlain by lacustrine deposits that provide a degre e of protection from contamination associated with metropolitan Mexico City. Solute transport behavior and the nature of chemical interactio ns with mineral components in these sediments is poorly understood. Th e objectives of this paper were to identify the clay mineral phases of the lacustrine sediments and to determine the significance of the exc hange properties of the day minerals on contaminant transport processe s. Samples obtained from two cores were separated into sand, silt, and clay-size fractions. The clay-size fraction was analyzed by X-ray dif fraction 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 t hat the clay was predominantly composed of a Si-rich allophane with ap proximately 30% montmorillonite. Halloysite was also present to a dept h 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 demonstr ate the impact of exchange properties imparted by the allophane compar ed to other possible clay mineral assemblages. The predominance of all ophane 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 ps -dependent cation exchange capacity. The pH of the pore water within t he lacustrine sediments of the Basin of Mexico is typically between 6. 5 and 12. Measured cation exchange values ranged from approximate to 4 50 meq kg(-1) at pH 6.5 to approximate to 650 meq kg(-1) at pH 12 whic h could produce variable cation mobility in the semiconfining aquitard . The simulations illustrated that allophane is very effective for ret arding the leaching of cationic metals compared to other possible clay mineral suites, particularly at high pH values. Simulation of long-te rm infiltration indicated that trace metals will become concentrated i n the clay matrix adjacent to fracture facts in the sediment near grou nd surface. As such, the aquitard behaves as an efficient barrier to t he infiltration of cationic metals into the underlying aquifer. (C) 19 97 Elsevier Science B.V.