Vertical variability of PCE sorption in the lacustrine clays of Mexico city

A surficial clay aquitard extends through the urban area where Mexico City is located. It has been assumed to function as a protective layer the under lying aquifer that provides 42 m(3)/s out of 63 m(3)/s of water used by 18 million inhabitants. To provide such protection, the aquitard must be imper meable to water flow and, ideally, have a signifi- capacity to sorb contami nants. The latter aspect was addressed, studying the vertical variability o f sorption of perchloroethylene (PCE), a widely much of used organic compou nd considered to pose health from 30 risks in groundwater. Batch sorption t ests were located in used and the clay-rich strata in the depth interval th ick, un from 8 to 75 m were studied. The results suggest terials. I that so rption depends mainly on the fraction of organic carbon (dc) present in the clayey materials. The sorption data were fit to the linear and serves a Fr eundlich models; many strata could be fit well by from either model, while some strata were distinctly ; non-linear. The linear isotherms showed a mea n tem sup value of 32.8 ml/g, and the Freundlich isotherm 96.6 ml/g, confir ming that the clay-rich media have significant sorption capacity for PCE, F rom the environmental perspective the clay-rich materials are serving as pr otection to the groundwater system.