Evaluation of soil erosion and characterization of indurated volcanic soils in central Mexico

In Central Mexico, soils of volcanic origin occupy an extensive area and contain indurated horizons within their profile locally called “tepetates”, which appear at the surface after the topsoil has been eroded. In some watersheds of this highly populated Valley the tepetates are quite extensive, representing an environmental hazard for the population due to the problems of erosion, flooding and sedimentation associated with them. Four types of tepetates were characterized in terms of their physical, chemical and mineralogical properties. A micromorphological study was performed to evaluate their structure under natural conditions. Soil erosion was evaluated in the laboratory using simulated rainfall and small erosion pans. The effect of surface application of gypsum on infiltration, runoff and sediment concentration was also evaluated. The climatic conditions for the formation of tepetates were associated with an annual rainfall no greater than 800 mm and a well defined dry season. The bulk densities of all the tepetates studied was greater than 1.5 Mg m -3 and their strength varied from 4.5 to 13.5 MPa. Micromorphological observations suggested that induration was largely pedogenic in the gray, brown and yellow tepetates and geologic in the pink tepetate. The clay fraction of the indurated volcanic soils studied was dominated by halloysite and smectite. The presence of amorphous silica and unweathered biogenic opal (phytoliths) suggested that there is Si-rich environment associated with the formation of tepetates. Total porosity of the tepetates was lesser than the overlying horizons. The micromorphology of tepetates on thin sections was characterized by a massive structure. Surface applications of gypsum increased steady state infiltration, and reduced total soil loss, runoff and sediment concentration in all soils studied. These results were related to a greater concentration of electrolytes and ionic strength of eroding water provided by the surface application of gypsum, which promotes clay flocculation and prevents aggregates from being destroyed and therefore reduces surface sealing. This practice appears to be a viable management method for reclaiming tepetates from the Valley of Mexico at lower cost than presently used techniques.