Dispersión de Ni y Cr en sedimentos y suelos superficiales derivados de piroxenitas, serpentinitas y basaltos de la cuenca San Juan de Otates, Estado de Guanajuato, México

Resumen

The particular aim of this work is to determine within the hydrologic San Juan de Otates system, State of Guanajuato, Mexico, the dispersion characteristics of total Ni and Cr in sediments and surficial soils derived from ultramaphic rocks, and also to confirm if there is dispersion of these elements towards neighboring subbasin. Usually both of these elements are found in nature associated to piroxenites, serpentinites and basalts, rocks which are all found in the upper part of the basin. For the sake of analysis the area under study was divided into five subbasins. In order to determine the concentration levels of these elements 56 sediment sites and 46 soil sites were chosen at a depth of 0-20 cm; the sampling sites were selected considering geology, geoform, slope, drenage, soil unities, land use, and civil engineering works. In order to stablish the association existing between mineralogy and metalic ions the macroscopy technique was used. The samples were digested in a microwave oven, utilizing aqua regia as extractant. Concentrations were determined by atomic absorption flame spectrophotometry, with detection limits of 0.004 mg/kg^-1 for Ni and 0.002 mg/kg^-1 for Cr. The ILWIS geographical information system was used for mapping the distribution of metalic ions. The statistical analysis did not show significant differences in the mean Ni and Cr contents both in soils and sediments. The grater Cr levels were found in the San Juan de Otates subbasin, while Ni showed a greater concentration in three subbasins in comparison with the rest of the area. The multivariate analysis pointed out that the two variables used (Ni and Cr) were not sufficient to make a proper choice of sampling sites in the different subbasins. According to mean world wide range reported for Ni and Cr, there are areas with natural contents higher than the ones found in San Juan de Otates basin. The Ni and Cr dispersion from its source point, shows geographical irregularity. In El Bajío, the channel of Santa Ana del Conde, which joins the arroyos of San Juan de Otates and El Juache, has lately increased the contents of the elements studied. Finally, the maps of Ni and Cr show, with respect to superficial soils and sediments, that there does not exist dispersion of these elements towards neighboring subbasins.