METHODOLOGY FOR A SOIL GEOCHEMICAL AND MINERALOGICAL SURVEY OF MEXICO

In 2008, the Servicio Geológico Mexicano (Mexican Geological Survey, SGM) started a program of collection of soil samples of the Mexican Republic. Sample sites were determined randomly with a density of about one site per 1,600 km² (1,283 planned sites). The sampling protocol at each site includes the collection of a sample from 0-5 cm depth, and composites of the A, B, and C horizons as long as they are at less than one meter depth (a total of 3,526 samples). From the total of planned sites, we have visited 1,259 with the remaining still pending due to social and security concerns mainly in the State of Michoacán. Chemical analyses include determinations of Al, Ca, Fe, K, Mg, Na, Ti, Ag, Au, As, Ba, Be, Bi, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Ga, Gd, Ho, La, Li, Lu, Mn, Mo, Nb, Nd, Ni, P, Pb, Pr, Re, Rb, Sb, Sc, Se, Sm, Sn, Sr, Ta, Tb, Th, Tl, Tm, U, V, W, Y, Yb, and Zn by inductively coupled plasma-mass spectrometry after near-total digestion in a mixture of HCl, HNO₃, HClO₄, and HF (591 samples analyzed) and inductively coupled plasma-optical emission spectrometry (2,633 samples analyzed) after partial digestion. Splits from the samples taken from the A, B, and C horizons are also currently being analyzed for mineralogy with X-ray diffraction (~50 of them have been analyzed so far). This study will serve to establish a geochemical baseline to monitor environmental changes, to locate areas impacted with toxic elements, and to support land management policies, environmental investigations, and mining or exploration projects, among other applications.