Color as a proxy for heavy metal pollution in soils of Mexico City

The objective was the evaluation of color as a proxy for soil contamination by heavy metals in Mexico City. The concentrations of heavy metals were measured by X-ray fluorescence, and the color was determined with the Munsell Tables and colorimeter using Red Green Blue (RGB) system. From the Hue, Value and Croma values, five soil groups were formed by color: I, Dark reddish brown, 5YR 4/2; II, Dark gray, 7.5YR 4/1; III, Olive gray, 5Y 5/2; IV, Gray, 5YR 5/1; V, Pinkish gray, 7.5YR 6/2. With the Red Green Blue colorimetric values, three color indexes were calculated: Hue index, Redness index and saturation index. Validation of soil groups was greater with color indexes than with heavy metals. Groups I and II of dark color had the highest concentrations of Cr, Cu, Pb and Zn; they also had the highest percentages of Fe2O3, MnO, and TiO2. Light colored groups IV and V had the highest percentages of Al2O3, MgO and SiO2. Group III had intermediate values of Cr, Cu, Pb, Zn, Fe2O3, MnO, and TiO2. For the first time, it is reported that the color of the urban soil of Mexico City can be used as a rapid analysis technique to identify sites contaminated with heavy metals.