Chromium (Cr) geochemical mapping based on fixed-values' method: Case studies in China

Geochemical map is a kind of hierarchical color contour map that can be expressed visually. The commonly used mapping methods are limited to a fixed area with fixed quantity of data and unfavorable for comparative study in different areas. A new method with fixed values to contour Cr geochemical map is presented here with case studies in China. The fixed-values' method of Cr geochemical map is colored hierarchically with 26 levels on 25 values of Cr concentration and six classes are classified on these 26 levels with increasing Cr concentrations. The cumulative-frequency method used commonly and the new presented method are compared in Lixian town of Daxing district in Beijing, China as a case study on the geochemical survey scale of 1:20 000. The results on the fixed-values' method show that Cr concentrations in this area belong to the high-background area of Cr con-centration. Based on the China Geochemical Baselines (CGB) project data of Cr concentrations on the geochemical survey scale of 1:8 000 000, the Cr geochemical map in Chinese mainland is contoured on the fixed-values' method. Two northeast lines of Lhasa-Changchun and Qinzhou-Hangzhou can be used to separate the whole mainland into three areas as the low-background area in the northwest, the high-background area in the middle, and the low-background area in the southeast. The results in Daxing district including the Lixian town in Beijing on the scale of 1:8 000 000 are consistent with the results from the Lixian town map on the scale of 1:20 000. This indicates that the fixed-values' method of Cr geochemical map provides a methodological technique for Cr geochemical mapping universally and a reference for other elements' geochemical mapping.