S1 S2 S3 Baoji Xi’an Guangzhou Hong Kong Ottawa Avil├ęs Calcutta Madrid
Cr 71 183 277 126.7 167.3 78.8 124 43.3 41.6 54 61
Mn 177 30 35 804 687 481 594 432 1661 619 362
Table 10: Comparison of the heavy metal (Cr, Mn) concentration (ppm) of the three sample points and other selected cites.