S1 S2 S3 Baoji Xi’an Guangzhou Hong Kong Ottawa Avilés 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.