Accumulation Status of Heavy Metals in Northwest China and Analysis of Causes in Typical Areas

Since 2004, a total of 290 700 km² of 1:250 000 land quality geochemical survey has been completed in Northwest China. The contents of eight heavy metals elements such as cadmium, mercury, arsenic, lead, chromium, copper, nickel and zinc in 66006 topsoils have been obtained by X-ray fluorescence spectrometry (XRF), atomic fluorescence spectrometry (AFS) and inductively coupled plasma mass spectrometry (ICP-MS/AES). The content and accumulation of heavy metals in topsoil in Northwest China were analyzed by using the methods of Geo-accumulation index, single factor index and Nemerow multi-factor index. On this basis, the causes of heavy metal accumulation in typical areas such as Baiyin and Laji mountain are discussed. The results show that:① the single pollution indexes of cadmium, mercury, arsenic, lead, chromium, copper, nickel and zinc in the soil are more than 99.26%, indicating that the soil in Northwest China is not polluted by heavy metals and they are as a whole in good condition. ② The sites above the screening value of soil heavy metals in Northwest China are up to 52.7% and dominated by geological accumulation, which only four sites reach the control value. It shows that, the content of heavy metals in soil is very difficult to reach the control value under natural conditions and pose low-risk threat to ecosystem health. The sites above the screening value caused by human factors are mainly distributed in Baiyin, Tongguan, Xi'an and other areas, and the points greater than the control value are concentrated in Baiyin and Tongguan. ③ It is recommended to carry out in-depth research, especially the bioavailability of soil heavy metals in the arid area of Northwest China, so as to further optimize the control value and screening value of heavy metals under different backgrounds.