Soil samples were obtained from along the roads in the urban
area of Lanzhou (Chengguan District, Qilihe District,
Anning District, and Xigu District). Urban soils are mostly
strongly influenced by human activities, always contain a
wealth of xenobiotics. In order to ensure the representativeness
of the soil samples within a small sampling range, the
soil within 10 m of each sampling point was measured with
a Bartington MS2 magnetometer, excluding the outliers of
magnetic susceptibility, and soil with relatively homogeneous
magnetic susceptibility was selected. A total of 117
representational samples were collected from a depth of 2
cm,and the sample for each soil sampling point was mixed
by multi-point acquisition (Figure 1). The sampling points
were selected based on the principle of combining the mesh
point with the actual situation. The samples were placed in
pocket-sized sealable plastic bags with known susceptibility,
and were allowed to dry at room temperature, before being
measured in the laboratory.
Previous works [31–33] have shown that the characteristics
of S0 loess on Jiuzhoutai Mountain are similar to urban
soils found in Lanzhou, therefore samples from Jiuzhoutai
could be used as a background of Lanzhou. An additional
21 samples were collected for S0 loess in the Jiuzhoutai
section at an interval of 5 cm as the background value for
this study.