2. Materials and methods
2.1. study area
Chengdu is a key hub city in China’s west, located in the hinterland the
Chengdu Plain. The area is mainly formed by the alluvium of Minjiang
River, Tuojiang River and its tributaries, and some gullies and valleys,
and the soil-forming parent material is mainly gray alluvium of Hori
River and gray-brown alluvium of Tuojiang River in the primary terrace,
and ice carbon and ice water sediment in the old alluvium of the Fourth
Pleistocene in the secondary and tertiary terraces, with flat terrain,
fertile soil and various parent materials, and rice soils and purple
soils are the main types of soil. (Figure 1 ). The area is dominated by
a humid monsoon climate in the subtropics, with an average annual
population of 15.2-16.5°C and 900-1300 mm of precipitation
correspondingly, making it a major production base for agricultural
products such as grain and fruits in the Sichuan basin (Zhang et al.,
1999; Hu et al., 2004; Huang, 2011).
2.2. sample collection and analysis
Samples of soils were sampled in September 2008 and September 2017,
respectively. Sampling points were set up according to the distance from
the central city and the economic development, which were divided into
suburban, suburban, and distant suburban strata. The suburban stratum
includes: Wenjiang District, Shuangliu County, Pixian County, and
Longquanyi; the suburban stratum includes: Qingbaijiang, Xindu; the
distant suburban stratum includes: Xinjin County, Jintang County,
Pujiang County, etc. In the two phases of data, 226 soil samples were
collected in September 2008, and the sampling points were collected
according to the three major circles of suburbs, suburbs, and distant
suburbs, with a density of about 1 sample per km; on this basis, to
further investigate the characteristics of spatial variability of heavy
metals, 389 soil samples were taken in September 2017, about 2 samples
per km. in the 2017 sampling, in addition to being able to collect soil
samples at the original sampling point positioning to collect soil
samples, but also fully considered urban development, land use type
change, spatial structure characteristics, and other factors, as far as
possible corresponding or close to the original sampling point of the
agricultural land to collect soil samples. Collection of soil samples
from the 0-10 cm surface layer of the tillage layer with a wooden
shovel, and collected according to the plum sampling method, and the
soil samples were thoroughly mixed and 1.5 kg of soil samples were
retained according to the quadrat method. The coordinates of the sample
points were located with a handheld GPS from Garmin, USA during the
sampling process, and sampling points are distributed as shown in Figure
2.
The samples were dried by natural air in indoor drying stalls, picked up
to remove stones, plant roots, and other foreign matters, ground and
sieved through 200 mesh nylon mesh for analytical tests. The samples
were determined by atomic fluorescence spectrometry (AFS) for As and Hg,
by flame atomic absorption spectrometry (AAS/ FAAS) for Cr and Pb, and
the Cd was detected by graphite furnace atomic absorption
spectrophotometry (GF-AAS).
2.3. data collection and analyses
In this study, Microsoft Excel 2007 was used to calculate and analyze
the elemental concentration data, SPSS 13.0 was used to carry out
statistical analysis, ArcGis 10.2 and Kriging interpolation methods were
used for spatial analysis, and use the positive matrix factorization
(PMF) model (USEPA PMF 5.0) to assess the origin of the resolved heavy
metals.