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.