## 2 Materials and Methods
### Sample Collection and Analysis  
In the primary wheat- and corn-growing areas of Hebei plain, 230 soil surface layer and agricultural samples were collected. The sample points were located using GPS. The soil sample points were selected by the diagonal method (the distance from the ground edge was greater than 1 m). Wheat grain samples of 500 g were collected at each point using a 1 m \(\times\) 1 m sample square. Within the crop sample points, the cultivated soil layer was collected between a depth of 0 to 20 cm. When the soil surface was being collected, weeds, grassroots, gravel, bricks, fertilizer clumps, and other debris were removed. To ensure that representative samples were taken from each sample site, during sampling one point was taken as a fixed point, and three to five sub-samples were collected 20 m around it. Sampling was conducted until at least 1.0 kg of material had been collected. Cultivated soil and wheat samples were collected at the same time, whereas corn samples were collected separately. The wheat and corn samples were dried and sent to a laboratory for analysis. Soil samples used for chemical analyses were first air-dried and passed through a 20-mesh nylon screen to remove roots, residue, and stones.  
The total concentrations of Cu, Zn, Cr, Cd, Ni, Pb, As, and Hg in soils were extracted by an acid digestion mixture (HNO\({}_{3}\) + HF+ HCl\({}_{4}\)). Ni, Cu, Zn, Cr, Cd, and Pb levels were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS,7500a, Agilent Technologies, Calif, USA). As and Hg levels were determined by atomic fluorescence spectrometry (AFS) [34].  
Soil pH was measured using a pH meter (1:2.5 soil:water mixture). Total organic carbon (TOC) was analyzed using the potassium dichromatic oxidation and REDOX volumetric method [35; 36; 37]. Next, the clay samples were determined using the hydrometer method [38]. CeC was determined with the ammonium acetate method and N was determined using the acid-base titration volumetric  
Figure 1: The sampling site map of soils and crops in the study area. (The city names Langfang, Baoding, Shijiazhuang, Cangzhou, Hengshui, Xingtai, and Handan have the abbreviations LF, BD, SJZ, CZ, HS, XT, and HD.).  
method. The content of Fe, P, K, Mn, Ca, Mg, and other soil properties was determined using the powder tablet XRF method [39].  
National level soil reference materials (GBW07402 and GBW07406) were used to check the data for accuracy and precision. Random soil samples were selected and checked for quality control. Abnormal points were identified in relation to the quality checks. Quality analyses were also made for replicated samples, where analyses were repeated to assess whether sampling or analysis errors had any significant influence on regional geochemical changes.  
The wheat and corn samples were cleaned, dried, and broken up before being passed through a 2-mm sieve. The samples were then homogenized before being analyzed. Crop grain samples were digested by a mixture of HNO\({}_{3}\) and HClO\({}_{4}\) (4:1, v:v). The metal(loid) concentrations in the samples were determined as described by [33]. Quality control was implemented in accordance with GBW08502 (rice flour), GBW08503b (wheat flour), GBW10010 (rice), GBW10011 (wheat), and GBW10013 (soybeans), which were also used for accuracy control. The relative error (RE) was less than 30%. The precision of the analyses for the crop metal(loid)s were As (98.5%), Cd (99.5%), Hg (99.6%), Pb (99.9%), Cr (99.9%), Ni (100%), Cu (100%), and Zn (100%).