## 2 Materials and Methods
### Analytical Methods  
The collected surface soil samples were first naturally air-dried. Then, the soil samples were sieved through a nylon sieve of 10 mesh. Finally, the soil samples were crushed to less than 200 mesh (\(\leq\)0.074 mm) using a pollution-free planetary ball mill before chemical analysis. The collected crop samples (maize, walnut, and rice) were preprocessed through dehulling or peeling, and rinsed with clean water and then deionized water three times. They were dried naturally and then crushed to pass through nylon sieves of 40-60 mesh in an agate mortar before elemental analysis. And they were digested into sample solution with nitric acid and hydrogen peroxide in a dedicated microwave digester at high temperature and pressure. Then, the contents of heavy metals were determined using an inductively coupled plasma spectrometer, which can scan the plasma with masses ranging between 5 u and 250 u, with the minimum resolution at a peak height of 5% and a peak  
Figure 1: Location of the study area in China (**a**) and its geological map modified after [PERSON] et al. [30] with sample locations of surface soils and crops (**b**). Notes in (**b**): 1—Jurassic sandstone; 2—Upper Triassic Waigucun Formation sandstone, conglomerate, limestone, and mari; 3—Triassic Ganbatang Formation metamorphic conglomerate, sandstone, and phyllite; 4—Middle-Permani Yangbazhai Formation mudstone, sandstone, and siltstone; 5—Lower-Permian Baliu Formation limestone, siltstone, fine-grained sandstone, mudstone, and silicous rock; 6—Carboniferous slate, phyllite, fused breccia, and tuff; 7—Devonian sandstone, siltstone, mudstone, limestone, and silicous rock; 8—Palaeozoic Waimaidi Formation metamorphic sandstone, siltstone, and phyllite; 9—Neoproterozoic Dahebian Formation quartz schist, and phyllite intercalated with leptynite; 10—Paleoproterozoic Qing-shuihe Formation of Ailaoshan Group schist, leptynite, and marble; 11—granite; 12—potassium-feldspathic granite; 13—peridotite; 14—petrological boundary; 15—fault; 16—main residential place; 17—locations of surface soil samples; 18—locations of maize samples; 19—locations of walnut samples; and 20—locations of rice samples.  
width of 1 u; and an atomic fluorescence spectrometer, which is equipped with a special mercury lamp.  
All chemical analyses were performed at the Center of Laboratory in Yunnan Exploration and Development Bureau of Geology and Mineral Resources (Kunming Testing and Quality Supervision Center for Geological and Mineral Products in the Ministry of Natural Resources, Yunnan Province). The analysis of soil and crop samples was strictly carried out in accordance with the requirements of the Specification of Land Quality Geochemical Assessment (DZ/T 0295-2016) [40]. The data of six items (As, Cd, Cr, Hg, Pb, and pH) were analyzed in this study.  
The analytical methods and detection limits of soil samples were as follows. The pH was determined using the ion selective electrode (ISE) with a detection limit of 0.01. The contents of As and Hg were analyzed using the atomic fluorescence spectroscopy (AFS) method with detection limits of 0.05 ug/g and 0.4 ng/g, respectively. Cadmium, Cr, and Pb were determined using inductively coupled plasma-mass spectrometry (ICP-MS) with detection limits of 10 ng/g, 0.82 ug/g, and 0.96 ug/g, respectively. With respect to the crop samples, Cd, As, Cr, and Pb were determined through ICP-MS with detection limits of 10 ng/g, 0.1 ug/g, 0.05 ug/g, and 0.02 ug/g, respectively. Mercury was determined using AFS with a detection limit of 1 ng/g.  
The analytical methods, detection limits, accuracy, and precision all met the requirements in Specification of Land Quality Geochemical Assessment (DZ/T 0295-2016) [40]. The analytical qualification rates, controlled by the first-grade standard samples and spiked recovery, were larger than 99.6% for soil samples and 99.4% for crop samples.