## 3 The Fixed-Value Method on Ni Geochemical Map  
To create a more objective geochemical map, we adopt the 19-level fixed-value method to contour the geochemical map of Ni. In this method, the key task is to determine the 18 fixed values, which divided the concentrations into 19 levels further categorized into six major classes. In the 19-level fixed-value method, [PERSON] et al. [11] and others [12; 14] determined 18 fixed values by relying on the different cumulative frequency values of stream sediments of the RGNR project [1] and soils of the NMPRGS project [28]. In this paper, these statistical parameters are also considered. Additionally, since Ni is one of the heavy metals [29; 30], and there are different screening values on different pH values for Ni concentrations in agricultural land specified in the Chinese Standard GB15618-2018 [31], the Ni screening values in GB15618-2018 are also taken into consideration when determining these 18 fixed values. In the 19-level fixed-value method, the first fixed value for Sn and Li were set at 1 ug/g and 5 ug/g corresponding to their detection limits in the RGNR and NMPRGS projects 12, respectively. Therefore, the first value for Ni is set at 2 ug/g which corresponds to the detection limits of Ni in the RGNR and NMPRGS projects. The second value is set at 6 ug/g, which corresponds to three times the detection limit and is just right to the value of the 2% cumulative frequency in stream sediments of the RGNR project [7]. The third value is 10 ug/g, corresponding to the 15% cumulative frequency value in the stream sediments. The fourth value is suggested to be 18 ug/g, which is the mean of the two values: the 25% cumulative frequency value of 14.5 ug/g in the stream sediments of the RGNR project and the 25% cumulative frequency value of 21 ug/g in soils in the NMPRGS project [28]. The fifth value is proposed as 25 ug/g, which is the mean of 23 ug/g of the 50% cumulative frequency value in the stream sediments and 27 ug/g of the 50% cumulative frequency value in the soils. The sixth and seventh fixed values are set at 32 and 39 ug/g, respectively, which correspond to the 75% and 85% cumulative frequency values in stream sediments of the RGNR project. The eighth fixed value is suggested to be 48 ug/g, which is close to the 92% cumulative frequency value of 46 ug/g in stream sediments from the RGNR project. The ninth fixed value is set at 60 ug/g, which corresponds to the minimum screening value for agricultural land in GB15618-2018 [31] and is equal to the mean of two values: the 97% cumulative frequency value of 68 ug/g in stream sediments of the RGNR project and the 97.5% cumulative frequency value of 52 ug/g in surface soils in the NMPRGS project. The tenth fixed value is 70 ug/g, which corresponds to the intermediate screening value of 70 ug/g for agricultural land specified in GB15618-2018 [31]. The eleventh fixed value is proposed as 100 ug/g, which corresponds to the intermediate screening value for agricultural land of 100 ug/g in GB15618-2018 and is close to the 98.8% cumulative frequency value of 102 ug/g in stream sediments of the RGNR project. The twelfth fixed value is set at 190 ug/g, which corresponds to the maximum screening value for agricultural land in GB15618-2018 [31]. In the 19-level fixed-value method, the fifteenth value of Sn is 200 ug/g, corresponding to the cutoff grade of the Sn placer [11], and the fifteenth value of Li is 232 ug/g, which is the cutoff grade of the clay-type Li deposit [12]. The cutoff grade for Ni as an associated economic metal in copper deposits (DZ/T0214-2002) [32] and iron-manganese deposits (DZ/T 0200-2020) [33] is 0.1%. Therefore, the fifteenth fixed value is suggested as 1000 ug/g, which corresponds to the concentration of Ni as an associated economic metal. Based on the twelfth and fifteenth fixed values, the thirteenth and fourteenth fixed values for Ni are interpolated to be 245 ug/g and 495 ug/g, respectively.