*3.1. Sample Collection and Analysis*

The study flowchart is shown below, which shows the overall idea of the study, from the background investigation, and data collection, to health risk assessment, using the Monte Carlo model and Visual Minteq, finally deriving the main ion morphology of carcinogenic factors.

In this study, 49 groups of monitoring points with relatively continuous and complete observations were selected as the analysis data (20–30 m below the ground surface); the location of all the monitoring points is shown in Figure 1. These data were collected by the Groundwater Dynamic Monitoring Network of the Ningxia Hui Autonomous Region in 2017–2018 (the dry season (March–April) and the rainy season (July–August)). The collection, sealing, and transportation of water samples were carried out in strict accordance with national technical regulations [31]. Detailed test methods, instrument specifications, and detection limits for each indicator are shown in Table 1. The pH and conductivity of groundwater samples were measured in the field, while other parameters were measured in the laboratory. Test parameters include Na<sup>+</sup> , Ca2+, Mg2+, K<sup>+</sup> , HCO<sup>3</sup> <sup>−</sup>, SO<sup>4</sup> <sup>2</sup>−, Cl−, As, Cr6+, total hardness (TH), and total dissolved solids (TDS), which were analyzed in the laboratory of the Hebei GEO University using standard methods recommended by technical specifications for environmental monitoring of groundwater. The accuracy of the test results was measured by the calculation of percent charge balance errors (%CBE), as follows:

**Table 1.** Analytical methods, instruments, and detection limits of physiochemical parameters.


The study flowchart is shown below (Figure 2), which shows the overall idea of the study, from background investigation, data collection, to health risk assessment, using the Monte Carlo model and Visual Minteq, finally deriving the main ion morphology of carcinogenic factors.

The final detection results show that the relative error of all samples was <±5%, indicating that all 49 samples could be used for analysis.

$$\% \text{CBE} = \frac{\Sigma cations - \Sigma anions}{\Sigma cations + \Sigma anions} \times 100\% \tag{1}$$

**Figure 1.** Study area, geological profile, and location of samplings. **Figure 1.** Study area, geological profile, and location of samplings.
