*3.3. Monte Carlo Simulation and Visual Minteq*

Due to individual differences and sampling limitations, the results of the health risk calculation were highly uncertain. In order to overcome this defect, this study used the Monte Carlo Simulation (MCS) method, which is a probabilistic statistical mathematical

method to evaluate uncertainty by random sampling each variable value [40]. This simulation method was combined with the U.S. Environmental Protection Agency risk assessment technology to quantify and minimize the uncertainty, and analyze the sensitivity of pollutant exposure in drinking water wells. Different numbers related to each variable were sampled for a large number of digital repetition operations. The main steps are as follows: (1) The respective distribution functions were fitted to the data of each variable, and the hypothetical variable was defined; (2) The calculation formula was inputted and the decision variable was defined; (3) A random sampling was conducted from the distribution of the hypothetical variables; (4) The randomly selected parameter sequence was used for a large number of repeated operations, to obtain as output the probability distribution of the operation results (Figure 3). In this study, the Monte Carlo Model with 10,000 iterations was used to evaluate the carcinogenic risk of exposure to As and Cr6+ for children and adults during the rainy and dry seasons from 2017 to 2018. Then, sensitivity analysis was carried out using the MCS method with 10,000 repetitions using Oracle Crystal Ball®. Finally, the frequency diagram and sensitivity analysis diagram drawn by it were used to identify the input parameters with the greatest impact on the output of the risk assessment model. After using the sensitivity analysis to obtain the main influencing factors of health risk assessment results, Visual Minteq was used to analyze the possible ion forms of the main factors in the hydrogeological environment of the study area. *Water* **2022**, *14*, x FOR PEER REVIEW 10 of 23 analyze the possible ion forms of the main factors in the hydrogeological environment of the study area.

**Figure 3.** Monte Carlo model operation flowchart.

**Figure 3.** Monte Carlo model operation flowchart.

4.1.2. Non‐Carcinogenic Risk Assessment

**4. Results and Discussion** *4.1. Health Risk Assessment* 4.1.1. Health Risk Assessment

risk of eight ions and the non‐carcinogenic risk of two heavy metal ions [42].

The concentrations of ions in water were used to assess the possible health risks from human exposure to drinking water through both oral intake and skin contact. This study considered two groups, namely adults and children [41], and evaluated the carcinogenic

The Hazard Index (HI) was used to measure non‐carcinogenic health risk as the sum of the hazard quotients (HQs) from both oral intake and dermal contact of several parameters including F−, NH4+, NO2−, NO3−, Cr6+, Mn, As, Fe (including Fe2+, Fe3+), and
