## 2 Materials and Methods
### Methods for HM Pollution Assessment
#### 2.3.2 Human Health Risk Assessment Based on Monte Carlo Simulation  
A comprehensive environmental health risk assessment was conducted using the four-step methodology endorsed by the U.S. Environmental Protection Agency [36; 37; 38; 39], encompassing both non-carcinogenic and carcinogenic risk quantification through oral ingestion and dermal exposure pathways for adult and pediatric populations. To enhance the scientific rigor and statistical reliability of the health risk evaluation, a probabilistic risk assessment was implemented through Monte Carlo simulations, with computational parameters standardized at 10,000 iterations and a 95% confidence interval for probabilistic determination of HM-induced health impacts [39; 40].  
The daily average exposure doses via hand-to-mouth ingestion and dermal contact are calculated using the following equations [41; 42]:  
\[CDD_{ing}=\frac{C_{i}\times IR_{ing}\times ED\times EF}{BW\times AT}\times K \tag{3}\]  
\[CDD_{derm}=\frac{C_{i}\times SA\times AF\times ABS\times ED\times EF}{BW\times AT }\times K \tag{4}\]  
Figure 1: Location of the study area and distribution of sampling sites.  
where \(CDD_{ing}\) and \(CDD_{derm}\) are the daily average exposure doses via hand-to-mouth ingestion and dermal contact (mg\(\cdot\)(kg\(\cdot\)d)\({}^{-1}\)), respectively; \(C_{i}\) is the measured concentration of HM \(i\) (mg\(\cdot\)kg\({}^{-1}\)); \(IR_{ing}\) is the hand-to-mouth ingestion rate (mg\(\cdot\)d\({}^{-1}\)); \(ED\) and \(EF\) are the exposure duration (a) and exposure frequency (d\(\cdot\)a\({}^{-1}\)), respectively; \(BW\) is the average body weight (kg); \(SA\) and \(AF\) represent the skin exposure surface area (m\({}^{2}\)) and skin adherence factor (mg\(\cdot\)(cm\({}^{2}\cdot\)d)\({}^{-1}\)), respectively; \(AT\) denotes the average exposure time (d); \(ABS\) is the dermal absorption factor (dimensionless); and \(K\) is the unit conversion factor. The probabilistic distributions of exposure parameters based on Monte Carlo simulation and the values of parameters involved in Equations (3) and (4) are presented in Table S1.  
HMs such as As, Cd, Cr, Cu, Ni, Pb, and Zn pose non-carcinogenic risks to human health, among which As, Cd, Cr, Ni, and Pb also have carcinogenic risks [43]. The non-carcinogenic risks associated with HM elements can be determined using the following calculation formula [36; 41]:  
\[HI=\sum HQ_{ij}=\sum\frac{CDD_{ij}}{RFD_{ij}} \tag{5}\]  
where \(HI\) is the total non-carcinogenic risk (dimensionless) and \(HQ_{ij}\) and \(RFD_{ij}\) are the non-carcinogenic risk value and reference dose value (mg\(\cdot\)(kg\(\cdot\)d)\({}^{-1}\)) of HM \(i\) via exposure pathway \(j\), respectively, with specific reference values presented in Table S2. Generally, when \(HQ\) or \(HI\leq 1\), it indicates no non-carcinogenic risk; when \(1<HQ\) or \(HI\leq 10\), it indicates the presence of non-carcinogenic risk; when \(HQ\) or \(HI>10\), it indicates a significant non-carcinogenic risk.  
The carcinogenic risk of the HMs As, Cd, Cr, and Pb in the sediments to different populations is calculated using the following equation [36]:  
\[TCR=\sum CR_{ij}=CDD_{ij}\times SF_{ij} \tag{6}\]  
where \(TCR\) is the total carcinogenic risk value (dimensionless) and \(CR_{ij}\) and \(SF_{ij}\) are the carcinogenic risk value and carcinogenic slope factor value (mg\(\cdot\)(kg\(\cdot\)d)\({}^{-1}\)) of HM \(i\) via exposure pathway \(j\), respectively. The RFD and SF values for different HMs via hand-to-mouth ingestion and dermal contact exposure pathways are presented in Table S2. In terms of carcinogenic risk, a \(TCR\) or \(CR\leq 10^{-6}\) is considered to pose no carcinogenic risk; when \(10^{-6}<TCR\) or \(CR\leq 10^{-4}\), the risk is deemed acceptable; a \(TCR\) or \(CR>10^{-4}\) indicates significantly elevated carcinogenic risk.