Fig. 3: Predictor importance and relationships between the observed and predicted values of heavy metal concentrations.

a The relative importance of predictors; b Comparison of predicted and observed concentrations of heavy metals in inland waters after log transformation. In regression plot (b), the dashed line indicates the 1:1 line, the blue line indicates the regression line between predicted and observed values, and the R2 represents the evaluation of random forest models. MAT mean annual temperature, AET actual evapotranspiration, LSR land surface runoff, HDI human development index, Primary primary treatment of wastewater treatment plants (WWTPs), Second secondary treatment of WWTPs, and Tertiary tertiary treatment of WWTPs.