Prediction of sulfate concentrations in groundwater in areas with complex hydrogeological conditions based on machine learning.

The persistent and increasing levels of sulfate due to a variety of human activities over the last decades present a widely concerning environmental issue. Understanding the controlling factors of groundwater sulfate and predicting sulfate concentration is critical for governments or managers to provide information on groundwater protection. In this study, the integration of self-organizing map (SOM) approach and machine learning (ML) modeling offers the potential to determine the factors and predict sulfate concentrations in the Huaibei Plain, where groundwater is enriched with sulfate and the areas have complex hydrogeological conditions. The SOM calculation was used to illustrate groundwater hydrochemistry and analyze the correlations among the hydrochemical parameters. Three ML algorithms including random forest (RF), support vector machine (SVM), and back propagation neural network (BPNN) were adopted to predict sulfate levels in groundwater by using 501 groundwater samples and 8 predictor variables. The prediction performance was evaluated through statistical metrics (R, MSE and MAE). Mine drainage mainly facilitated increase in groundwater SO while gypsum dissolution and pyrite oxidation were found another two potential sources. The major water chemistry type was Ca-HCO. The dominant cation was Na while the dominant anion was HCO. There was an intuitive correlation between groundwater sulfate and total dissolved solids (TDS), Cl, and Na. By using input variables identified by the SOM method, the evaluation results of ML algorithms showed that the R, MSE and MAE of RF, SVM, BPNN were 0.43-0.70, 0.16-0.49 and 0.25-0.44. Overall, BPNN showed the best prediction performance and had higher R values and lower error indices. TDS and Na had a high contribution to the prediction accuracy. These findings are crucial for developing groundwater protection and remediation policies, enabling more sustainable management.