Integration of self-organizing maps, statistical analysis, and hydrogeochemical modeling methods to identify spatio-seasonal variations in mine water quality

Abstract

Monitoring the quality of mine drainage is an important task in the Ruhr coalfield in western Germany, especially after decommissioning all mining activities in 2018. In this study, seasonal and spatial variations in mine water quality of abandoned coal mines around the lower Ruhr river area were investigated. Statistical analysis, self-organizing maps (SOMs), and hydrogeochemical modeling were used to reveal changes in the water quality of 103 water samples collected between January 2018 and November 2019. An analysis of physical and chemical properties using SOM subdivided mine water samples into five groups: freshwater (groups 1, 2, 3, and 4) and brackish water (group 5) with different physical and chemical characteristics. Freshwater samples were mostly distributed in the southern and central parts, whereas brackish water samples were mostly distributed in the northeastern part of the study area. Approximately 18% of the sampling locations experienced significant seasonal changes during the study period, while 82% of the remaining locations showed only slight variations. Across all groups, seasonal variations in the water quality were less pronounced than differences between southern and northeastern sampling locations. A significant increase was observed in major ion concentrations, TDS, and EC values from southern to northern and northeastern sampling points during the monitoring period. A general trend of approaching saturation state from studied mineral phases was detected with increasing TDS concentrations from groups 1, 2, 3, and 4 to group 5. Trace element concentrations were relatively low, except for several streams with higher Fe and Mn concentrations, and varied between seasons. The SOM approach helped to identify the seasonal changes in mine water properties in the area. The evaluation of changes in mine water quality is useful for the management and protection of water resources, especially drinking water resources, in the densely populated Ruhr area.