Understanding hydrodynamics in stratified underground mines is critical for effective mine water management. In Tyrol’s Georgi-Unterbau mine, density stratification in a subvertical shaft between levels 20 and 40 enabled the study of flow and tracer dispersion using four solid tracers and a fluorescent dye introduced at varying depths. An inclined rise connected to the shaft required adding table salt as a tracer, disrupting the delicate density layering with a 0.1–0.2 K difference. While flow paths and velocities were measured, the stratification breakdown caused by the injected brine highlighted the sensitivity of such systems. This study underscores the challenges of external interventions and provides insights for managing stratified mine water.

The needs of today's mines require the use of waste rocks for the construction of components, i.e. tailings dams, so having a good system to monitor and predict the geochemical behaviour of the components where these materials have been used is essential, especially regarding the acid rock drainage and metal leaching. The application of this methodology strengthens the use of geochemistry to add value to operations and reduce closure costs in mining projects.

The use of cells in the field for more than six years has allowed the evaluation of acid rock drainage and metal leaching, both of isolated materials and of combinations between different lithologies, which has allowed defining the ranges of acceptable chemical and mineralogical compositions for construction materials; as well as the optimal combinations of rocks. The installation of oxygen, temperature, electrical conductivity and water content sensors has allowed these parameters to be monitored inside the component for a period of more than three years. Using the above results, together with data on drainage flow rates and water quality, a reactive transport model has been calibrated under unsaturated conditions and with multiphase flow (water and air) in non-isothermal conditions, to predict the long-term behaviour of the materials used in the construction and to assess the future drainage water quality of the component.

The good correlation between the field cell tests and the evaluated behaviour of the different materials inside the component, demonstrate the value of these field tests to determine the compositions of those materials that can or cannot be used in the construction of certain mining components. Furthermore, the information generated by the sensors, especially those of water content and temperature, has proven to be of great value in understanding the flow of water inside the component and the generation of acid drainage associated with the oxidation of pyrite (exothermic reaction), allowing to corroborate the compositional ranges of the materials used in the construction.

The use of this methodology, in line with the mine planning, allows for better use of waste rock instead of considering it as a liability, minimising the volume of rock sent to the waste dump and, therefore, minimising its handling and the costs associated with mine closure. Likewise, the establishment of long-term geochemical monitoring and control programmes is of great help in optimising the use of these materials and planning mine closure activities.

The primary objective of the study was to develop and implement a coupled groundwater-surface water flow and solute transport modelling workflow to evaluate the influence of mine water rebound from an abandoned underground coal mine on the overlying aquifers and nearby streams. This workflow is crucial as it enhances the accuracy of simulations, improves risk assessments, informs environmental management strategies, and supports sustainable development.

The developed workflow relies on the industry standard groundwater modelling code MODFLOW 6 (MF6) and the FloPy Python library and is able to represent groundwater-surface water interactions dynamically in space and time both for flow and solute transport. This comprehensive approach ensures accurate simulation of complex hydrological and hydrogeological processes. The modelling workflow aims to identify contaminant migration pathways and potential receptors of mine water discharge, by estimating increases of concentration for four key chemical species: chloride, sulfate, total iron, and ammoniacal nitrogen.

The modelling results indicate that important areas of the aquifer and nearby streams could potentially be affected by discharge from the abandoned mine, under the considered assumptions associated with a worst-case scenario. The model also suggests substantial groundwater-surface water interaction, where contaminants migrate from the aquifer into the stream and vice versa depending on the location and time of the year, with relatively fast travel times in the streams.

The ability to model groundwater-surface water interactions dynamically provides a more accurate representation of real-world hydrological and hydrogeological processes, enhancing the reliability of risk assessments, more effectively informing environmental management strategies, and supporting the development of effective remediation plans. In particular, it has been observed that coupled modelling allows to present solute transport results in easy-to-understand figures and animations, which promote understanding and engagement of the different stakeholders. By engaging stakeholders through clear visualizations and predictions, it fosters informed decision-making, ultimately contributing to the preservation of natural resources and the promotion of a sustainable future. The additional effort involved in the development of the coupled approach is marginal when compared to traditional modelling approaches, making it a valuable tool for environmental scientists and policymakers.