Modelling the hydrogeochemical evolution of mine water in a decommissioned opencast coal mine

Abstract

Highlights• Kinetic geochemical modelling calibration method proposed-calibration against leaching test data was successful.• Transient source term transport modelling successful -calibrated against existing groundwater monitoring data.• Improved confidence in numerical transport modelling of the initial flush from opencast mines• Contaminant plume calibration in numerical models is possible using the proposed methodology. AbstractA method for the geochemical modelling of transient contaminant release from rehabilitated opencast coal mines, including calibration against existing data, is proposed. The need for such a methodology is illustrated by a directive received by a decommissioned opencast coal mine in Mpumalanga, South Africa. Groundwater monitoring data, geochemical analyses, numerical flow modelling and geochemical modelling are used to model the hydrogeochemical evolution of mine water over time. Models presented in this study are based on a conceptual model detailing groundwater levels and flow directions, hydraulic conductivities, groundwater chemistry, precipitation, evaporation, surface water bodies and potential sources. Additional to this, mineralogical analyses, leaching tests and acid-base accounting were performed to obtain a better understanding of the site geochemistry. A geochemical model was constructed which was used to obtain a statistically representative mineral assemblage based on laboratory data which was calibrated against leaching test data. This assemblage was simulated in field conditions as input to a numerical flow and transport model. The transport of sulfate was modelled accordingly and sulfate concentrations from monitoring data were used for chemical calibration. Following this, long term contaminant release was simulated. Calibration graphs from the transport model indicated concentrations within a 20 mg/L error margin, showing that the proposed methodology can be used to calculate contaminant concentrations in an aquifer over time within an acceptable range. This approach could provide an improved estimate of the duration of the first flush 2 which, upon completion, will transform decommissioned collieries into large scale reservoirs of utilisable groundwater.

Characteristics of Pores in Coals Samples Exposedto Acid Mine Drainage

“…Acid mine drainage interacts with coal because of its complex composition 11 . In addition, because coal contains several minerals 12 , its physical and chemical properties change significantly after long immersion in water [13][14][15] .…”

Groundwater Chemistry and Stratification in the Flooded Hard-Coal Mine Shaft “Nowy I” (Nowa Ruda Region, SW Poland)

“…Geochemical processes in flooded mines along with water stratification are described in detail by Younger [14][15][16][17][18], Nutatall [19] and Wolkersdorfer [20]. Some papers also present mine water issues in opencast mines [21,22].…”

“…Mine water stratification also causes a decrease in mineralization of the excess water flowing to the surface and limiting of the water mixing zone range resulting from the flooding rate of individual mining levels. For several years, research has also been carried out worldwide on modelling the stratification phenomenon to forecast hydrogeological changes and minimize the environmental impact of mine closures, and also to establish the best practices in the long-term management of a closed mine [22,33]. Monitoring of the water in a flooded mine in the years following its closure allows for tracking of the flooding process and changes in hydrogeological conditions.…”

SIMPL: A Simplified Model-Based Program for the Analysis and Visualization of Groundwater Rebound in Abandoned Mines to Prevent Contamination of Water and Soils by Acid Mine Drainage

“…Similarly, Sherwood [ 7 ] applied the MODFLOW model to predict the groundwater rebound associated with a large-scale coal field in the United Kingdom. Huisamen and Wolkersdorfer [ 8 ] estimated the hydrogeochemical evolution of mine water over time, based on MODFLOW model using groundwater monitoring data and geochemical analyses. However, as the groundwater environment in mines is physically and hydraulically different from a “typical” groundwater environment, analysis of the mine groundwater rebound phenomenon using a conventional groundwater flow model, such as MODFLOW, presents many challenges [ 4 ].…”