Quantification methods and management of hydrogeochemistry in decommissioned collieries of the Mpumalanga Coalfields

Abstract

Mine water chemistry and its evolution has been the focus of various studies. However, these studies were primarily based on data from underground mines, pit lakes and mining waste dumps. Backfilled opencast mines have received limited attention in this regard and were postulated to undergo an initial flush, in a similar fashion to underground mines. Previous work in the prediction of mine water quality focussed on the fitting of an idealised decay rate curve to existing mine water chemistry data, analytical calculations, simplified geochemical modelling approaches and numerical transport models. This study has taken components from these approaches to predict the evolution of mine water quality from backfilled opencast mines, during the initial flush, but with an additional component, defining it as a new approach. This component is calibration of geochemical modelling data and numerical flow and transport modelling data, with existing groundwater monitoring data over a short term, relative to the duration of the initial flush. Laboratory analyses were further used to augment the calibration process in various steps. Results obtained show that the initial flush in backfilled opencast mines is likely to last 20 to 100 years, depending on site specific conditions. To further understand this duration, speciation modelling and statistical analysis was undertaken to determine controlling mineral phases in solution at backfilled opencast coal mines. Pyrite, gypsum, calcite, kaolinite and possible smectite were identified as the major controlling mineral phases in mine water chemistry of backfilled opencast coal mines at the study sites used for this thesis. Based on the understanding of the controlling mineral phases in solution and the calibrated mine water quality predictions made, the study also proposed the maximisation of water addition to backfilled opencast coal mines as a means to accelerate the initial flush, potentially turning contaminated mine water into a resource.