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.
