The
design of the settling pond for the Malvern
Hills opencast coal project was based on a primary requirement to
control runoff and sediment generated from the immediate 6 ha mine catchment,
which included two seepages from old underground workings. The pond has been
particularly effective in controlling turbidity in the outflow water, with the
consent limit of 50 NTU being met on all but one occasion and normal levels of
discharge less than 10 NTU. Water quality testing carried out at the time of
consenting indicated that there were no acid-generation issues with the proposed
mine site in dominant mudstone strata of the Broken River Formation, and that
water from the old mine workings was near-neutral in terms of pH and
well-buffered. However, it became apparent over the very dry summer of 2003 -
2004 that slaking of the exposed mudrocks in the mine batters was resulting in
significant acidity generation, and outflow pH levels were well below the
consent limit of 6.5 during and following periods of rainfall. Although these
acidity problems have been remediated in the short-term by the use of lime
products, it is clear that a programme of lithotype characterisation for AMD
should have been undertaken and this has now been implemented. Design of
long-term acid drainage control measures is being carried out, and will probably
be based around the use of hydrated lime for pH control with enhancement of
natural wetlands.