Geochemical change following pH remediation in Mangatini Stream, Stockton Coal Mine, New Zealand

Davies, Hugh Gabriel.

Underground and opencast coal mining has occurred since 1896 on the Stockton Plateau, West Coast, New Zealand, and as a result Acid Mine Drainage (AMD) from both historic and current mine workings has had an adverse impact on the surrounding aquatic environment. Mangatini Stream, a waterway at the mine affected by AMD, receives high volume flows provided by a high annual rainfall (> 6 m/yr). The pH of this stream is typically low (2.7 - 3.1) and the concentrations of dissolved metals are elevated (Al: 12 - 82 mg/L). The mine owner, Solid Energy New Zealand Limited has commenced remediation at Stockton Coal Mine through both minimisation (e.g. capping overburden dumps to reduce oxygen and water ingress) and treatment of AMD. Dosing of ultrafine limestone (> 90 % passing 100 ?m) is now being used for constant pH remediation of the Mangatini Stream. Following the introduction of ultrafine limestone dosing, a rapid change in stream water chemistry was observed and a pH increase (from ~3 to > 5) resulted in the dissolved concentrations of Al, Fe and a number of other metals decreasing significantly. Turbidity has increased slightly subsequent to the dosing of limestone in both the Mangatini Stream, and the final receiving waterway, the Ngakawau River, due to unreacted limestone and the solid Al and Fe oxides that formed in the water as pH increased. Over the first 500 m from the source of the neutralising agent, the solid elemental composition of this suspended solid material changed from predominantly un-dissolved limestone (calcite) to a mixture of metal (mainly Al and Fe) oxides, sulphates, and carbonates with trace elements (As, Cu, Cr, Pb, Sr, Th, U) that had strong positive correlations to the Al and Fe composition. The relative amounts of each component changed from day to day with the variable flow conditions. Long-range ordering of molecules into crystalline structures was not observed in the precipitate products, with the products instead forming largely amorphous structures that could not be identified using x-ray spectroscopic techniques. Geochemical modelling was used to suggest the saturation indices of secondary minerals typical of mine drainage environment, and determined that the saturation of Al was possibly controlled by alunite at low pH, and then boehmite following pH adjustment. The Fe saturation was determined both with and without the influence of the slowly-oxidised Fe2+ ion, and appeared to be controlled by schwertmannite at low pH and then ferrihydrite following pH adjustment.

xviii, 181 leaves : col. ill., map ; 30 cm. + 1 CD-ROM (12 cm.)

2009Davies

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