The Water Quality and Geochemistry of Legacy Acid Mine Drainage on the Denniston Plateau, West Coast

Abstract

The Eocene Brunner Coal Measures on the West Coast of New Zealand are pyritic, paired with low neutralisation capacity in the coal measures, resulting in a high acid producing potential. Historic mining located on the elevated high-rainfall Denniston Plateau left a legacy of acid mine drainage (AMD). The aim of this study is to characterise and quantify the AMD discharges from the historic mines, in order to create a potential plan for downstream water quality management and ecosystem recovery.

Characterisation of the water flow rates and water chemistry was undertaken with continuously-operating data-loggers at strategically-placed v-notch monitoring points. Chemical parameters from data-loggers were calibrated with periodic water sampling and laboratory analysis over a 16-month period. Response of the water chemistry to rainfall events was a particular focus of the study.

Increases in stream flow rates in response to rainfall events initially causes some dilution of dissolved load and increase in pH from ~3.3 to ~3.6. However, as stream flows decrease during waning rainfall, there is a lag of AMD discharge over 1-2 hours because of enhanced flushing of historic mine workings by infiltrating rain. This flushing results in the persistence of low pH (ca. 3.2) and elevated dissolved load, which persists for up to 24 hours during high rainfall events. Some of this enhanced flushing is contributable to secondary minerals acting as stored acidity in historic workings.

As acid waters are diluted and mixed with pure tributary the pH increases in the stream discharge system and a series of precipitates form on the streambeds. Scanning electron microscope (SEM) imagery and associated semi quantitative analysis of precipitates has shown that most consist of ferrihydrite and schwertmannite that form agglomerations of nanometre scale particles. These precipitates form at pH > 3. In waters with pH near 5, basaluminite is also visible with the ferrihydrite. Geochemical modelling of mineral saturation levels in the waters are consistent with the SEM mineralogical observations.

The combination of rainfall dilution and dilution by incoming side stream waters is effective at raising the pH of the AMD discharges and the resulting precipitates facilitate removal of dissolved metals and sulphur. There is a total discharge acid load of 845 t CaCO3 yr-1, and total metal load is lowered by precipitation and dilution from up to 30 g/m3 Al, 15 g/m3 Fe and 360 g/m3 SO42-, down to ca. <3 g/m3 Al, <1 g/m3 Fe and <75 g/m3 SO42-. Extrapolation of the acid load on the Plateau led to an overall acid load of 1,654 t CaCO3/yr. This information will be used in a further study to develop a mine drainage release decay curve for the entire Denniston Plateau.