Coal mine drainage chemistry on the West Coast of the South Island is highly variable; pH ranges from about 2Á8, and chemical concentrations vary by several orders of magnitude. Factors that influence mine drainage chemistry on the West Coast include regional geology, mine type, hydrogeology, and local geology. At a regional scale, mine drainage chemistry is bimodal and relates to geological formations. Mines within the Paparoa Coal Measures have neutral mine drainage, whereas those within the Brunner Coal Measures have acid mine drainage. This is related to the availability of SO 4 during coal measures deposition and, in combination with Fe and organic material, the subsequent formation of pyrite during burial. Paparoa Coal Measures were deposited in alluvial to lucustrine environments where SO 4 was relatively unavailable, whereas Brunner Coal Measures were deposited in alluvial to estuarine to marginal marine settings where marine SO 4 was abundant. Brunner Coal Measures acid mine drainage chemistry is influenced by mine type; open cast mines have Al-and trace element-rich acid mine drainage compared to underground mines. Acid mine drainage forming reactions that release trace elements and Al proceed more rapidly and completely at open cast mines where mine waste has a higher reactive surface area compared to waste rock at underground mines. Brunner coal mine drainage chemistry is also influenced by hydrogeology where flooded underground mines release less acid than free-draining mines because there is less oxygen available to react with pyrite. In addition, local geology overprints mine drainage chemistry where differences in acid mine drainage chemistry arise from changes in contributing lithologies either within a single mine or between different coalfields. Identification of factors that control mine drainage chemistry enables prediction of mine drainage chemistry. These predictions have application to the mining industry for managing, mitigating, monitoring, and remediation of mine drainages that would otherwise cause negative environmental impact.

Assessment of the potential for waste rock at coal mine sites to produce acid mine drainage is an important part of mine planning and operations and is commonly assessed using acid base accounting analyses. The underlying factor that controls mine drainage chemistry is the mineralogical composition of the coal measures sequences which in turn is controlled by several geological factors including provenance, depositional environment, diagenetic processes and tectonic setting. Therefore, coal mine drainage chemistry is directly linked to the geology of coal measures sequences.Our research uses acid base accounting data to identify relationships between coal measures geology and acid production potential for Brunner, Paparoa and Morley Coal Measures as well as Gore Lignite Measures. Our data is from several sources and reflects areas in these sets of coal measures where mining or exploration is underway. In general Brunner Coal Measures are strongly acid producing especially fine grained rocks and this relates to a coastal depositional environment, common diagenetic pyrite, overlying transgressive marine rocks and compositional maturity. Morley and Paparoa Coal Measures are mostly nonacid forming relating to fluvial -lucustrine depositional environments, overlying coal measures, diagenetic carbonates, and possibly carbonates from source rocks. Gore Lignite Measures are mostly non-acid forming, however, some acid forming rocks are present. Gore Lignite Measures are delta plane deposits with occasional marine influence.Datasets from Brunner, Paparoa and Morley Coal Measures as well as Gore Lignite Measures also demonstrate some of the limitations of acid base accounting analyses. For example, oxidation steps in acid base accounting analyses designed to dissolve pyrite also react with organic material and this causes a false positive analysis. Other assumptions common in acid base accounting such as use of total S content to calculate pyrite related acidity are inappropriate for some samples because of other forms of S in these samples. Despite the interferences, standard acid base accounting tests and procedures are very useful for first pass predictions of mine drainage chemistry and datasets from commonly mined New Zealand coal measures can be successfully related to geological processes.

Dissolved As can be strongly adsorbed to fine grained Fe(III) minerals such as hydroxides, oxyhydroxides and hydroxysulphates. Therefore precipitates that form during neutralisation or treatment of acid mine drainage have potential to be useful for treatment of As-contaminated water because acid mine drainage is often Fe rich. We tested the adsorption properties of Fe(III) rich precipitates from two West Coast coal mines with As-contaminated water from an historic gold ore processing site near Reefton. Precipitates were collected from distinctly different settings, an active acid mine drainage treatment plant at Stockton mine and the neutralisation/oxidation zone of acid mine drainage discharge at the abandoned Blackball Coal Mine. The two mine sites produce precipitates with different compositions and mineralogy. Arsenic adsorption onto precipitates from each site was determined in batch and column tests under laboratory conditions. Batch experiments indicate As adsorption occurs rapidly during the first 5 h and reaches equilibrium after 24 h. At equilibrium, and for a dosing ratio of 50 g of precipitate per litre of water, As concentrations decreased from 99 mg/L to 0.0080 mg/L with precipitates from Stockton and to 0.0017 mg/L with precipitates from Blackball. Arsenic adsorption capacity is up to 12 mg/g on precipitates from Stockton sludge and 74 mg/g on precipitates from Blackball. The Blackball precipitate adsorbs more As than precipitates from Stockton which is probably due to the higher Fe oxide content but pH and surface structure could also play a role. The column experiment confirmed that adsorption of As from a continuous waste stream onto these precipitates is possible, and that passive remediation using this waste product mixed with gravel to enhance permeability could be a viable approach at As-contaminated mine sites.