Hydrochemistry, mineralogy and sulphur isotope geochemistry of acid mine drainage at the Mt Morgan mine environment, Queensland, Australia

Mineralogical, hydrochemical and S isotope data were used to constrain hydrogeochemical processes that produce acid mine drainage from sulfidic waste at the historic Mount Morgan Au–Cu mine, and the factors controlling the concentration of SO4 and environmentally hazardous metals in the nearby Dee River in Queensland, Australia. Some highly contaminated acid waters, with metal contents up to hundreds of orders of magnitude greater than the Australia–New Zealand environmental standards, by-pass the water management system at the site and drain into the adjacent Dee River. Mine drainage precipitates at Mt. Morgan were classified into 4 major groups and were identified as hydrous sulfates and hydroxides of Fe and Al with various contents of other metals. These minerals contain adsorbed or mineralogically bound metals that are released into the water system after rainfall events. Sulfate in open pit water and collection sumps generally has a narrow range of S isotope compositions (δ34S = 1.8–3.7‰) that is comparable to the orebody sulfides and makes S isotopes useful for tracing SO4 back to its source. The higher δ34S values for No. 2 Mill Diesel sump may be attributed to a difference in the source. Dissolved SO4 in the river above the mine influence and 20 km downstream show distinctive heavier isotope compositions (δ34S = 5.4–6.8‰). The Dee River downstream of the mine is enriched in 34S (δ34S = 2.8–5.4‰) compared with mine drainage possibly as a result of bacterial SO4 reduction in the weir pools, and in the water bodies within the river channel. The SO4 and metals attenuate downstream by a combination of dilution with the receiving waters, SO4 reduction, and the precipitation of Fe and Al sulfates and hydroxides. It is suggested here that in subtropical Queensland, with distinct wet and dry seasons, temporary reducing environments in the river play an important role in S isotope systematics

Increasing the efficiency of solute leaching: impacts of flow interruption with drainage of the "preferential flow paths"

Most soils contain preferential flow paths that can impact on solute mobility. Solutes can move rapidly down the preferential flow paths with high pore-water velocities, but can be held in the less permeable region of the soil matrix with low pore-water velocities, thereby reducing the efficiency of leaching. In this study, we conducted leaching experiments with interruption of the flow and drainage of the main flow paths to assess the efficiency of this type of leaching. We compared our experimental results to a simple analytical model, which predicts the influence of the variations in concentration gradients within a single spherical aggregate (SSA) surrounded by preferential flow paths on leaching. We used large (length: 300 mm, diameter: 216 mm) undisturbed field soil cores from two contrasting soil types. To carry out intermittent leaching experiments, the field soil cores were first saturated with tracer solution (CaBr2), and background solution (CaCl2) was applied to mimic a leaching event. The cores were then drained at 25- to 30-cm suction to empty the main flow paths to mimic a dry period during which solutes could redistribute within the undrained region. We also conducted continuous leaching experiments to assess the impact of the dry periods on the efficiency of leaching. The flow interruptions with drainage enhanced leaching by 10-20% for our soils, which was consistent with the model's prediction, given an optimised equivalent aggregate radius for each soil. This parameter quantifies the time scales that characterise diffusion within the undrained region of the soil, and allows us to calculate the duration of the leaching events and interruption periods that would lead to more efficient leaching. Application of these methodologies will aid development of strategies for improving management of chemicals in soils, needed in managing salts in soils, in improving fertiliser efficiency, and in reclaiming contaminated soils. (C) 2000 Elsevier Science B.V. All rights reserved.

Sudden drawdown and drainage of a horizontal aquifer

Drainage of a saturated horizontal aquifer following a sudden drawdown is reanalyzed using the Boussinesq equation. The effect of the finite length of the aquifer is considered in detail. An analytical approximation based on a superposition principle yields a very good estimate of the outflow when compared to accurate numerical solutions. An illustration of the new analytical approach to analyze basin-scale field data is used to demonstrate possible field applications of the new solution.

Numerical modelling of tide-induced beach water table fluctuations

Field studies have shown that the elevation of the beach groundwater table varies with the tide and such variations affect significantly beach erosion or accretion. In this paper, we present a BEM (Boundary Element Method) model for simulating the tidal fluctuation of the beach groundwater table. The model solves the two-dimensional flow equation subject to free and moving boundary conditions, including the seepage dynamics at the beach face. The simulated seepage faces were found to agree with the predictions of a simple model (Turner, 1993). The advantage of the present model is, however, that it can be used with little modification to simulate more complicated cases, e.g., surface recharge from rainfall and drainage in the aquifer may be included (the latter is related to beach dewatering technique). The model also simulated well the field data of Nielsen (1990). In particular, the model replicated three distinct features of local water table fluctuations: steep rising phase versus flat falling phase, amplitude attenuation and phase lagging.

Effect of particle hydrophobicity on particle and water transport across a flotation froth

Froth recovery measurements have been conducted in both the presence (three-phase froth) and absence (two-phase froth) of particles of different contact angles in a specially modified laboratory flotation column. Increasing the particle hydrophobicity increased the flow rate of particles entering the froth, while the recovery of particles across the froth phase itself also increased for particle contact angles to 63 and at all vertical heights of the froth column. However, a further increase in the contact angle to 69 resulted in lower particle recovery across the froth phase. The reduced froth recovery for particles of 69 contact angle was linked to significant bubble coalescence within the froth phase. The reduced froth recovery occurred uniformly across the entire particle size range, and was, presumably, a result of particle detachment from coalescing bubbles. Water flow rates across the froth phase also varied with particle contact angle. The general trend was a decrease in the concentrate flow rate of water with increasing particle contact angle. An inverse relationship between water flow rate and bubble radius was also observed, possibly allowing prediction of water flow rate from bubble size measurements in the froth. Comparison of the froth structure, defined by bubble size, gas hold-up and bubble layer thickness, for two- and three-phase froths, at the same frother concentration, showed there was a relationship between water flow rate and froth structure. (c) 2005 Elsevier B.V. All rights reserved.

Modelling of entrainment in industrial flotation cells: Water recovery and degree of entrainment

Entrainment in flotation can be considered as a two-step process, including the transfer of the suspended solids in the top of the pulp region just below the pulp-froth interface to the froth phase and the transfer of the entrained particles in the froth phase to the concentrate. Both steps have a strong classification characteristic. The degree of entrainment describes the classification effect of the drainage process in the froth phase. This paper briefly reviews two existing models of degree of entrainment. Experimental data were collected from an Outokumpu 3 m(3) tank cell in the Xstrata Mt. Isa Mines copper concentrator. The data are fitted to the models and the effect of cell operating conditions including air rate and froth height on the degree of entrainment is examined on a size-by-size basis. It is found that there is a strong correlation between the entrainment and the water recovery, which is close to lineal. for the fines. The degree of entrainment decreases with increase in particle size. Within the normal range of cell operating conditions, few particles coarser than 50 mu m are recovered by entrainment. In general, the degree of entrainment increases with increase in the ail rate and decreases with increase in the froth height. Air rate and froth height strongly interact with each other and affect the entrainment process mainly via changes in the froth retention time, the froth structure and froth properties. As a result, other mechanisms such as entrapment may become important in recovering the coarse entrained particles. (c) 2005 Elsevier Ltd. All rights reserved.

The agricultural water supply challenge - the need for improved water use efficiency

This paper examines the challenges of water supply in agriculture, with particular emphasis on requirements of field crops, including maize. It places the issue of water supply to agriculture in the context of increasing demands for water from alternatives users, declining quality water, pressure of increasing population, all of which are placing stresses on water availability at local, regional and national levels. The paper also examines existing freshwater resources and the potential impact of climate change on water supply and distribution and consequential impact on water stress incidence in various parts of the globe. It examines competition for water in both industrialized and developing countries, with particular emphasis on the impacts on agriculture and food supplies. The challenge of water use efficiency (WUE) in agriculture is explored with discussion of agronomic, economic and physiological WUE concepts, with specific reference to maize.

Air Entrainment Processes in a Full-Scale Rectangular Dropshaft at Large Flows

Rectangular dropshafts, commonly used in sewers and storm water systems, are characterised by significant flow aeration. New detailed air-water flow measurements were conducted in a near-full-scale dropshaft at large discharges. In the shaft pool and outflow channel, the results demonstrated the complexity of different competitive air entrainment mechanisms. Bubble size measurements showed a broad range of entrained bubble sizes. Analysis of streamwise distributions of bubbles suggested further some clustering process in the bubbly flow although, in the outflow channel, bubble chords were in average smaller than in the shaft pool. A robust hydrophone was tested to measure bubble acoustic spectra and to assess its field application potential. The acoustic results characterised accurately the order of magnitude of entrained bubble sizes, but the transformation from acoustic frequencies to bubble radii did not predict correctly the probability distribution functions of bubble sizes.

Free-Surface Aeration and Momentum Exchange at a Bottom Outlet

This study aims to provide some new understanding of the air-water flow properties in high-velocity water jets discharging past an abrupt drop. Such a setup has been little studied to date despite the relevance to bottom outlets. Downstream of the step brink, the free-jet entrains air at both upper and lower air-water interfaces, as well as along the sides. An air-water shear layer develops at the lower nappe interface. At the lower nappe, the velocity redistribution was successfully modelled and the velocity field was found to be similar to that in two-dimensional wake flow. The results highlighted further two distinct flow regions. Close to the brink (Wex < 5000), the flow was dominated by momentum transfer. Further downstream (Wex > 5000), a strong competition between air bubble diffusion and momentum exchanges took place.

Water Flow Through the Gills of Port Jackson Sharks

Evidence is presented for the existence of a countercurrent flow between water and blood at the respiratory surfaces of the Port Jackson shark gill.