Geochemical changes during neutralisation of acid mine drainage in a dynamic mountain stream, New Zealand

The Mangatini Stream drains a coal mining area in the mountains of northwestern South Island of New Zealand. Abundant rainfall on pyritic rocks yields acid mine drainage (AMD) to the stream, which flows through a steep gorge at discharges that rapidly increase from <1 to >100 m³/s during frequent rain events. The AMD is treated with finely ground limestone, which is discharged as a slurry at a point in the middle of the gorge. The limestone slurry mixes and reacts with the AMD during flow ∼4 km downstream over ∼12 h. Neutralisation reactions increase stream pH from near 3 (untreated Mangatini Stream water impacted by AMD) to 5–6 in the first 250 m downstream, although mixing is commonly incomplete in this zone. Large stream discharge volumes in rain events dilute the neutralising material input, thus driving the pH back towards 4 downstream of treatment. More complete neutralisation is achieved 4 km downstream, even in major rain events, and pH can rise to >7. Partial neutralisation is sufficient to remove most of the dissolved Fe(III) (typically ∼30 mg/L) from the Mangatini Stream in the first 10 m, and remaining dissolved Fe is essentially all Fe(II), which decreases over time as it oxidises and precipitates. Dissolved Al in the Mangatini Stream (typically ∼50 mg/L) decreases steadily downstream over ∼100 m in the limestone mixing zone. Precipitated Fe and Al form amorphous oxyhydroxides that are transported as suspended solids and deposited on the stream bed with excess limestone in zones of low flow velocity. Dissolved Zn is removed from solution by adsorption to Fe oxyhydroxide when pH reaches ∼5, but dissolved Ni remains in solution despite the neutralisation process. Gypsum precipitation occurs throughout the limestone mixing zone, resulting in at least 30% decrease in dissolved . Minor ettringite forms in the first 100 m, but then probably redissolves. The limestone dosing system is an effective method of neutralising the effects of AMD and removing most dissolved metals in a steep mountain stream with frequent rain events where this dynamic environment places many constraints on treatment options.     

Sea-level change and paleogeographic reconstructions,southern Vancouver Island,British Columbia,Canada

Forty-eight new and previously published radiocarbon ages constrain deglacial and postglacial sea levels on southern Vancouver Island, British Columbia. Sea level fell rapidly from its high stand of about +75 m elevation just before 14 000 cal BP (12 000 radiocarbon yrs BP) to below the present shoreline by 13 200 cal BP (11 400 radiocarbon years BP). The sea fell below its present level 1000 years later in the central Strait of Georgia and 2000 years later in the northern Strait of Georgia, reflecting regional differences in ice sheet retreat and downwasting. Direct observations only constrain the low stand to be below ?11 m and above ?40 m. Analysis of the crustal isostatic depression with equations utilizing exponential decay functions appropriate to the Cascadia subduction zone, however, places the low stand at ?30 ± 5 m at about 11 200 cal BP (9800 BP). The inferred low stand for southern Vancouver Island, when compared to the sea-level curve previously derived for the central Strait of Georgia to the northwest, generates differential isostatic depression that is consistent with the expected crustal response between the two regions. Morphologic and sub-bottom features previously interpreted to indicate a low stand of ?50 to ?65 m are re-evaluated and found to be consistent with a low stand of ?30 ± 5 m. Submarine banks in eastern Juan de Fuca Strait were emergent at the time of the low stand, but marine passages persisted between southern Vancouver Island and the mainland. The crustal uplift presently occurring in response to the Late Pleistocene collapse of the southwestern sector of the Cordilleran Ice Sheet amounts to about 0.1 mm/yr. The small glacial isostatic adjustment rate is a consequence of low-viscosity mantle in this tectonically active region.     

Partitioning of metals (Fe,Pb, Cu,Zn) in urban run‐off from the Kaikorai Valley,Dunedin, New Zealand

Urban run‐off from a catchment in Dunedin, New Zealand was sampled and chemically characterised (iron (Fe), lead (Pb), copper (Cu), zinc (Zn) in total and <0.4 (im fractions, suspended solids, nitrogen (N), phosphorus (P), dissolved organic carbon, major ions, pH) during base flows, and storm flows from five rainfall events. Fe and Pb were found to be predominantly particle‐associated (>0.4 μm) and their concentrations increased significantly at the beginning of storm run‐off. In contrast, the majority of Cu and Zn was found in the <0.4μm fraction before rainfall events but during the initial period of storm flows a significant proportion of Cu and Zn was also present in the >0.4 μm fraction. The results indicate that Cu and Zn may be more bio‐available, and more difficult to remove by storm‐water treatment, than Pb. The pH level and the concentration of major ions (Ca⁺², Na⁺, Mg⁺², K⁺), dissolved reactive phosphorus, and nitrate generally decreased during storm flows as a result of dilution by rainwater. Concentrations of total N and P often increased during the initial period of storm run‐off, which was likely because of wash‐off of particulate plant material.     

Transpressive duplex and flower structure: Dent Fault System, NW England

Revised mapping along the Dent Fault (northwest England) has improved the resolution of folds and faults formed during Variscan (late Carboniferous) sinistral transpression. A NNE-trending east-down monocline, comprising the Fell End Syncline and Taythes Anticline, was forced in Carboniferous cover above a reactivated precursor to the Dent Fault within the Lower Palaeozoic basement. The Taythes Anticline is periclinal due to interference with earlier Acadian folds. The steep limb of the monocline was eventually cut by the west-dipping Dent Fault. The hangingwall of the Dent Fault was dissected by sub-vertical or east dipping faults, together forming a positive flower structure in cross-section and a contractional duplex in plan view. The footwall to the Dent Fault preserves evidence of mostly dip-slip displacements, whereas strike-slip was preferentially partitioned into the hangingwall faults. This pattern of displacement partitioning may be typical of transpressive structures in general. The faults of the Taythes duplex formed in a restraining overlap zone between the Dent Fault and the Rawthey Fault to the west. The orientations of the duplex faults were a response to kinematic boundary conditions rather than to the regional stress field directly. Kinematic constraints provided by the Dent and neighbouring Variscan faults yield a NNW–SSE regional shortening direction in this part of the Variscan foreland.     

Measurements of Photolyzable Halogen Compounds and Bromine Radicals During the Polar Sunrise Experiment 1997

As part of the Polar Sunrise Experiment (PSE) 1997, concentrations of halogen species thought to be involved in ground level Arctic ozone depletion were made at Alert, NWT, Canada (82.5°N, 62.3°W) during the months of March and April, 1997. Measurements were made of photolyzable chlorine (Cl₂ and HOCl) and bromine (Br₂ and HOBr) using the Photoactive Halogen Detector (PHD), and bromine radicals (BrO_x) using a modified radical amplifier. During the sampling period between Julian Day 86 (March 27) and Day 102 (April 12), two ozone depletion episodes occurred, the most notable being on days 96-99, when ozone levels were below detectable limits (1 ppbv). Concentrations of BrO_x above the 4 pptv detection limit were found for a significant part of the study, both during and outside of depletion events. The highest BrO_x concentrations were observed at the end of the depletion event, when the concentration reached 15 pptv. We found substantial amounts of Br₂ in the absence of O₃, indicating that O₃ is not a necessary requirement for production of Br₂. There is also Br₂ present when winds are from the south, implying local scale (e.g. from the snowpack) production. During the principal O₃ depletion event, the HOBr concentration rose to 260 pptv, coincident with the BrO_x maximum. This implies a steady state HO₂ concentration of 6 pptv. During a partial O₃ depletion event, we estimate that the flux of Br₂ from the surface is about 10 times greater than that for Cl₂.     

Trends and Variability in Spring and Autumn 0 °C-Isotherm Dates over Canada

In cold-regions climates, numerous environmental processes and socio-economic activities are significantly impacted by the timing of the seasonal advance and retreat of the 0 °C isotherm. This investigation examines 20th century trends and variability in spring and autumn 0 °C-isotherm dates over Canada. Results reveal considerable variability across the country. Significant trends toward earlier springs are observed over most of western Canada including a dramatic shift to earlier dates during the last 20–30 years. Central regions areassociated with smaller, generally insignificant earlier spring trends. Conversely, extreme eastern areas experience later springs. During autumn, isotherm dates show little change over the majority of the country. The observed spatial and temporal characteristics in the 0 °C-isotherm trends are reflected in past variations in several hydro-cryospheric variables over many regions of Canada including the timing of snowmelt and the dates of freshwater ice break-up. Investigation into the potential causes of observed variability in 0 °C-isotherm dates reveals that large-scale oscillations representing atmospheric/oceanic variations in the north Pacific and north Atlantic significantly relate to isotherm dates over western and eastern Canada, respectively, during both spring and autumn. Although significant, a relatively small amount of overall variance in isotherm dates is explained by the oscillations indicating the influence of other factors. Results from this study improve our knowledge of past trends and variability in 0°C-isotherm dates and resultant impacts on hydro-cryospheric processesover Canada. They also provide insight into potential future climatologic impacts given the possibility of climate change.     

Progressive hybridisation between Late Devonian mafic-intermediate and felsic magmas in the Meguma Zone of Nova Scotia, Canada

Two felsic plutons of Late Devonian (385–370 Ma) age in the Meguma Zone of southwestern Nova Scotia contain three circa 376 Ma synplutonic mafic-intermediate intrusions that collectively record progressive stages of in situ hybridisation. A 5 m wide spessartite dyke in the Port Mouton Pluton probably underwent rapid cooling and crystallisation immediately after intrusion, which heated and coarsened the adjacent tonalite. An 85 m long sheet of pillowed kersantite (also in tonalite of the Port Mouton Pluton) presumably contained residual magma after quenching and obtained K, P, Ba, Rb, more radiogenic Sr, Zr, Nb, and light REE from the tonalite during magma mingling. The third synplutonic body, a >100 m wide diorite sheet, intrudes granodiorite of the adjacent Shelburne Pluton and has a circa 45 m wide gradational contact of metaluminous hornblende-tonalite. This tonalite dominantly records magma mixing by the transfer of Ti, Mg, Fe, Ca, and V in hornblende, biotite, plagioclase, and (at least in part) apatite xenocrysts derived from dioritic pillows that were originally disaggregated in the granodiorite, probably in response to convection. Scattered data points, unusual “concave-down” variation trends for Al₂O₃, P₂O₅, and Sr, and non-hyperbolic Sr-Nd isotopic characteristics in the tonalite, apparently reflect syn- or post-mixing fractionation and accumulation of xenocrysts from residual magma. Phosphorus may have assisted diffusion of Sr, Zr, Nb, and light REE, and caused premature quenching of the hybrids at Mcleods Cove and Birchtown, during magma mingling and mixing. Received: 1 January 1996 / Accepted: 3 August 1996