Coupled thermal, hydraulic and geochemical evolution of pyritic tailings in unsaturated column experiments

The evolution of pore-water and the composition of solid phases in the vadose zone of pyritic tailings was studied by means of unsaturated column experiments. Several columns of water-saturated mine tailings were dried during 125 days under controlled laboratory conditions. The columns were dismantled at four successive drying stages and the evolution of pore-water, mineralogy, water content and temperature was characterized.Sulfide and aluminosilicate minerals present in the waste dissolved, releasing sulfate and other solutes (mainly Fe, Zn, Cu, Al, Mg and Ca) to the pore-water. Evaporation caused a crust of efflorescent, water-soluble sulfates to develop over the complete top surface of the columns and into the pores of the underlying waste material. This crust, which has also been identified in the field, changed the hydraulic properties of the tailings and produced a decrease in the evaporation rate of the columns. Moreover, these water-soluble precipitates (mainly rozenite, szomolnokite, halotrichite, hexahydrite, mirabilite and gypsum) acted as temporary sinks for Cd, Pb, Co and Ni, which could be released to the surface run-off or the groundwaters during rainfall events under field conditions.Pore-water evolution was determined not only by geochemical processes (dissolution of sulfides and aluminosilicates, precipitation of secondary phases) but also by thermal and hydraulic processes. Progressive dilution was observed in the lower part of the columns. Dilution was caused by the thermally driven vapor flux from the top of the column to its colder bottom and subsequent condensation therein. This process, which may also occur in tailings under sub-arid climate, played a key role on the evolution of pore-water with increasing drying.     

Crustal attenuation in the Southern Andean retroarc (38°–39°30′ S) determined from tectonic and gravimetric studies: The Lonco-Luán asthenospheric anomaly

The western retroarc of the Southern Andes between 38° and 40° S is formed by a NNW-elongated ridge not associated with stacked thrust sheets. On the contrary, during the last 4–3 Ma this ridge was affected by extensional deformation, regional uplift and related folding on a very broad scale. Receiver function analysis shows that the drainage divide area and adjacent retroarc lie over an attenuated crust. Expected crustal thickness at these latitudes is around 38 km, whereas in this part of the retroarc the thickness is less than 32 km. The causes for such attenuation have been linked to a moderate steepening of the subducted Nazca plate beneath the South American plate, which is suggested by a westward shift and narrowing of the magmatic arc during the last 4 to 5 Ma. Gravimetric studies show that the upper plate did not react homogeneously to slab steepening, but ancient sutures and lithospheric discontinuities deeply buried under Mesozoic to Cenozoic sequences in the retroarc were locally reactivated. These processes resulted in an asthenospheric anomaly that correlates at the surface with the area of Pliocene to Quaternary doming, widespread extension and three radial troughs. Two of the troughs have accommodated substantial amounts of extension, but the third was probably aborted at an early stage. Moreover, the presence of an anomalous concentration of calderas and large volcanic centers over the proposed asthenospheric anomaly, and their age distribution, may indicate minor migration of the asthenospheric anomaly between 4 and 2 Ma through the western South American plate.     

Separation of local and regional information in distorted GDS response functions by hypothetical event analysis

Electromagnetic investigations are usually intended to examine regional structures where induction takes place at a given period range. However, the regional information is often distorted by galvanic effects at local conductivity boundaries. Bahr (1985) and Groom & Bailey (1989) developed a physical distortion model for decomposing the MT impedance tensor, based upon local galvanic distortion of a regional 2-D electromagnetic field. We have extended their method to predict the magnetic variation fields created at an array of sites. The magnetic response functions at periods around 1000 s may be distorted by large-scale inhomogeneities in the upper or middle crust. In this period range, the data measured by a magnetometer array contain common information that can be extracted if the data set is treated as a unit, for example by using hypothetical event analysis. With this technique it is always possible to recover the regional strike direction from distorted data, even if a strong, spatially varying regional vertical field component is present in the data set. The determination of the regional impedance phases, on the other hand, is far more sensitive to deviations from the physical distortion model.
The approach has been used to investigate the Iapetus data set. For the array, which covers an area of 200  km × 300  km in northern England/southern Scotland, the technique revealed a common regional strike azimuth of ca . N125° E in the period range 500–2000  s. This direction differs from the strike indicated by the induction arrows, which seem influenced mainly by local current concentrations along the east–west-striking Northumberland Trough and a NE–SW-striking mid-crustal conductor. Both impedance phases are positive and differ by ca . 10°, which supports the assumptions of distortion fields in the data set and that the regional structure is 2-D.     

Coda Q before and after the eruptions of 13 November 1985, and 1 September 1989, at Nevado del Ruiz Volcano, Colombia

 Coda Q^–1 was calculated at Nevado del Ruiz Volcano (NRV) before and after two phreatomagmatic eruptions (November 1985, September 1989) and for a period of stability (May 1987) using a functional form for coda derived from a single scattering model (Sato 1977). Substantial changes were found before and after the eruptions. The highest value of Q^–1 was found during the November 1985 eruption, an intermediate value for the September 1989 eruption, and the lowest value for May 1987. It seems that the changes in coda Q^–1 at NRV have a still-unknown relationship with the volume or magnitude of the eruption. A relatively strong frequency dependence was found for all periods. Also Q^–1 clearly changed with time, suggesting that the scattering was strong for the eruption of November 1985 and decreased for the eruption of September 1989, and that the intrinsic absorption probably increased. This suggests the possibility that crystallization is taking place in the NRV magma. The clear change of coda Q^–1 before and after the eruptions at NRV also suggests the possibility that coda Q^–1 is a premonitory tool of activity at this volcano. Received: 25 October 1996 / Accepted: 21 January 1998     

The Cepheid period–luminosity zero-point from radial velocities and Hipparcos proper motions

A value for the zero-point (ρ) of the Cepheid period–luminosity relation, <  M _V  >= 2.81 log P  + ρ, is deduced by comparing the value of the Oort constant, A , derived from radial velocities with that derived from Hipparcos proper motions. We find in this way that ρ =−1.47 ± 0.13, in excellent agreement with the value derived from Hipparcos trigonometrical parallaxes, ρ = −1.43 ± 0.10, by Feast &38; Catchpole in a recent paper.     

Relative influence of vegetation on urban energy budgets and surface temperatures

Two deterministic models were combined: one for canopy leaf energy budgets and one for street canyon energy budgets. The effects of street parks and roof gardens in contrast to non-vegetated city blocks were examined by the use of four typical urban morphologies, which were exposed latitudinally to summer and winter simulations. A variety of increases and decreases in shortwave radiation, net radiation, sensible heat flux, and system reradiation resulted. These changes appear to represent the generalized limits of the possible responses to the addition of vegetation to non-vegetated city blocks.     

Uranium and thorium series nuclides in oriented ferromanganese nodules: growth rates,turnover times and nuclide behavior

Three ferromanganese nodules handpicked from the tops of 2500 cm² area box cores taken from the north equatorial Pacific have been analysed for their U-Th series nuclides.²³⁰Th_exc concentrations in the surface 1–2 mm of the top side of the nodules indicate growth rates of 1.8–4.6 mm/10⁶ yr. In two of the nodules a significant discontinuity in the²³⁰Th_exc depth profile has been observed at ～0.3 m.y. ago, suggesting that the nodule growth has been episodic. The concentration profiles of²³¹Pa_exc (measured via²²⁷Th) yield growth rates similar to the²³⁰Th_exc data. The bottom sides of the nodules display exponential decrease of²³⁰Th_exc/²³²Th activity ratio with depth, yielding growth rates of 1.5–3.3 mm/10⁶ yr.The²³⁰Th_exc and²³¹Pa_exc concentrations in the outermost layer of the bottom face are significantly lower than in the outermost layer of the top face. Comparison of the extrapolated²³⁰Th_exc/²³²Th and²³⁰Th_exc/²³¹Pa_exc activity ratios for the top and bottom surfaces yields an “age” of (5?15) × 10⁴ yr for the bottom relative to the top. This “age” most probably represents the time elapsed since the nodules have attained the present orientation.The²¹⁰Pb concentration in the surface ～0.1 mm of the top side is in large excess over its parent²²⁶Ra. Elsewhere in the nodule, up to ～1 mm depth in both top and bottom sides,²¹⁰Pb is deficient relative to²²⁶Ra, probably due to²²²Rn loss. The absence of²¹⁰Pb_exc below the outermost layer of the top face rules out the possibility of a sampling artifact as the cause of the observed exponentially decreasing²³⁰Th_exc and²³¹Pa_exc concentration profiles. The flux of²¹⁰Pb_exc to the nodules ranges between 0.31 and 0.58 dpm/cm² yr. The exhalation rate of²²²Rn, estimated from the²²⁶Ra-²¹⁰Pb disequilibrium is ～570 dpm/cm² yr from the top side and >2000 dpm/cm² yr from the bottom side.²²⁶Ra is deficient in the top side relative to²³⁰Th up to ～0.5–1 mm and is in large excess throughout the bottom. The data indicate a net gain of²²⁶Ra into the nodule, corresponding to a flux of (24?46) × 10^?3 dpm/cm² yr. On a total area basis the gain of²²⁶Ra into the nodules is <20% of the²²⁶Ra escaping from the sediments. A similar gain of²²⁸Ra into the bottom side of the nodules is reflected by the high²²⁸Th/²³²Th activity ratios observed in the outermost layer in contact with sediments.     

Comparative discharges of fresh and waste water along the California coast

As the population of California grows, the discharge of domestic and industrial waste water and power-plant cooling water into coastal waters continuously increases. About 0.75 × 10⁶ m³ of industrial waste water, 3.75 × 10⁶ m³ of domestic waste water, and 26.5 × 10⁶ m³ of thermal effluent are discharged daily into California's nearshore waters. These waters do not have an infinite capacity to break down and disperse the pollutants. Waste water discharged near shore may remain there for considerable time and have a significant effect on water quality and marine life. The low population and wet climate of the northern coastal region have led to small discharges of waste water but very large inputs of fresh water to the ocean. Southward along the Pacific Coast the climate becomes more arid, and population increases. This combination leads to a large excess of waste-water over fresh-water discharge. Many individual waste-water discharges exceed the average daily discharge of most of the rivers which enter the coastal waters.