Influence of dissolved organic carbon on photochemically mediated cycling of hydrogen peroxide in rainwater

The influence of sunlight and dissolved organic carbon (DOC) on the photochemically mediated cycling of hydrogen peroxide (H₂O₂) was investigated in rainwater samples collected in Wilmington, North Carolina USA. Upon exposure to simulated sunlight 14 of 19 authentic rainwater samples exhibited significant decreases in H₂O₂. The concentration of hydrogen peroxide did not change significantly in organic-free synthetic rainwater spiked with H₂O₂ in the light or in dark controls suggesting that the loss was not due to direct photolysis or dark mediated reactions. There was a significant correlation between pseudo-first order rate constants of H₂O₂ decay and initial H₂O₂ concentrations. There was also a significant correlation between the rate constant and the abundance of DOC suggesting that rainwater organic carbon plays an important role during photolytic decay either via direct reaction or indirectly through production of peroxide reactive species or scavenging of peroxide generating radicals. Several rain samples exhibited an initial increase in H₂O₂ during the first 2 h of irradiation. These increases were generally small and most likely do not represent a significant input of peroxide in precipitation. The photo-induced destruction of H₂O₂ is important because it may partly explain the late afternoon decrease of peroxide concentrations observed in earlier field studies and the substantial under saturation (<10%) of this oxidant in rainwater compared with gas phase concentrations.     

Daily scale modelling of aquifer–river connectivity in the urban alluvial aquifer in Langreo, Spain

Evaluating aquifer–river interactions is naturally complex, particularly within urban settings. This is largely due to the difficulties involved in quantifying most elements of the water balance. The ability of numerical models to deal with several dynamic variables simultaneously makes them valuable tools to address this kind of problem. An applied, modeling-based approach to investigate the spatial and temporal variations of aquifer–river connectivity within a shallow urban aquifer is presented. Model development is based on comprehensive field campaigns in Langreo, Spain. Two calibration runs (for summer and winter conditions) were carried out in order to evaluate the spatial distribution of recharge rates. The model suggests that baseflows are largely negligible in comparison with total streamflows. This is mostly attributed to the abrupt nature of the catchment, which prevents the existence of sufficiently large alluvial systems to a great extent. Modelling results also show that aquifer–river connectivity at the study site is constrained by urban pumping as well as by seasonal fluctuations.     

Effects of aging on silica solubility: A laboratory study

In three separate experiments, one using a bicarbonate buffered aqueous NaCl solution and two using natural seawater. the solubility of chemically pure X-ray amorphous precipitated silica decreased by approximately 20‰ after aging in contact with solution for several months. The specific surface area of the silica decreased by approximately half during the same experimental time periods. The solubility decrease in buffered NaCl solution occurred at both 2 and 22°C and over the pressure range from 1.01 × 10⁵ to 7.45 × 10⁷ Pa (1–740 atm). The aging process which causes the solubility and surface area decreases may be caused by the formation of a smooth surface silica layer that is more dense than the original surface. No change in the bulk chemistry of the silica could be defined after aging. This short term aging process contributes to the scatter in published solubilities for amorphous silica in seawater.     

A mechanism for pyroxene-pyroxenoid and pyroxenoid-pyroxenoid transformations

A mechanism for the solid state replacement of clinopyroxene by pyroxenoids has been derived from consideration of the crystal structures. The mechanism involves the propagation of partial dislocations, with Burgers vectors of 1/4 \([2\bar 3\bar 1]\) , through the clinopyroxene matrix to generate a resultant shear; the passage of one dislocation gives rise to a single chain-repeat unit of wollastonite in each silicate chain. Pyroxenoids may thus be formed from clinopyroxene by the periodic introduction of these dislocations on parallel slip-planes, and pyroxenoid to pyroxenoid inversions may proceed by the introduction and removal of such shears by the propagation of the correct combination of partial dislocations. The Burgers vectors for these dislocations in each of the pyroxenoid mineral structures has been calculated. A two step mechanism is proposed for clinopyroxene to bustamite, and pyroxenoid to bustamite inversions, which utilises the same shear as above which generates a wollastonite structure. The second step is a shear of 1/2[001] on the plane \((1\bar 10)\) of this wollastonite intermediate, to give a bustamite structure.     

The experimental determination of the johannsenite-bustamite equilibrium inversion boundary

The CaMnSi₂O₆ clinopyroxene, johannsenite, inverts to bustamite at elevated temperatures. The inversion boundary has been reversibly determined at five pressures up to 22 kbar, using synthetic phases, and shown to have the equation; P(kbar)=0.0411T(°C)–10.7The determination allows the calculation of the enthalpies and free energies of formation for both clinopyroxene and bustamite of CaMnSi₂O₆ composition, and these are compared with data available for the CaFeSi₂O₆ composition which shows analagous inversion behaviour. Johannsenite appears to be of comparable stability to hedenbergite at STP, but at elevated temperatures hedenbergite may be stabilised by crystal field effects associated with the Fe²⁺ ion. Together with the stabilisation of the bustamite structure by manganese, this accounts for the low inversion temperature of johannsenite compared to that for hedenbergite.     

Scale effects on modelling the seismic signature of gas: results from an outcrop analogue

Numerous examples of reservoir fields from continental and marine environments involve thin‐bedded geology, yet, the inter‐relationship between thin‐bedded geology, fluid flow and seismic wave propagation is poorly understood. In this paper, we explore the 4D seismic signature due to saturation changes of gas within thin layers, and address the challenge of identifying the relevant scales and properties, which correctly define the geology, fluid flow and seismic wave propagation in the field. Based on the study of an outcrop analogue for a thin‐bedded turbidite, we model the time‐lapse seismic response to fluid saturation changes for different levels of model scale, and explore discrepancies in quantitative seismic attributes caused by upscaling. Our model reflects the geological complexity associated with thin‐bedded turbidites, and its coupling to fluid flow, which in turn affects the gas saturation distribution in space, and its time‐lapse seismic imprint. Rock matrix and fluid properties are modelled after selected fields to reproduce representative field models with realistic impedance contrasts. In addition, seismic modelling includes multiples, in order to assess their contribution in seismic propagation through thin gas layers. Our results show that multiples could contribute significantly to the measured amplitudes in the case of thin‐bedded geology. This suggests that forward/inverse modelling involving the flow simulation and seismic domains used in time‐lapse seismic interpretation should account for thin layers, when these are present in the geological setting.     

Paradigmatic responses of marine benthic communities to different anthropogenic pressures, using M-AMBI, within the European Water Framework Directive

The use of AMBI and M-AMBI in benthic quality assessments, within the European Water Framework Directive (WFD), has increased dramatically in recent years. M-AMBI is a multivariate tool, which incorporates AMBI, richness and Shannon diversity within the assessment. The response of this approach to different human pressures in coastal and estuarine waters has been investigated for the Basque Country (Northern Spain). In this contribution, several paradigmatic examples of these applications to different water bodies are presented. The time-series extend to samples collected between 1995 and 2007, showing the evolution of M-AMBI values under different pressures, i.e. urban and industrial discharges, dredging and disposal of sediments, and engineering works (such as land reclamation or marina construction), and, in addition, for illustrating the benthic quality recovery after positive actions have been undertaken, i.e. the removal of point-source discharges or water treatment programmes. In most cases, M-AMBI responds to these pressures as expected, with decreases in the ecological status immediately following the pressure. Conversely, when a pressure is removed, the recovery takes between 2 and 15 years, depending upon the intensity of the pressure and the characteristics of the water body. M-AMBI has been intercalibrated previously in coastal waters. After intercalibration in transitional waters, M-AMBI will be able to be used in the integrative quality assessment of European water bodies.     

Is delta-aminolevulinic acid dehydratase activity in bivalves from south-west Iberian Peninsula a good biomarker of lead exposure?

The coast of Huelva is considered as a heavily contaminated area where the Tinto and Odiel rivers discharge after running through a metalliferous mining area in the Iberian Pyrite Belt and end in common estuary called Ría of Huelva. Lead is a highly toxic and widely distributed element in the aquatic environment; therefore there is a great interest in assessing the impact of this contaminant on aquatic organisms. To study the bioavailability and sources of lead, the bivalve species Chamelea gallina was collected from six sampling sites along the coast of Huelva. Besides the effect of this metal on the enzyme activity, delta-aminolevulinic acid dehydratase (ALA-D) was determined in the whole tissue. Results show the inverse relationship between the lead concentration and the activity of ALA-D measured in whole tissues (r2=0.7). Individuals collected from the Ría of Huelva estuary had the highest levels of total lead concentrations and the lowest activity of ALA-D. Lead isotope analyses demonstrated the different sources of lead contamination and also confirmed the influence of Ría of Huelva on the metal input to the marine environment.     

On the use and abuse of Newton's second law for variable mass problems

We clarify some misunderstandings currently found in the literature that arise from improper application of Newton's second law to variable mass problems. In the particular case of isotropic mass loss, for example, several authors introduce a force that actually does not exist.Facultad de Ciencias Astronómicas y Geofisicas de la Universidad Nacional de La PlataOn a Fellowship from the Comisión de Investigaciones Cientificas de la Provincia de Buenos Aires.Member of the Carrera del Investigador Cientifico del Consejo Nacional de Investigaciones Cientificas y Técnicas de la República Argentina.