Natural Gradient Tracer Test to Evaluate Natural Attenuation of MTBE Under Anaerobic Conditions

A natural gradient tracer test using perdeuterated MTBE was conducted in an anaerobic aquifer to determine the relative importance of dispersion and degradation in reducing MTBE concentrations in ground water. Preliminary ground water chemistry and hydraulic conductivity data were used to place the tracer within an existing dissolved MTBE plume at Port Hueneme, California. Following one year of transport, the tracer plume was characterized in detail.
Longitudinal dispersion was identified as the dominant mechanism for lowering the perdeuterated MTBE concentrations. The method of moments was used to determine the longitudinal and lateral dispersion coefficients (0.85 m²/day and 0.08 m²/day, respectively). A mass-balance analysis, carried out after one year of transport, accounted for 110% of the injected mass and indicated that no significant mass loss occurred. The plume structure created by zones of higher and lower hydraulic conductivity at the site was complex, consisting of several localized areas of high tracer concentration in a lower concentration plume. This is important because the aquifer has generally been characterized as exhibiting fairly minor heterogeneity. In addition, the tracer plume followed a curved flowpath that deviated from the more macroscopic direction of ground water flow inferred from local ground water elevation measurements and the behavior of the existing plume. Understanding the mass balance, plume structure, curvature of the tracer plume, and consequently natural attenuation behavior required the detailed sampling approach employed in this study. These data imply that a detailed understanding of site hydrogeology and an extensive sampling network may be critical for the correct interpretation of monitored natural attenuation of MTBE.     

Neoarchaean magmatism and metamorphism of the western granulites in the central domain of the Mozambique belt, Tanzania: U–Pb shrimp geochronology and PT estimates

U–Pb sensitive high resolution ion microprobe (SHRIMP) dating of zircons from charnockitic and garnet–biotite gneisses from the central portion of the Mozambique belt, central Tanzania indicate that the protolith granitoids were emplaced in a late Archaean, ca. 2.7 Ga, magmatic event. These ages are similar to other U–Pb and Pb–Pb ages obtained for other gneisses in this part of the belt. Zircon xenocrysts dated between 2.8 and 3.0 Ga indicate the presence of an older basement. Major and trace element geochemistry of these high-grade gneisses suggests that the granitoid protoliths may have formed in an active continental margin environment. Metamorphic zircon rims and multifaceted metamorphic zircons are dated at ca. 2.6 Ga indicating that these rocks were metamorphosed some 50–100 my after their emplacement. Pressure and temperature estimates on the charnockitic and garnet–biotite gneisses were obscured by post-peak metamorphic compositional homogenisation; however, these estimates combined with mineral textures suggest that these rocks underwent isobaric cooling to 800–850 °C at 12–14 kbar. It is considered likely that the granulite facies mineral assemblage developed during the ca. 2.6 Ga event, but it must be considered that it might instead represent a pervasive Neoproterozoic, Pan African, granulite facies overprint, similar to the ubiquitous eastern granulites further to the east.     

The Global Distribution of Secondary Particulate Matter in a 3-D Lagrangian Chemistry Transport Model

A global 3-D Lagrangian chemistry-transport model STOCHEM is used to describe the tropospheric distributions of four components of the secondary atmospheric aerosol: nitrate, sulphate, ammonium and organic compounds. The model describes the detailed chemistry of the formation of the acid precursors from the oxidation of SO₂, DMS, NO_x, NH₃ and terpenes and their uptake into the aerosol. Model results are compared in some detail with the available surface observations. Comparisons are made between the global budgets and burdens found in other modelling studies. The global distributions of the total mass of secondary aerosols have been estimated for the pre-industrial, present day and 2030 emissions and large changes have been estimated in the mass fractions of the different secondary aerosol components.     

Tropospheric Ozone in a Global-Scale Three-Dimensional Lagrangian Model and Its Response to NOX Emission Controls

A three-dimensional Lagrangian tropospheric chemistry modelis used toinvestigate the impact of human activities on the tropospheric distributionofozone and hydroxyl radicals. The model describes the behaviour of 50 speciesincluding methane, carbon monoxide, oxides of nitrogen, sulphur dioxide andnineorganic compounds emitted from human activities and a range of other sources.Thechemical mechanism involves about 100 chemical reactions of which 16 arephotochemical reactions whose diurnal dependence is treated in full. The modelutilises a five minute chemistry time step and a three hour advection timestepfor the 50,000 air parcels. Meteorological data for the winds, temperatures,clouds and so on are taken from the UK Meteorological Office global model for1994 onwards. The impacts of a 50% reduction in European NO_Xemissions onglobal ozone concentrations are assessed. Surface ozoneconcentrations decrease in summertime and rise in wintertime, but to differentextents.     

Influence of landscape position and vegetation on long-term weathering rates at the Hubbard Brook Experimental Forest, New Hampshire, USA

The spatial variability of long-term chemical weathering in a small watershed was examined to determine the effect of landscape position and vegetation. We sampled soils from forty-five soil pits within an 11.8-hectare watershed at the Hubbard Brook Experimental Forest, New Hampshire. The soil parent material is a relatively homogeneous glacial till deposited ∼14,000 years ago and is derived predominantly from granodiorite and pelitic schist. Conifers are abundant in the upper third of the watershed while the remaining portion is dominated by hardwoods. The average long-term chemical weathering rate in the watershed, calculated by the loss of base cations integrated over the soil profile, is 35 meq m⁻² yr⁻¹—similar to rates in other ∼10 to 15 ka old soils developed on granitic till in temperate climates. The present-day loss of base cations from the watershed, calculated by watershed mass balance, exceeds the long-term weathering rate, suggesting that the pool of exchangeable base cations in the soil is being diminished. Despite the homogeneity of the soil parent material in the watershed, long-term weathering rates decrease by a factor of two over a 260 m decrease in elevation. Estimated weathering rates of plagioclase, potassium feldspar and apatite are greater in the upper part of the watershed where conifers are abundant and glacial till is thin. The intra-watershed variability across this small area demonstrates the need for extensive sampling to obtain accurate watershed-wide estimates of long-term weathering rates.     

Imaging the ramp–décollement geometry of the Chelungpu fault using coseismic GPS displacements from the 1999 Chi-Chi, Taiwan earthquake

We use coseismic GPS data from the 1999 Chi-Chi, Taiwan earthquake to estimate the subsurface shape of the Chelungpu fault that ruptured during the earthquake. Studies prior to the earthquake suggest a ramp–décollement geometry for the Chelungpu fault, yet many finite source inversions using GPS and seismic data assume slip occurred on the down-dip extension of the Chelungpu ramp, rather than on a sub-horizontal décollement. We test whether slip occurred on the décollement or the down-dip extension of the ramp using well-established methods of inverting GPS data for geometry and slip on faults represented as elastic dislocations. We find that a significant portion of the coseismic slip did indeed occur on a sub-horizontal décollement located at 8 km depth. The slip on the décollement contributes 21% of the total modeled moment release. We estimate the fault geometry assuming several different models for the distribution of elastic properties in the earth: homogeneous, layered, and layered with lateral material contrast across the fault. It is shown, however, that heterogeneity has little influence on our estimated fault geometry. We also investigate several competing interpretations of deformation within the E/W trending rupture zone at the northern end of the 1999 ground ruptures. We demonstrate that the GPS data require a 22- to 35-km-long lateral ramp at the northern end, contradicting other investigations that propose deformation is concentrated within 10 km of the Chelungpu fault. Lastly, we propose a simple tectonic model for the development of the lateral ramp.