Effects of changing land use on the microbial water quality of tidal creeks

Population growth along the southeastern United States coast has precipitated the conversion of forested watersheds to suburban and urban ones. This study sampled creeks representing forested, suburban, and urban watersheds along a longitudinal gradient for indicators of water quality, including traditional indicator bacteria (fecal coliforms and enterococci) and alternative viral indicators (male-specific and somatic coliphages). Tested microorganisms were generally distributed with highest concentrations in creek headwaters and in more developed watersheds. The headwaters also showed the strongest predictive relationship between indicator concentrations and urbanization as measured by impervious cover. A seasonal pattern was observed for indicator bacteria but not for indicator viruses. Coliphage typing indicated the likely source of contamination was nonhuman. Results suggest that headwater creeks can serve as sentinel habitat, signaling early warning of public health concerns from land-based anthropogenic activities. This study also implies the potential to eventually forecast indicator concentrations under land use change scenarios.     

Shear effects on hollow section piers under seismic actions: experimental and numerical analysis

Shear effects are often a very important issue on the seismic behaviour of piers, particularly for hollow section bridge piers. In fact, for this type of piers the cyclic response is similar to that of a structural wall in which both the transverse reinforcement ratio and the detailing can play an important role on its performance, even likely to be determinant in terms of the failure mechanism. On the other hand, codes and design guidelines are usually very conservative concerning shear capacity in order to avoid any shear failure mechanism likely to trigger well known catastrophic consequences. Therefore, research studies on this topic are still needed for a better understanding of pier cyclic shear response and also for improvement of the performance under seismic actions. Pursuing this general objective, this paper partially reports on an experimental/numerical campaign carried out on 1:4 reduced scale bridge piers in order to highlight and investigate shear-type problems. Within the scope of this paper, two specimens types were selected having equal rectangular hollow section (900 × 450 mm², 75 mm thick) but different transverse reinforcement detailing, namely one with a single stirrup per wall (representative of typical bridge construction without seismic design requirements) and another with multiple stirrups, according to Eurocode 8 provisions. Numerical simulations of the experimental results were also conducted aiming at contributing for complete and consistent interpretations of experimental results. Detailed modelling was performed allowing for realistic simulations of the non linear behaviour, particularly suitable when a significant shear component is involved. Therefore, the numerical strategy was based on a detailed 3D FEM discretization using a two-scalar variable damage model for the concrete constitutive law and a suitable cyclic behaviour law for steel bars represented by truss elements. Results have shown that shear deformation and failure modes are well simulated, while providing detailed insight concerning concrete damage pattern and distribution of yielding on the transverse and longitudinal reinforcement.     

Experimental synthesis and phase relations of phengitic muscovite from 6.5 to 11 GPa in a calcareous metapelite from the Dabie Mountains, China

The stability and phase relations of phengitic muscovite in a metapelitic bulk composition containing a mixed H₂O+CO₂ fluid were investigated at 6.5–11 GPa, 750–1050°C in synthesis experiments performed in a multianvil apparatus. Starting material consisted of a natural calcareous metapelite from the coesite zone of the Dabie Mountains, China, ultrahigh-pressure metamorphic complex that had experienced peak metamorphic pressures greater than 3 GPa. The sample contains a total of 2.1 wt.% H₂O and 6.3 wt.% CO₂ bound in hydrous and carbonate minerals. No additional fluid was added to the starting material. Phengite is stable in this bulk composition from 6.5 to 9 GPa at 900°C and coexists with an eclogitic phase assemblage consisting of garnet, omphacite, coesite, rutile, and fluid. Phengite dehydrates to produce K-hollandite between 8 and 11 GPa, 750–900°C. Phengite melting/dissolution occurs between 900°C and 975°C at 6.5–8 GPa and is associated with the appearance of kyanite in the phase assemblage. The formation of K-hollandite is accompanied by the appearance of magnesite and topaz-OH in the phase assemblage as well as by significant increases in the grossular content of garnet (average X_grs=0.52, X_py=0.19) and the jadeite content of omphacite (X_jd=0.92). Mass balance indicates that the volatile content of the fluid phase changes markedly at the phengite/K-hollandite phase boundary. At P≤8 GPa, fluid coexisting with phengite appears to be relatively CO₂-rich (XCO₂/XH₂O=2.2), whereas fluid coexisting with K-hollandite and magnesite at 11 GPa is rich in H₂O (XCO₂/XH₂O=0.2). Analysis of quench material and mass balance calculations indicate that fluids at all pressures and temperatures examined contain an abundance of dissolved solutes (approximately 40 mol% at 8 GPa, 60 mol% at 11 GPa) that act to dilute the volatile content of the fluid phase. The average phengite content of muscovite is positively correlated with pressure and ranges from 3.62 Si per formula unit (pfu) at 6.5 GPa to 3.80 Si pfu at 9 GPa. The extent of the phengite substitution in muscovite in this bulk composition appears to be limited to a maximum of 3.80–3.85 Si pfu at P=9 GPa. These experiments show that phengite should be stable in metasediments in mature subduction zones to depths of up to 300 km even under conditions in which aH₂O1. Other high-pressure hydrous phases such as lawsonite, MgMgAl-pumpellyite, and topaz-OH that may form in subducted sediments do not occur within the phengite stability field in this system, and may require more H₂O-rich fluid compositions in order to form. The wide range of conditions under which phengite occurs and its participation in mixed volatile reactions that may buffer the composition of the fluid phase suggest that phengite may significantly influence the nature of metasomatic fluids released from deeply subducted sediments at depths of up to 300 km at convergent plate boundaries.     

Application of neural networks in natural periods identification and simulation of prefabricated buildings response

The paper deals with an application of neural networks for detection of natural periods of vibrations of prefabricated, medium height buildings. The neural network technique is also used to simulate the dynamic response at selected floor of one of the analysed buildings subject to seismic loading induced by explosives in a nearby quarry. Both the training and testing patterns were formulated on the basis of measurements performed on actual structures. The results of neural network identification of natural periods of the considered buildings obtained with different soil, geometrical and stiffness parameters are compared with the results of experiments. The application of back-propagation neural networks enables us to identify the natural periods of the buildings with accuracy quite satisfactory for engineering practice. The experimental and generated data of vibration displacements are compared and much clearer comparison is given on the phase plane: displacements versus velocities. It was stated that a good generalization takes place both with respect to displacements and velocities.     

EROSION AND SEDIMENTATION PROBLEMS IN SOME ASIAN COUNTRIES

1 INTRODUCTIONFrom 1989 to 1993, as project managers of the UNDP/UNESCO project entitled Regional TrainingProgramme on Erosion and Sedimentation for Asia, the writers had the privilege of meeting manyoutstanding hydraulic engineers of this region and came to learn of some important erosion andsedimentation problems thereof. Some issues as noted by the writers are presented in the followingalong the some comments. These issues are mainly concerned with rivers. It should be stated tha…     

Effect of Fe on garnet–melt trace element partitioning: experiments in FCMAS and quantification of crystal-chemical controls in natural systems

Garnet–melt trace element partitioning experiments were performed in the system FeO–CaO–MgO–Al₂O₃–SiO₂ (FCMAS) at 3 GPa and 1540°C, aimed specifically at studying the effect of garnet Fe²⁺ content on partition coefficients (D^Grt/Melt). D^Grt/Melt, measured by SIMS, for trivalent elements entering the garnet X-site show a small but significant dependence on garnet almandine content. This dependence is rationalised using the lattice strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994. Prediction of crystal–melt partition coefficients from elastic moduli. Nature 372, 452–454], which describes partitioning of an element i with radius r_i and valency Z in terms of three parameters: the effective radius of the site r₀(Z), the strain-free partition coefficient D₀(Z) for a cation with radius r₀(Z), and the apparent compressibility of the garnet X-site given by its Young's modulus E_X(Z). Combination of these results with data in Fe-free systems [Van Westrenen, W., Blundy, J.D., Wood, B.J., 1999. Crystal-chemical controls on trace element partitioning between garnet and anhydrous silicate melt. Am. Mineral. 84, 838–847] and crystal structure data for spessartine, andradite, and uvarovite, leads to the following equations for r₀(3+) and E_X(3+) as a function of garnet composition (X) and pressure (P):

r₀(3+) [Å]=0.930X_Py+0.993X_Gr+0.916X_Alm+0.946X_Spes+1.05(X_And+X_Uv)−0.005(P [GPa]−3.0)(±0.005 Å)

E_X(3+) [GPa]=3.5×10¹²(1.38+r₀(3+) [Å])^−26.7(±30 GPa)

Accuracy of these equations is shown by application to the existing garnet–melt partitioning database, covering a wide range of P and T conditions (1.8 GPa<P<5.0 GPa; 975°C<T<1640°C). D^Grt/Melt for all 3+ elements entering the X-site (REE, Sc and Y) are predicted to within 10–40% at given P, T, and X, when D^Grt/Melt for just one of these elements is known. In the absence of such knowledge, relative element fractionation (e.g. D_Sm^Grt/Melt/D_Nd^Grt/Melt) can be predicted. As an example, we predict that during partial melting of garnet peridotite, group A eclogite, and garnet pyroxenite, r₀(3+) for garnets ranges from 0.939±0.005 to 0.953±0.009 Å. These values are consistently smaller than the ionic radius of the heaviest REE, Lu. The above equations quantify the crystal-chemical controls on garnet–melt partitioning for the REE, Y and Sc. As such, they represent a major advance en route to predicting D^Grt/Melt for these elements as a function of P, T and X.     

The effect of Ca-Tschermaks component on trace element partitioning between clinopyroxene and silicate melt

We have studied the influence of Ca-Tschermaks (Calcium Tschermaks or CaTs) content of clinopyroxene on the partitioning of trace elements between this phase and silicate melt at fixed temperature and pressure. Ion probe analyses of experiments carried out in the system Na₂O–CaO–MgO–Al₂O₃–SiO₂, at 0.1 MPa and 1218°C, produced crystal-melt partition coefficients (D) of 36 trace elements (Li, Cl, Sc, Ti, V, Cr, Fe, Co, Ge, Sr, Y, Zr, Nb, Mo, Ru, Rh, In, Sn, Sb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta and W), for clinopyroxene compositions between 10 and 32 mol% CaTs. Partition coefficients for 2+ to 5+ cations show, for each charge, a near parabolic dependence of log D on ionic radius of the substituting cation, for partitioning into both the M1 and M2 sites of clinopyroxene. Fitting the results to the elastic strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994. Prediction of crystal-melt partition coefficients from elastic moduli. Nature 372, 452–454] we obtain results for the strain-free partition coefficients of theoretical cations (D₀), with site radius r₀, and for the site's Young's Modulus (E).

In agreement with earlier data our results show that increasing ^ivAl concentration in cpx is matched by increasing D, E_M1, E_M2 and D₀ for tri-, tetra- and pentavalent cations. The degree of fractionation between chemically similar elements (i.e. Ta/Nb, Zr/Hf) also increases. In contrast, D values for mono-, di- and hexavalent cations decrease with increasing ^ivAl in the cpx. The large suite of trace elements used has allowed us to study the effects of cation charge on D₀, r₀ and E. We have found that D₀ and r₀ decrease with increasing cation charge, e.g. r₀=0.66 Å for 4+ cations and 0.59 Å for 5+ cations substituting into M1. Values of E_M1 and E_M2 increase with cation charge as well as with increasing ^ivAl content. The increase in E_M2 is linear and close to the trend set by Hazen and Finger [Hazen, R.M., Finger, L.W., 1979. Bulk modulus-volume relationship for cation–anion polyhedra. J. Geophys. Res. 84 (10) 6723–6728] for oxides. E_M1 values are much higher and do not fit the trend predicted by the Hazen and Finger relationship.     

The influence of Cr on the garnet–spinel transition in the Earth''s mantle: experiments in the system MgO–Cr2O3–SiO2 and thermodynamic modelling

The position of the transition from spinel peridotite to garnet peridotite in a simplified chemical composition has been determined experimentally at high pressures and high temperatures. The univariant reaction MgCr₂O₄+2Mg₂Si₂O₆=Mg₃Cr₂Si₃O₁₂+Mg₂SiO₄, has a negative slope in P–T space between 1200 °C and 1600 °C. The experimental results, combined with assessed thermodynamic data for MgCr₂O₄, MgSiO₃ and Mg₂SiO₄ give the entropy and enthalpy of formation of knorringite garnet (Mg₃Cr₂Si₃O₁₂). Thermodynamic calculations in simplified chemical compositions indicate that Cr shifts the garnet-in reaction to much higher pressures than previously anticipated. Moreover, in Cr-bearing systems a pressure–temperature field exists where garnet and spinel coexist. The width of this divariant field strongly depends on the Cr/(Cr+Al) of the system.