Upscaling microbial chemotaxis in porous media

Biodegradation is an important mechanism for contaminant reduction in groundwater environments; in fact, in situ bioremediation and bioaugmentation methods represent alternatives to traditional methods such as pump-and-treat. Microbial chemotaxis has been shown to significantly increase contaminant degradation in subsurface environments. In this work, the method of volume averaging is used to upscale the microscale chemotactic microbial transport equations in order to obtain the corresponding effective medium models for the mass balance of bacteria and the chemical attractant to which they respond. As a first approach, cellular growth/death and consumption of the attractant by chemical reaction are assumed to be negligible with respect to convective and diffusive transport mechanisms. For microorganisms, two effective coefficients are introduced, namely a total motility tensor and a total velocity vector. Our results show that, under certain conditions, these coefficients can differ considerably from the values corresponding to non-chemotactic transport. These transport coefficients show strong dependence of the microstructure of the porous medium, the fluid flow fields and the distribution of the attractant.     

Quantification of seismic potential failure modes in concrete dams

A comprehensive framework for potential failure modes (PFM) identification and quantification of concrete dams subjected to seismic excitation is presented. A quantifiable indicator of PFM is presented in the context of both linear and nonlinear analyses. As an illustrative example, a thin arch dam subjected to a set of ground motions at different seismic intensity levels is investigated and corresponding PFM quantified. An outcome of this analysis is the probabilistic‐based correlation between linear and nonlinear analyses and identification of the optimal intensity measure parameter. This study, is an adaptation and extension of well‐accepted procedures defined by the performance‐based earthquake engineering paradigm in buildings. Copyright © 2015 John Wiley & Sons, Ltd.     

An Unsteady State Tracer Method for Characterizing Fractures in Bedrock Wells

Evaluating contaminants impacting wells in fractured crystalline rock requires knowledge of the individual fractures contributing water. This typically involves using a sequence of tools including downhole geophysics, flow meters, and straddle packers. In conjunction with each other these methods are expensive, time consuming, and can be logistically difficult to implement. This study demonstrates an unsteady state tracer method as a cost‐effective alternative for gathering fracture information in wells. The method entails introducing tracer dye throughout the well, inducing fracture flow into the well by conducting a slug test and then profiling the tracer concentration in the well to locate water contributing fractures where the dye has been diluted. By monitoring the development of the dilution zones within the wellbore with time, the transmissivity and the hydraulic head of the water contributing fractures can be determined. Ambient flow conditions and the contaminant concentration within the fractures can also be determined from the tracer dilution. This method was tested on a large physical model well and a bedrock well. The model well was used to test the theory underlying the method and to refine method logistics. The approach located the fracture and generated transmissivity values that were in excellent agreement with those calculated by slug testing. For the bedrock well tested, two major active fractures were located. Fracture location and ambient well conditions matched results from conventional methods. Estimates of transmissivity values by the tracer method were within an order of magnitude of those calculated using heat‐pulse flow meter data.     

Real Time Analysis and Display of Scanning Lidar Scattering Data

We describe software for an IBM compatible personal computer (PC) that we have developed and used to collect and analyze backscatter data from a multi-wavelength Mie-Rayleigh scanning lidar system. Both one and two-dimensional distributions of optical scattering coefficients are calculated from the lidar return signals and displayed in real time. Although we are using the software for analyzing marine aerosols, the modular nature of the software and data acquisition drivers make it straightforward to adapt the software to different systems, such as differential absorption lidar (DIAL) and oceanic lidars. We show examples of both horizontal and vertical Mie-Rayleigh lidar scans collected at the Makai Pier and Bellows Beach on the northeast side of the island of Oahu.     

Ocean-bottom seismograph observations on the Mid-Atlantic Ridge near 45° N

Summary. Rayleigh-wave phase velocities at very long periods (185–290 s) are investigated and regionalized, taking into account the lateral heterogeneities within ocean plates revealed by earlier studies at shorter periods. The two-station method is applied to a few 'pure-age' oceanic paths, and is shown to be compatible with the average Earth model C2 (Anderson & Hart 1976) below depths of 180 km. Under this assumed oceanic model, regionalized for age above 180 km, continental velocities are then derived from a set of experimental great-circle values, both new or taken from previously published studies. The results basically agree with earlier studies (Dziewonski 1970; Kanamori 1970), although they exhibit less scatter than Kanamori's model. Results are successfully checked against a set of values derived by the two-station method from a pure continental path.
Although the shield velocities are substantially different from the mean oceanic ones, they still fall within the range of variation of oceanic velocities with the age of the plate. This makes velocities derived theoretically from Jordan's (1975a, b) models of deep continent—ocean lateral heterogeneities, inconsistent with the present set of experimental data. Finally, we show that Dziewonski's (1971) model S2 reconciles all experimental seismic data relative to shields, without being significantly different from oceanic models below 240 km.     

The impact of resuspension on sediment mercury dynamics, and methylmercury production and fate: A mesocosm study

The objective of this study was to investigate the effects of resuspension on the fate and bioaccumulation of mercury (THg) and methylmercury (MeHg) in shallow estuarine environments, using mesocosms. Two 4-week experiments were conducted in July (Experiment 1) and October (Experiment 2) of 2001 with Baltimore Harbor sediments. Hard clams, Mercenaria mercenaria, were introduced into sediments for Experiment 2. Tidal resuspension (4 h on and 2 h off cycles) was simulated, with 3 replicate tanks for each treatment—resuspension (R) and non-resuspension (NR). Sediment cores were collected during the experiments for THg, MeHg, organic content and AVS analyses, and for the determination of methylation/demethylation using Hg stable isotopes (¹⁹⁹Hg(II) and CH₃¹⁹⁹Hg(II)). Zooplankton samples were collected once a week while clams were taken before and after Experiment 2 for THg and MeHg analyses. Our results suggest that the interplay between Hg methylation and MeHg degradation determines the overall MeHg pool in sediments. Sediment resuspension does not appear to directly impact the Hg transformations but can lead to changes in the association to Hg binding phases, influencing Hg methylation. The bioaccumulation results indicate that sediment resuspension can play an important role in transferring sediment MeHg into organisms.     

Hydrodynamic coefficients of a harmonically oscillated tower

A tower hinged at the bottom was oscillated mechanically in a sinusoidal fashion in a plane in still water. An instrumented section in the tower measured the inline and transverse forces locally on the tower due to the hydrodynamic effects. These forces are analyzed for the added mass, drag and lift coefficients which are presented as functions of Keulegan-Carpenter and Reynolds number. The lift force frequencies are also investigated. The measured overall reactions on the tower are used to verify the values of the local coefficients. The results presented here are not only applicable to articulated towers but to other moving elements of an offshore structure, e.g. risers, tension-legs, etc.     

Stable isotope evidence for regional-scale fluid migration in a Barrovian metamorphic terrane,Vermont, USA

We measured the C- and O-isotopic composition of carbonate minerals in the Waits River Formation, eastern Vermont, to determine the extent of fluid infiltration during regional metamorphism over an approx. 2000 km² area in a deep-seated (>25 km) Barrovian terrane. From a petrologic study of this terrane, Ferry proposed the existence of a large regional metamorphic hydrothermal system with two first-order features: (1) on the scale of the entire terrane fluid flow was focused into the axes of two major antiforms of regional extent; (2) on a smaller scale (about 100 km²) flow was further focused around synmetamorphic granitic plutons that intruded along the axes of the antiforms. We find isotopic evidence for both the regional hydrothermal activity along the antiforms and the more intense fluid flow around synmetamorphic plutons. The evidence for hydrothermal activity around the plutons is large heavy isotope depletions, up to 6–9 in ¹⁸O_carb and ¹³C_carb, in diopside zone rocks adjacent to the plutons. These isotopic shifts are greater than can be explained solely by prograde metamorphic reactions. We find two lines of evidence for the more diffuse regional flow that was focused into axes of the antiforms. First, ¹⁸O_carb and ¹³C_carb, within individual outcrops become increasingly homogeneous with increasing grade towards antiform axes, indicating that the rocks equilibrated with a permeating fluid. Second, there are depletions in ¹⁸O near the margins of the Waits River Formation which can be interpreted as a dispersed, advective infiltration front displaced toward the antiform axes. These fronts were modelled using Eq. 13 of Bickle and Baker and imply time-integrated fluid of 10⁵–10⁶ cm³/cm², which are consistent with values derived by Ferry from measured progress of prograde devolatilization reactions.