The Effect of Bottle Fill Rate and Pour Technique on the Recovery of Volatile Organics

This field study was conducted to examine whether the method or the flow rate (fill rate) used to fill a sample vial affects recovery of volatile organics (VOCs). To our knowledge, there have not been any systematic studies that have examined this issue. For this field study, three fill rates (50 mL/min, 250 mL/min, and ~1 L/min) and three filling methods (top‐pour, side‐pour, and bottom‐fill) were used to fill sample vials. We found that the bottom‐fill method, with the tubing submerged in the sample as it fills, yielded the greatest recovery (i.e., highest concentrations) of VOCs. Little improvement was observed by pouring down the side of a vial vs. simply pouring straight down from the top. We also found that filling the vials at the fastest fill rate (~1 L/min) yielded higher recovery than the slowest fill rate (50 mL/min) using all three filling methods. These results are counter to prevailing guidance and conventional wisdom that slower filling is preferable to faster filling and that pouring down the side of a sample vial is the best practice for VOC sampling. However, because we were unable to randomize the order the samples were collected with respect to fill rate, we recommend a follow‐on study be conducted that will allow us to confirm our findings and better determine which fill rates minimize losses of VOCs     

2011年日本东北地区太平洋近海地震对亚洲板块及韩国大地控制网的影响分析

本研究分析了2011年3月11日发生的M_w9.0日本东北地区太平洋近海地震对亚洲地区和韩国国内GPS卫星常年跟踪站的位移影响.为此,利用了日本东北地区太平洋近海地震发生前后两周(2011年3月4日到3月18日)的GPS站点数据,包括震中附近地区(韩国,中国,中国台湾地区,日本和俄罗斯)55个GPS卫星常年跟踪站和284个IGS 全球跟踪站,并采用GAMIT/GLOBK软件进行处理和平差,估算出所有GPS站点的同震形变.结果显示,日本东北地区太平洋近海地震引起的同震形变影响在亚洲地区比较明显,包括日本和附近国家,距离震中2702 km的中国武汉(WUHN)站也观测到同震形变.为精确分析日本东北地区太平洋近海地震对韩国国家大地控制网的影响,通过GAMIT/GLOBK软件计算出韩国GPS卫星常年跟踪站之间的基线长度变形,并分析出弹性变形量.结果表明:大部分GPS站点均向震中方向膨胀,且向震中的垂直方向收缩.由日本东北地区太平洋近海地震导致的最大剪应变达到韩国国家大地控制网年均变形率的约7倍,对韩国的地壳产生14.5~57.7 mm的水平位移,并导致韩国国家大地控制网产生弹性变形.因此,在不及时更新维护韩国国家大地控制网的情况下,GPS测量成果将会发生最大20 mm的位置误差.     

An analytical model for a transient vapor plume on the Moon

Naturally occurring lunar transients have been observed for many centuries. Artificially induced impacts on airless near-Earth objects are now being used as a test bed to study a wide range of geomorphic phenomena. In this paper an analytical theory is used to predict the flash event that signals the initial phase of a surface impact. The model predicts the wave form, duration, and radiated energy content of the vapor plume starting only with the mass and kinetic energy of the impactor. The theory is applied to the planned 2009 LCROSS lunar impact where the impact flash event is predicted. The transient radiation emitted by the impact can be used to obtain some relevant impact parameters, and it is predicted to be energetic enough to possibly be captured by terrestrial observers.     

Tracer mass recovery in fractured aquifers estimated from multiple well tests

Forced-gradient tracer tests in fractured aquifers often report low mass recoveries. In fractured aquifers, fractures intersected by one borehole may not be intersected by another. As a result (1) injected tracer can follow pathways away from the withdrawal well causing low mass recovery and (2) recovered water can follow pathways not connected to the injection well causing significant tracer dilution. These two effects occur along with other forms of apparent mass loss. If the strength of the connection between wells and the amount of dilution can be predicted ahead of time, tracer tests can be designed to optimize mass recovery and dilution. A technique is developed to use hydraulic tests in fractured aquifers to calculate the conductance (strength of connection) between well pairs and to predict mass recovery and amount of dilution during forced gradient tracer tests. Flow is considered to take place through conduits, which connect the wells to each other and to distant sources or sinks. Mass recovery is related to the proportion of flow leaving the injection well and arriving at the withdrawal well, and dilution is related to the proportion of the flow from the withdrawal well that is derived from the injection well. The technique can be used to choose well pairs for tracer tests, what injection and withdrawal rates to use, and which direction to establish the hydraulic gradient to maximize mass recovery and/or minimize dilution. The method is applied to several tracer tests in fractured aquifers in the Clare Valley, South Australia.     

Wave-forced resuspension of upper Chesapeake Bay muds

Moored instruments were used to make observations of near bottom currents, waves, temperature, salinity, and turbidity at shallow (3.5 m and 5.5 m depth) dredged sediment disposal sites in upper Chesapeake Bay during the winters of 1990 and 1991 to investigate time-varying characteristics of resuspension processes over extended periods. Resulting time series data show the variability of two components of the suspended sediment concentration field. Background suspended sediment concentrations varied inversely with salinity and in direct relation to Susquehanna River flow. Muddy bottom sediments were also resuspended locally by both tidal currents and wind-wave forcing, resulting in short-term increases and decreases in suspended concentration, with higher peak concentrations near the bottom. In both years, episodes of wave-forced resuspension dominated tidal resuspension on an individual event basis, exceeding most tidal resuspension peaks by a factor of 3 to 5. The winds that generated the waves responsible for the observed resuspension events were not optimal for wave generation, however. Application of a simple wind-wave model showed that much greater wave-forced resuspension than that observed might be generated under the proper conditions. The consolidated sediments investigated in 1990 were less susceptible to both tidal and wave-forced resuspension than the recently deposited sediments investigated in 1991. There was also some indication that wave-forced resuspension increased erodibility of the bottom sediments on a short-term basis. Wave-forced resuspension is implicated as an important part of sediment transport processes in much of Chesapeake Bay. Its role in deeper, narrower, and more tidally energetic estuaries is not as clear, and should be investigated on a case-by-case basis.     

Mechanisms of Storm-Related Loss and Resilience in a Large Submersed Plant Bed

There is a growing emphasis on preserving ecological resilience, or a system’s capacity to absorb or recover quickly from perturbations, particularly in vulnerable coastal regions. However, the factors that affect resilience to a given disturbance are not always clear and may be system-specific. We analyzed and synthesized time series datasets to explore how extreme events impacted a large system of submersed aquatic vegetation (SAV) in upper Chesapeake Bay and to identify and understand associated mechanisms of resilience. We found that physical removal of plants around the edge of the bed by high flows during a major flood event as well as subsequent wind-driven resuspension of newly deposited sediment and attendant light-limiting conditions were detrimental to the SAV bed. Conversely, it appears that the bed attenuated high flows sufficiently to prevent plant erosion at its inner core. The bed also attenuated wind-driven wave amplitude during seasonal peaks in plant biomass, thereby decreasing sediment resuspension and increasing water clarity. In addition, clear water appeared to “spill over” into adjacent regions during ebb tide, improving the bed’s capacity for renewal by creating more favorable growing conditions in areas where plant loss had occurred. These analyses demonstrate that positive feedback processes, whereby an SAV bed modifies its environment in ways that improve its own growth, likely serve as mechanisms of SAV resilience to flood events. Although this work focuses on a specific system, the synthetic approach used here can be applied to any system for which routine monitoring data are available.     

Contribution of deviatoric stresses to metamorphic P-T paths: an example appropriate to low-P, high-T metamorphism

P-T paths for a simple situation appropriate to many low- P , high- T (LPHT) terranes in which metamorphism and deformation are localized by advection of heat in magmas, has been modelled assuming a medium with a power-law rheology with an inverse exponential dependence of stress and temperature and capable of sustaining deviatoric stress, τ, in the order of 100 MPa at 400 C and strain rates of up to 10^-13s^-1. Numerical simulations and analytical approximations for P-T histories appropriate for simple convergent deformation histories show that the destruction of the deviatoric stress field around large intrusions may result in significant decompression near the metamorphic temperature peak. Moreover, for a specified strain rate and temperature evolution, P-T paths may vary from clockwise to anticlockwise merely as a function of vertical distance from the heat source. Inasmuch as mounting independent evidence suggests that the crust can support deviatoric stresses of up to about 100 MPa at temperatures of 400-500 C, and that the shear strength of the crust is strongly temperature-dependent in the range 400-800 C, these results suggest that caution should be taken in the tectonic interpretation of P-T paths involving decompression of the order of 100 MPa or less in LPHT terranes. The results illustrate a plausible mechanism for the close spatial association of both clockwise and anticlockwise P-T paths documented in some LPHT terranes.     

Internal recycling of particle reactive organic chemicals in the Chesapeake Bay water column

Hydrophobic organic contaminants (HOCs) may be used as tracers of particle dynamics in aquatic systems. Internal cycling of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) were studied in the mesohaline Chesapeake Bay to assess the role of resuspension in maintaining particle and contaminant inventories in the water column, and to compare settling and suspended particle characteristics. Direct measurements of sediment resuspension and settling conducted in conjunction with one of the sediment trap deployments indicate reasonable agreement between measurements of particle flux using the two different methods. Organic carbon and PCB concentrations in settling solids collected in near-surface sediment traps were remarkably lower than concentrations in suspended particles collected by filtration during the trap deployments, but higher PAH concentrations were found in the settling particles. The different behaviors of PAHs and PCBs in the settling particles are due to their different source types and association to different types of particles. Sediment trap collections in near bottom waters were dominated by resuspension. Resuspension fluxes of HOCs measured 2 m above the bay bottom were as high as 2.5 μg/m² day for total PCBs and 15 μg/m² day for fluoranthene, and were 25 and 10 times higher than their settling fluxes from surface waters, respectively. HOC concentrations in the near bottom traps varied much less between trap deployments than HOC concentrations in the surface traps, indicating that the chemical composition of the resuspended particles collected in the near bottom traps was more time-averaged by repeated resuspension than the surface particles.     

A nearshore model to investigate the effects of seagrass bed geometry on wave attenuation and suspended sediment transport

The effects of seagrass bed geometry on wave attenuation and suspended sediment transport were investigated using a modified Nearshore Community Model (NearCoM). The model was enhanced to account for cohesive sediment erosion and deposition, sediment transport, combined wave and current shear stresses, and seagrass effects on drag. Expressions for seagrass drag as a function of seagrass shoot density and canopy height were derived from published flume studies of model vegetation. The predicted reduction of volume flux for steady flow through a bed agreed reasonably well with a separate flume study. Predicted wave attenuation qualitatively captured seasonal patterns observed in the field: wave attenuation peaked during the flowering season and decreased as shoot density and canopy height decreased. Model scenarios with idealized bathymetries demonstrated that, when wave orbital velocities and the seagrass canopy interact, increasing seagrass bed width in the direction of wave propagation results in higher wave attenuation, and increasing incoming wave height results in higher relative wave attenuation. The model also predicted lower skin friction, reduced erosion rates, and higher bottom sediment accumulation within and behind the bed. Reduced erosion rates within seagrass beds have been reported, but reductions in stress behind the bed require further studies for verification. Model results suggest that the mechanism of sediment trapping by seagrass beds is more complex than reduced erosion rates alone; it also requires suspended sediment sources outside of the bed and horizontal transport into the bed.