How Effective Is Thermal Remediation of DNAPL Source Zones in Reducing Groundwater Concentrations?

Dense nonaqueous phase liquid (DNAPL) source areas containing chlorinated volatile organic compounds (cVOCs) such as trichloroethene (TCE) and perchloroethene (PCE) often give rise to significant dissolved plumes in groundwater, leading to the closure of downgradient water supply wells and creating vapor intrusion issues in buildings located above the plume. Hydraulic containment via pump‐and‐treat has often been implemented to limit migration but must continue indefinitely. Removal of the DNAPL source area by means such as in situ thermal remediation (ISTR) offers the potential to diminish or end the need for hydraulic containment if the associated dissolved plume attenuates sufficiently following source removal. A question often raised is whether this occurs or whether the back diffusion of contaminants from secondary sources such as low‐permeability lenses in the dissolved plume precludes it. The authors conducted DNAPL source removal using ISTR at dozens of sites. This paper presents a compilation of cases—10 separate DNAPL source areas at five project sites—where data indicate that the implementation of a thorough ISTR in a DNAPL source area can result in the attenuation of the associated dissolved plume, such that in several cases, long‐standing pump‐and‐treat systems could be turned off. Our findings contrast with recent assertions that aggressive source remediation may not be justifiable because dissolved plume concentrations will not decline sufficiently. We show that the application of ISTR can result in the thorough removal of the DNAPL source, effective diminution of dissolved plume groundwater concentrations, and achievement of drinking water standards.     

Velocity and Intensity Power and Cross Spectra in Numerical Simulations of Solar Convection

Fitting observed power and cross spectra of medium-degree p modes in velocity (V) and intensity (I) has been widely used for getting information about the p-mode excitation process and, in particular, for trying to determine the type and location of the acoustic sources. Numerical simulations of solar convection allow one to “observe” velocity and temperature (T, used as proxy for I) fluctuations in different reference frames. Sampling the oscillations on planes of constant optical depth (τ-frame) closely corresponds to the observer’s point of view, whereas sampling the oscillations at constant geometrical height (z-frame) is more appropriate for comparison with predictions from theoretical models based on Eulerian hydrodynamics. The results of the analysis in the two frames show significant differences. Considering the effects introduced on oscillations by the steep temperature gradient of the photosphere and by the temperature- and pressure-dependent continuum opacity, we develop a new model for fitting the simulated V and T power and cross spectra both in the τ- and z-frames and discuss its merits and limitations.     

Operator-oriented CRS interpolation

In common‐reflection‐surface imaging the reflection arrival time field is parameterized by operators that are of higher dimension or order than in conventional methods. Using the common‐reflection‐surface approach locally in the unmigrated prestack data domain opens a potential for trace regularization and interpolation. In most data interpolation methods based on local coherency estimation, a single operator is designed for a target sample and the output amplitude is defined as a weighted average along the operator. This approach may fail in presence of interfering events or strong amplitude and phase variations. In this paper we introduce an alternative scheme in which there is no need for an operator to be defined at the target sample itself. Instead, the amplitude at a target sample is constructed from multiple operators estimated at different positions. In this case one operator may contribute to the construction of several target samples. Vice versa, a target sample might receive contributions from different operators. Operators are determined on a grid which can be sparser than the output grid. This allows to dramatically decrease the computational costs. In addition, the use of multiple operators for a single target sample stabilizes the interpolation results and implicitly allows several contributions in case of interfering events. Due to the considerable computational expense, common‐reflection‐surface interpolation is limited to work in subsets of the prestack data. We present the general workflow of a common‐reflection‐surface‐based regularization/interpolation for 3D data volumes. This workflow has been applied to an OBC common‐receiver volume and binned common‐offset subsets of a 3D marine data set. The impact of a common‐reflection‐surface regularization is demonstrated by means of a subsequent time migration. In comparison to the time migrations of the original and DMO‐interpolated data, the results show particular improvements in view of the continuity of reflections events. This gain is confirmed by an automatic picking of a horizon in the stacked time migrations.     

Factors Influencing the Stream‐Aquifer Flow Exchange Coefficient

Knowledge of river gain from or loss to a hydraulically connected water table aquifer is crucial in issues of water rights and also when attempting to optimize conjunctive use of surface and ground waters. Typically in groundwater models this exchange flow is related to a difference in head between the river and some point in the aquifer, through a “coefficient.” This coefficient has been defined differently as well as the location for the head in the aquifer. This paper proposes a new coefficient, analytically derived, and a specific location for the point where the aquifer head is used in the difference. The dimensionless part of the coefficient is referred to as the SAFE (stream‐aquifer flow exchange) dimensionless conductance. The paper investigates the factors that influence the value of this new conductance. Among these factors are (1) the wetted perimeter of the cross‐section, (2) the degree of penetration of the cross‐section, and (3) the shape of the cross‐section. The study shows that these factors just listed are indeed ordered in their respective level of importance. In addition the study verifies that the analytical correct value of the coefficient is matched by finite difference simulation only if the grid system is sufficiently fine. Thus the use of the analytical value of the coefficient is an accurate and efficient alternative to ad hoc estimates for the coefficient typically used in finite difference and finite element methods.     

青藏高原东北缘冬给错纳湖全新世湖面波动

青藏高原处于东亚季风、印度季风和西风环流交互作用区.末次冰消期以来,太阳辐射对该地区的古气候环境产生了重要影响,湖泊随着季风系统的变化发生了明显的水位升降,对湖岸阶地的形成起到了直接作用.本项研究重建了青藏高原东北缘冬给错纳湖湖岸阶地记录的湖面波动历史,试图了解青藏高原季风系统演变过程.通过湖泊北岸265 cm厚湖岸阶地沉积物粒度、碳酸盐、矿物、元素和介形虫古环境指标,结合OSL年代模式,分析表明在约10. 2 ka B. P.之前水体较浅;约10. 2~9. 0 ka B. P.湖面开始上升,气候凉湿;9. 0~8. 5 ka B. P.为印度季风强盛期,湖面明显上升,降雨量增高、温度上升;8. 5~7. 9 ka B. P.湖面降低与气候变冷有关;7. 9~7. 0 ka B. P.印度季风开始减弱,气温、降雨下降,但有效湿度较大,湖面降低;7. 0~6. 1 ka B. P.湖面上升可能与低蒸发作用有关,印度季风仍然影响该地区;6. 1~5. 2 ka B. P.,印度季风衰退,气候逐渐变冷、降雨量减小、水体变浅;5. 2~4. 6 ka B. P.气候冷干,有效湿度减小,湖面进一步下降;4. 6 ka B. P.至今气候干冷,东亚季风衰退,湖面下降,期间也可能受西风环流影响而有短暂的降雨增加时期.     

Carbon and Oxygen Isotopic Composition of Surface‐Sediment Carbonate in Bosten Lake (Xinjiang,China) and its Controlling Factors

Bosten Lake is a mid-latitude lake with water mainly supplied by melting ice and snow in the Tianshan Mountains. The depositional environment of the lake is spatially not uniform due to the proximity of the major inlet and the single outlet in the western part of the lake. The analytical results show that the carbon and oxygen isotopic composition of recent lake sediments is related to this specific lacustrine depositional environment and to the resulting carbonate mineralogy. In the southwestern lake region between the Kaidu River inlet and the Kongqi River outlet, carbon isotope composition (δ13C) values of the carbonate sediment (?1‰ to ?2‰) have no relation to the oxygen isotope composition of the carbonate (δ18O) values (?7‰ to ?8‰), with both isotopes showing a low variability. The carbonate content is low (<20%). Carbonate minerals analyzed by X-ray diffraction are mainly composed of calcite, while aragonite was not recorded. The salinity of the lake water is low in the estuary region as a result of the Kaidu River inflow. In comparison, the carbon and oxygen isotope values are higher in the middle and eastern parts of the lake, with δ13C values between approximately +0.5‰ and +3‰, and δ18O values between ?1‰ and ?5‰. There is a moderate correlation between the stable oxygen and carbon isotopes, with a coefficient of correlation r of approximately 0.63. This implies that the lake water has a relatively short residence time. Carbonate minerals constitute calcite and aragonite in the middle and eastern region of the lake. Aragonite and Mg–calcite are formed at higher lake water salinity and temperatures, and larger evaporation effects. More saline lake water in the middle and eastern region of the lake and the enhanced isotopic equilibrium between water and atmospheric CO2 cause the correlating carbon and oxygen isotope values determined for aragonite and Mg–calcite. Evaporation and biological processes are the main reasons for the salinity and carbonate mineralogy influence of the surface-sediment carbonate in Bosten Lake. The lake water residence time and the CO2 exchange between the atmosphere and the water body control the carbon and oxygen isotope composition of the carbonate sediment. In addition, organic matter pollution and decomposition result in the abnormally low carbon isotope values of the lake surface-sediment carbonate.     

Polyphase scheelite and stanniferous silicates in a W-(Sn) skarn close to Felbertal tungsten mine,Eastern Alps

Mineralogy and Petrology - The scheelite exploration target Messelingscharte (Eastern Tyrol, Austria) is located in vicinity of the world-class Felbertal tungsten deposit. W-(Sn) mineralisation...     

Sustainable Capture: Concepts for Managing Stream‐Aquifer Systems

Most surface water bodies (i.e., streams, lakes, etc.) are connected to the groundwater system to some degree so that changes to surface water bodies (either diversions or importations) can change flows in aquifer systems, and pumping from an aquifer can reduce discharge to, or induce additional recharge from streams, springs, and lakes. The timescales of these interactions are often very long (decades), making sustainable management of these systems difficult if relying only on observations of system responses. Instead, management scenarios are often analyzed based on numerical modeling. In this paper we propose a framework and metrics that can be used to relate the Theis concepts of capture to sustainable measures of stream‐aquifer systems. We introduce four concepts: Sustainable Capture Fractions, Sustainable Capture Thresholds, Capture Efficiency, and Sustainable Groundwater Storage that can be used as the basis for developing metrics for sustainable management of stream‐aquifer systems. We demonstrate their utility on a hypothetical stream‐aquifer system where pumping captures both streamflow and discharge to phreatophytes at different amounts based on pumping location. In particular, Capture Efficiency (CE) can be easily understood by both scientists and non‐scientist alike, and readily identifies vulnerabilities to sustainable stream‐aquifer management when its value exceeds 100%.     

Importance of Spatial Resolution in Global Groundwater Modeling

Global-scale gradient-based groundwater models are a new endeavor for hydrologists who wish to improve global hydrological models (GHMs). In particular, the integration of such groundwater models into GHMs improves the simulation of water flows between surface water and groundwater and of capillary rise and thus evapotranspiration. Currently, these models are not able to simulate water table depth adequately over the entire globe. Unsatisfactory model performance compared to well observations suggests that a higher spatial resolution is required to better represent the high spatial variability of land surface and groundwater elevations. In this study, we use New Zealand as a testbed and analyze the impacts of spatial resolution on the results of global groundwater models. Steady-state hydraulic heads simulated by two versions of the global groundwater model G³M, at spatial resolutions of 5 arc-minutes (9 km) and 30 arc-seconds (900 m), are compared with observations from the Canterbury region. The output of three other groundwater models with different spatial resolutions is analyzed as well. Considering the spatial distribution of residuals, general patterns of unsatisfactory model performance remain at the higher resolutions, suggesting that an increase in model resolution alone does not fix problems such as the systematic overestimation of hydraulic head. We conclude that (1) a new understanding of how low-resolution global groundwater models can be evaluated is required, and (2) merely increasing the spatial resolution of global-scale groundwater models will not improve the simulation of the global freshwater system.     

River Seepage Conductance in Large‐Scale Regional Studies

Flow exchange between surface and groundwater is of great importance be it for beneficial allocation and use of water resources or for the proper exercise of water rights. In large‐scale regional studies, most numerical models use coarse grid sizes, which make it difficult to provide an accurate depiction of the phenomenon. In particular, a somewhat arbitrary leakance coefficient in a third type (i.e., Cauchy, General Head) boundary condition is used to calculate the seepage discharge as a function of the difference of head in the river and in the aquifer, whose value is often found by calibration. A different approach is presented to analytically estimate that leakance coefficient. It is shown that a simple equivalence can be deduced from the analytical solution for the empirical coefficient, so that it provides the accuracy of the analytical solution while the model maintains a very coarse grid, treating the water‐table aquifer as a single calculation layer. Relating the empirical leakance coefficient to the exact conductance, derived from physical principles, provides a physical basis for the leakance coefficient. Factors such as normalized wetted perimeter, degree of penetration of the river, presence of a clogging layer, and anisotropy can be included with little computational demand. In addition the river coefficient in models such as MODFLOW, for example, can be easily modified when grid size is changed without need for recalibration.