DNAPL Characterization Methods and Approaches, Part 2: Cost Comparisons

Contamination from the use of chlorinated solvents, often classified as dense nonaqueous phase liquids (DNAPLs) when in an undissolved state, pose environmental threats to ground water resources worldwide. DNAPL site characterization method performance comparisons are presented in a companion paper (Kram et al. 2001). This study compares the costs for implementing various characterization approaches using synthetic unit model scenarios (UMSs), each with particular physical characteristics. Unit costs and assumptions related to labor, equipment, and consumables are applied to determine costs associated with each approach for various UMSs. In general, the direct-push sensor systems provide cost-effective characterization information in soils that are penetrable with relatively shallow (less than 10 to 15 m) water tables. For sites with impenetrable lithology using direct-push techniques, the Ribbon NAPL Sampler Flexible Liner Underground Technologies Everting (FLUTe) membrane appears to be the most cost-effective approach. For all scenarios studied, partitioning interwell tracer tests (PITTs) are the most expensive approach due to the extensive pre-and post-PITT requirements. However, the PITT is capable of providing useful additional information, such as approximate DNAPL saturation, which is not generally available from any of the other approaches included in this comparison.     

Sampling characteristics and biases of enclosure traps for sampling fishes in estuaries

We evaluated the sampling characteristics of enclosure traps in estuaries in southern California, USA. Using enclosure traps that sampled 0.25, 0.5, and 1-m² footprints, we found that enclosure trap size significantly affected estimates of fish density and the precision of these estimates. The highest estimates were produced by the 0.5-m² trap and the lowest by the 0.25-m² trap. Precision of the density estimates improved with increasing trap size, while the proportion of zero values in the data sets decreased and estimates of species richness increased. The largest trap was difficult to use in the field and often did not function properly; thus we concluded that intermediate enclosure trap sizes offered the best compromise between statistical and logistical considerations. By examination of burrows in sediment cores taken in fished out enclosure traps, we found no evidence to support the widely held view that burrow-dwelling fishes evaded capture by hiding in burrows. We also used mark-recapture techniques to estimate recovery efficiency in 0.43-m² enclosure traps. Recovery efficiency averaged 91% and did not differ significantly among estuaries or sampling stations within estuaries. Based on extensive netting within enclosure traps, we determined that in areas with dense fish populations (>90 fish 0.43-m⁻²), netting could be ceased after the first sweep that captured no fish with only a trivial effect on the estimate of density. In more sparsely populated areas, netting had to continue until 2–3 sweeps had captured no fish in order to obtain estimates of density that were within 90% of the actual values present. Overall, we found enclosure traps to be effective tools for sampling small, abundant fishes in shallow estuaries in southern California, but we recommend that care be taken when choosing trap size and sampling (netting) effort within traps in order to optimize their sampling characteristics.     

Dublin days: the 2003 NAM

Mark E Bailey and Tom Ray give an overview of the National Astronomy Meeting in Dublin, where record numbers of astronomers went to talk, listen and network.     

The Ledsjö end moraine—a subaquatic push moraine composed of glaciomarine clay in central Sweden

During the Younger Dryas cold event, the Scandinavian ice sheet readvanced in southwest Sweden and formed the Middle Swedish end-moraine zone (MSEMZ). Recent highway construction near Skara has created an exposure through the prominent ridge at Ledsjö. Through sketching and measurement of structural information, we have documented the internal character of the Ledsjö moraine. The moraine consists predominantly of clay with numerous sand pods and lenses, which show undeformed, brittle deformed, or fluidized structures. Based on geomorphology and structural geology, it is clear the moraine was made during two advances. As ice advanced, proglacial marine clay was subglacially mobilized by the ice and extruded at the ice margin forming a ramp of debris-flow sediment. Contemporaneously, subglacial meltwater transported sand to the margin, where the meltwater became a buoyant plume, and sand was deposited near the ice margin by currents moving away from as well as toward the ice margin. These processes resulted in interbedded sand and clay. Continued advance of the ice margin deformed this package and further pushed the assemblage into a ridge form with gravity sliding of portions of the ridge. Prior to the second advance, sand was deposited on the proximal side of the initial ridge. During readvance, this sand was thrust faulted and intruded by mobilized clay. Up ice of the intruded sands, subglacial, extensional deformation created a complex shear zone of faulted sand and clay. The Ledsjö moraine represents a subaerial example of submarine push moraines like the submerged moraines recently documented in Svalbard.     

Lattice-Boltzmann simulations of the capillary pressure–saturation–interfacial area relationship for porous media

Hysteresis in the relationship between capillary pressure (P_c), wetting phase saturation (S_w) and nonwetting–wetting interfacial area per volume (a_nw) is investigated using multiphase lattice-Boltzmann simulations of drainage and imbibition in a glass bead porous system. In order to validate the simulations, the P_c–S_w and a_nw–S_w main hysteresis loops were compared to experimental data reported by Culligan et al. [Culligan KA, Wildenschild D, Christensen BS, Gray WG, Rivers ML, Tompson AB. Interfacial area measurements for unsaturated flow through porous media. Water Resour Res 2004;40:W12413]. In general, the comparison shows that the simulations are reliable and capture the important physical processes in the experimental system. P_c–S_w curves, a_nw–S_w curves and phase distributions (within the pores) show good agreement during drainage, but less satisfactory agreement during imbibition. Drainage and imbibition scanning curves were simulated in order to construct P_c–S_w–a_nw surfaces. The root mean squared error (RMSE) and mean absolute error (MAE) between drainage and imbibition surfaces was 0.10 mm⁻¹ and 0.03 mm⁻¹, respectively. This small difference indicates that hysteresis is virtually nonexistent in the P_c–S_w–a_nw relationship for the multiphase system studied here. Additionally, a surface was fit to the main loop (excluding scanning curves) of the drainage and imbibition P_c–S_w–a_nw data and compared to the surface fit to all of the data. The differences between these two surfaces were small (RMSE = 0.05 mm⁻¹ and MAE = 0.01 mm⁻¹) indicating that the P_c–S_w–a_nw surface is adequately represented without the need for the scanning curve data, which greatly reduces the amount of data required to construct the non-hysteretic P_c–S_w–a_nw surface for this data.     

Quantifying volcanic ash fall hazard to electricity infrastructure

Quantifying the potential ash fall hazards from re-awakening volcanoes is a topic of great interest. While methods for calculating the probability of eruptions, and for numerical simulation of tephra dispersal and fallout exist, event records at most volcanoes that re-awaken sporadically on decadal to millennial cycles are inadequate to develop rigorous forecasts of occurrence, much less eruptive volume. Here we demonstrate a method by which eruption records from radiocarbon-dated sediment cores can be used to derive forecasting models for ash fall impacts on electrical infrastructure. Our method is illustrated by an example from the Taranaki region of New Zealand. Radiocarbon dates, expressed as years before present (B.P.), are used to define an age-depth model, classifying eruption ages (with associated errors) for a circa 1500–10 500 year B.P. record at Mt. Taranaki (New Zealand). In addition, data describing the youngest 1500 years of eruption activity is obtained from directly dated proximal deposits. Absence of trend and apparent independence in eruption intervals is consistent with a renewal model using a mix of Weibulls distributions, which was used to generate probabilistic forecasts of eruption recurrence. After establishing that interval length and tephra thickness were independent in the record, a thickness–volume relationship (from [Rhoades, D.A., Dowrick, D.J., Wilson, C.J.N., 2002. Volcanic hazard in New Zealand: Scaling and attenuation relations for tephra fall deposits from Taupo volcano. Nat. Hazards, 26:147–174]) was inverted to provide a frequency–volume relationship for eruptions. Monte Carlo simulation of the thickness–volume relationship was then used to produce probable ash fall thicknesses at any chosen site. Several critical electrical infrastructure sites in the Taranaki Region were analysed. This region, being the only gas and condensate-producing area in New Zealand, is of national economic importance, with activities in and around the area depending on uninterrupted power supplies. Forecasts of critical ash thicknesses (1 mm wet and 2 mm dry) that may cause short-circuiting, surges or power shutdowns in substations show that the annual probabilities of serious impact are between ~ 0.5% and 27% over a 50 year period. It was also found that while large eruptions with high ash plumes tend to affect “expected” areas in relation to prevailing winds, the direction impacts of small ash falls are far less predictable. In the Taranaki case study, areas out of normal downwind directions, but close to the volcano, have probabilities of impact for critical thicknesses of 1–2 mm of around half to 60% of those in downwind directions and therefore should not be overlooked in hazard analysis. Through this method we are able to definitively show that the potential ash fall hazard to electrical infrastructure in this area is low in comparison to other natural threats, and provide a quantitative measure for use in risk analysis and budget prioritisation for hazard mitigation measures.     

Monitoring Intraurban temperature patterns and associated land cover in phoenix,Arizona using Landsat thermal data

Abstract

This study investigates urban climatologic modification associated with development and changing land use in the relatively arid urban environment of Phoenix, Arizona. An analysis of surface temperatures, as portrayed on Landsat thermal remotely sensed data, were compared to current land use patterns in regions of the rapidly expanding urban landscape. A second focus of this study involved investigation of the surface temperatures of this environment, as extracted from the radiometric data of the Landsat thermal band, to provide insights into the complexities of the relationship to the near‐surface atmospheric temperature, a parameter used extensively in climate change analyses and in models for energy and water demand in this desert region. The near surface air temperature is usually measured approximately two meters above the ground surface. In general, spatial temperature patterns of the metropolitan region were strongly correlated with the presence of open water or biomass which provide an evapotranspirative heat sink.     

Dune field development,interactions and boundary conditions for crescentic and stellate megadunes of the Al Liwa Basin,the Empty Quarter

Within the greater Ar Rub' al Khali (Empty Quarter) sand sea lies an internal depocentre, the Al Liwa Basin, which comprises a variety of mega‐scale dune types. Crescentic dunes dominant the north of the basin while megadunes of stellate or star form are a major landform of the south‐eastern reaches. Their development into dune fields is determined by the style and rate of dune–dune interactions, the boundary conditions imposed by a multi‐modal wind regime, fluctuating groundwater levels, and sediment availability under an assortment of climatic conditions throughout the Quaternary. As a result, dune field patterns are a collective response to these perturbations in space, time and environment. The R‐statistic is a collective measure of these responses, and is a metric capable of identifying the degree of pattern maturity or self‐organization of the aeolian system, and the pathways from which patterns evolve. The spatial signature of the southerly located star dunes is characterized by two definitive patterns of organization: the first, one of complete spatial randomness, the second, a low degree of spatial uniformity. In isolation, these results appear to be unrelated to those for crescentic dunes of the region in which a significantly higher degree of pattern dispersion is the norm. However, when spatial statistical measures are integrated with the theoretical understanding of dune–dune interactions and the involvement of environmental agents, the complex morphodynamic pathways and linkages between regional dune fields is better understood. In this case, both constructive (e.g. merging, lateral linking) and regenerative activity (e.g. calving) have played important roles in the development of dune size, and associated adjustments in spacing, and dune numbers, and subsequently dune field patterns. Synergetic patterns are emblematic of this vast dunescape, whereby transitional geographic, morphologic, dimensional and environmental modifications exist between the mega‐crescentic and mega‐stellate dunes of the Empty Quarter. Copyright © 2012 John Wiley & Sons, Ltd.     

A Mass Balance Approach to Resolving LNAPL Stability

This article explores the hypothesis that natural losses of light nonaqueous phase liquids (LNAPLs) through dissolution and evaporation can control the overall extent of LNAPL bodies and LNAPL fluxes observed within LNAPL bodies. First, a proof‐of‐concept sand tank experiment is presented. An LNAPL (methyl tert‐butyl ether) was injected into a sand tank at five constant injection rates that were increased stepwise. Initially, for each injection rate the LNAPL bodies expanded quickly. With time the rate of expansion of the LNAPL bodies slowed and at extended times the extent of the LNAPL became constant. Attainment of a stable LNAPL extent is attributed to rates of LNAPL addition being equal to rates of LNAPL losses through dissolution and evaporation. Secondly, analytical solutions are developed to extrapolate the processes observed in the proof‐of‐concept experiment to dimensions and time frames that are consistent with field‐scale LNAPL bodies. Three LNAPL body geometries that are representative of common field conditions are considered including one‐dimensional, circular, and oblong shapes. Using idealized conditions, the solutions describe volumetric LNAPL fluxes as a function of position in LNAPL bodies and the overall extent of LNAPL bodies as a function of time. Results from both the proof‐of‐concept experiment and the mathematical developments illustrate that natural losses of LNAPL can play an important role in governing LNAPL fluxes within LNAPL bodies and the overall extent of LNAPL bodies.