Stephen H. Conrad

Soil‐water flux in the Southern Great Basin, United States: Temporal and spatial variations over the last 120,000 years

The disposal of hazardous and radioactive waste in arid regions requires a thorough understanding of the occurrence of soil-water flux and recharge. Soil-water chemistry and isotopic data are presented from three deep vadose zone boreholes (>230 m) at the Nevada Test Site, located in the Great Basin geographic province of the southwestern United States, to quantify soil-water flux and its relation to climate. The low water contents found in the soils significantly reduce the mixing of tracers in the subsurface and provide a unique opportunity to examine the role of climate variation on recharge in arid climates. Tracing techniques and core data are examined in this work to reconstruct the paleohydrologic conditions existing in the vadose zone well beyond the timescales typically investigated. Stable chloride and chlorine 36 profiles indicate that the soil waters deep in the vadose zone range in age from approximately 20,000 to 120,000 years. Secondary chloride bulges that are present in two of the three profiles support the concept of recharge occurring at or near the last two glacial maxima, when the climate of the area was considerably wetter and cooler. The stable isotopic composition of the soil water in the profiles is significantly more depleted in heavy isotopes than is modern precipitation, suggesting that recharge under the current climate is not occurring at this arid site. Past and present recharge appears to have been strongly controlled by surface topography, with increased incidence of recharge where runoff from the surrounding mountains may have been concentrated. The data obtained from this detailed drilling and sampling program shed new light on the behavior of water in thick vadose zones and, in particular, show the sensitivity of arid regions to the extreme variations in climate experienced by the region over the last two glacial maxima.

System dynamics modeling for community-based water planning: Application to the Middle Rio Grande

Abstract.The watersheds in which we live are comprised of a complex set of physical and social systems that interact over a range of spatial and temporal scales. These systems are continually evolving in response to changing climatic patterns, land use practices and the increasing intervention of humans. Management of these watersheds benefits from the development and application of models that offer a comprehensive and integrated view of these complex systems and the demands placed upon them. The utility of these models is greatly enhanced if they are developed in a participatory process that incorporates the views and knowledge of relevant stakeholders. System dynamics provides a unique mathematical framework for integrating the physical and social processes important to watershed management, and for providing an interactive interface for engaging the public. We have employed system dynamics modeling to assist in community-based water planning for a three-county region in north-central New Mexico. The planning region is centered on a ~165-km reach of the Rio Grande that includes the greater Albuquerque metropolitan area. The challenge, which is common to other arid/semi-arid environments, is to balance a highly variable water supply among the demands posed by urban development, irrigated agriculture, river/reservoir evaporation and riparian/in-stream uses. A description of the model and the planning process are given along with results and perspectives drawn from both.

Gravity‐destabilized nonwetting phase invasion in macroheterogeneous porous media: Experimental observations of invasion dynamics and scale analysis

The authors designed and conducted experiments in a heterogeneous sand pack where gravity-destabilized nonwetting phase invasion (CO{sub 2} and TCE) could be recorded using high resolution light transmission methods. The heterogeneity structure was designed to be reminiscent of fluvial channel lag cut-and-fill architecture and contain a series of capillary barriers. As invasion progressed, nonwetting phase structure developed a series of fingers and pools; behind the growing front they found nonwetting phase saturation to pulsate in certain regions when viscous forces were low. Through a scale analysis, they derive a series of length scales that describe finger diameter, pool height and width, and regions where pulsation occurs within a heterogeneous porous medium. In all cases, they find that the intrinsic pore scale nature of the invasion process and resulting structure must be incorporated into the analysis to explain experimental results. The authors propose a simple macro-scale structural growth model that assembles length scales for sub-structures to delineate nonwetting phase migration from a source into a heterogeneous domain. For such a model applied at the field scale for DNAPL migration, they expect capillary and gravity forces within the complex subsurface lithology to play the primary roles with viscous forces forming a perturbation on the inviscid phase structure.

Bench-scale visualization of DNAPL remediation processes in analog heterogeneous aquifers: surfactant floods and in situ oxidation using permanganate.

We have conducted well-controlled DNAPL remediation experiments within a 2-D, glass-walled, sand-filled chamber using surfactants (Aerosol MA and Tween 80) to increase solubility and an oxidant (permanganate) to chemically degrade the DNAPL. Initial conditions for each remediation experiment were created by injecting DNAPL as a point source at the top of the chamber and allowing the DNAPL to migrate downward through a water-filled, heterogeneous, sand-pack designed to be evocative of a fluvial depositional environment. This migration process resulted in the DNAPL residing as a series of descending pools. Lateral advection across the chamber was used to introduce the remedial fluids. Photographs and digital image analysis illustrate interactions between the introduced fluids and the DNAPL. In the surfactant experiments, we found that DNAPL configured in a series of pools was easily mobilized. Extreme reductions in DNAPL/water interfacial tension occurred when using the Aerosol MA surfactant, resulting in mobilization into low permeability regions and thus confounding the remediation process. More modest reductions in interfacial tension occurred when using the Tween 80 surfactant resulting in modest mobilization. In this experiment, capillary forces remained sufficient to exclude DNAPL migration into low permeability regions allowing the excellent solubilizing properties of the surfactant to recover almost 90% of the DNAPL within 8.6 pore volumes. Injection of a potassium permanganate solution resulted in precipitation of MnO2, a reaction product, creating a low-permeability rind surrounding the DNAPL pools. Formation of this rind hindered contact between the permanganate and the DNAPL, limiting the effectiveness of the remediation. From these experiments, we see the value of performing visualization experiments to evaluate the performance of proposed techniques for DNAPL remediation.

Critical national infrastructure reliability modeling and analysis

One of the top 10 priorities of the U.S. Department of Homeland Security is protection of our critical national infrastructures including power, communications, transportation, and water. This paper presents models to quantify the interdependencies of critical infrastructures in the U.S. and evaluate plans to compensate for vulnerabilities. Communications is a key infrastructure, central to all others, so that understanding and modeling the risk due to communications disruptions is a high priority in order to enhance public safety and infrastructure resiliency. This paper discusses reliability modeling and analysis at a higher level than usual. Reliability analysis typically deals at the component or sub-system level and talks about “mean time to failure” and “mean time to repair” to derive availability estimates of equipment. Here, we deal with aggregate scales of failures, restoration, and mitigation across national infrastructures. This aggregate scale is useful when examining multiple infrastructures simultaneously with their interdependencies. System dynamics simulation models have been created for both communication networks and for the infrastructure interaction models that quantify these interactions using a risk-informed decision process for the evaluation of alternate protective measures and investment strategies in support of critical infrastructure protection. We will describe an example development of these coupled infrastructure consequence models and their application to the analysis of a power disruption and its cascading effect on the telecommunications infrastructure as well as the emergency services infrastructure. The results show significant impacts across infrastructures that can become increasingly exacerbated if the consumer population moves more and more to telecom services without power lifeline.

A behavioral theory of insider-threat risks: A system dynamics approach

The authors describe a behavioral theory of the dynamics of insider-threat risks. Drawing on data related to information technology security violations and on a case study created to explain the dynamics observed in that data, the authors constructed a system dynamics model of a theory of the development of insider-threat risks and conducted numerical simulations to explore the parameter and response spaces of the model. By examining several scenarios in which attention to events, increased judging capabilities, better information, and training activities are simulated, the authors theorize about why information technology security effectiveness changes over time. The simulation results argue against the common presumption that increased security comes at the cost of reduced production.

Gravity-destabilized nonwetting phase invasion in macroheterogeneous porous media: Near-pore-scale macro modified invasion percolation simulation of experiments

The authors reconceptualize macro modified invasion percolation (MMIP) at the near pore (NP) scale and apply it to simulate the non-wetting phase invasion experiments of Glass et al [in review] conducted in macro-heterogeneous porous media. For experiments where viscous forces were non-negligible, they redefine the total pore filling pressure to include viscous losses within the invading phase as well as the viscous influence to decrease randomness imposed by capillary forces at the front. NP-MMIP exhibits the complex invasion order seen experimentally with characteristic alternations between periods of gravity stabilized and destabilized invasion growth controlled by capillary barriers. The breaching of these barriers and subsequent pore scale fingering of the non-wetting phase is represented extremely well as is the saturation field evolution, and total volume invaded.

IMPACT OF GEOLOGIC HETEROGENEITY ON RECHARGE ESTIMATION USING ENVIRONMENTAL TRACERS : NUMERICAL MODELING INVESTIGATION

This paper presents a numerical modeling approach for assessing the impacts of geologic heterogeneity on groundwater recharge estimates derived from environmental tracers. Common to many of the environmental tracer methods used to infer recharge in arid environments is an assumption of one-dimensional, vertical downward flow of water and solutes. However, in recent years there has been a growing recognition that fluid flux rates through geologic materials can be spatially variable owing to heterogeneities in porous media properties. Previous studies have suggested that local flow directions are dependent upon the stratigraphic and sedimentologic characteristics of the medium and may not be vertical even when application of water at the surface is spatially uniform and hydraulic gradients are vertical. Consequently, environmental tracer movement will also be spatially variable, and the one-dimensional assumption invoked to interpret unsaturated environmental tracer concentration profiles may be unrealistic. Two different numerical simulation experiments were performed to investigate the problems with using environmental tracers to infer recharge. The first involves simulation of flow and tracer transport through heterogeneous flat-lying media, and the second considers dipping layered anisotropic media. The results of these simulations show that recharge inferred using environmental tracer methods is also highly spatially variable and that recharge estimates obtained by tracer profiles tend to overestimate recharge and should be considered accurate only to within an order of magnitude, particularly in situations with significant media heterogeneity. Consequently, recharge estimates obtained from tracer profiles should be critically evaluated with regard to impacts of spatially variable flow.

Critical Infrastructure Analysis of Telecom for Natural Disasters

Critical national infrastructures for power, emergency services, finance, and other basic industries rely heavily on information and telecommunications networks (voice, data, Internet) to provide services and conduct business. While these networks tend to be highly reliable, severe, large scale outages do occur, especially at times of unfolding disasters, which can lead to cascading effects on other dependent infrastructures. This paper describes recent natural disasters in the USA, namely hurricanes Katrina, Rita, and Wilma, and the impacts they have had on power outages and flooding leading to failures on the dependent critical infrastructure. In particular we consider the impact of these disasters on the affected telecommunication networks. We quantify the level of availability of wireline and wireless services during these network failures. We also discuss studies that were performed in preparation for a hurricane impact. The studies have been performed using the network simulation modeling and analysis research tool (N-SMART), which has been developed to support detailed wireline and wireless network simulations under varying network conditions and degrees of failures. We analyze the levels of outages and recovery times for these disaster events as well as possible mitigations to prepare in advance for these and other potential future disasters

Inter-infrastructure modeling — Ports and telecommunications

During the past year, Bell Laboratories and Sandia National Laboratories have been modeling and simulating cross-industry interactions between infrastructures and the cascading of impacts under disruption scenarios. Critical national infrastructures for importing and exporting goods and materials (e.g., seaboard shipping through ports on the U.S. East and West Coasts) require the support of other industries to conduct business. For example, ports rely on the grid of information networks (voice, data, Internet) to communicate; they also rely on the power grid to operate machinery and the transportation grid to distribute the goods and materials. While information networks, power networks, and transportation networks tend to be highly reliable, disruptions can lead to extended outages requiring days/weeks to repair. These outages can cause shutdown of port operations, resulting in severe financial losses for the economy. This paper describes just one of those inter-infrastructure dependencies: by simulating a port and the interactions with the telecommunications infrastructure, it describes the impacts on both the flow of goods and materials through ports and the economic impact on the ports under a telecommunications disruption scenario.