Munjed A. Maraqa

Effects of residence time and degree of water saturation on sorption nonequilibrium parameters

This study reports the impact of the degree of water saturation on sorption nonequilibrium parameters. Two nonionic organic compounds (benzene and dimethylphthalate) and three nonaggregated sandy soils were utilized. Local equilibrium assumptions were found to be invalid for describing the transport of these compounds even at pore-water velocities as low as 0.7 cm/h. Sorption nonequilibrium appeared to be of a diffusive nature rather than due to a slow chemical reaction. Sorption mass-transfer coefficients varied proportionally with pore-water velocity. A strong correlation between the mass-transfer coefficient and residence time was found utilizing present and previously reported laboratory data. A similar relationship was also found for the mass-transfer coefficient between mobile and immobile water regions. Field data indicate that the sorption mass-transfer coefficient may continue to decrease in a consistent way even at residence times as large as 5×103 h. Variations in the degree of water saturation had no impact on the value of the sorption mass-transfer coefficient other than what would be expected due to changes in the residence time. This suggested that movement into the solid grains of the large emptied pores through diffusion from the water-filled pores into stagnant water covering these grains was relatively fast compared to the sorption rate.

Comparison of nonideal sorption formulations in modeling the transport of phthalate esters through packed soil columns

Sorption of dimethyl phthalate (DMP), diethyl phthalate (DEP) and dipropyl phthalate (DPP) to two soil materials that vary in organic matter content was investigated using miscible displacement experiments under saturated flow conditions. Generated breakthrough curves (BTCs) were inversely simulated using linear, equilibrium sorption (LE), nonlinear, equilibrium sorption (NL), linear, first-order nonequilibrium sorption (LFO), linear, radial diffusion (LRD), and nonlinear, first-order nonequilibrium sorption (NFO) models. The Akaike information criterion was utilized to determine the preferred model. The LE model could not adequately describe phthalate ester (PE) BTCs in higher organic matter soil or for more hydrophobic PEs. The LFO and LRD models adequately described the BTCs but a slight improvement in curve-fitting was gained in some cases when the NFO model was used. However, none of the models could properly describe the desorptive tail of DPP for the high organic matter soil. Transport of DPP through this soil was adequately predicted when degradation or sorption hysteresis was considered. Using the optimized parameter values along with values reported by others it was shown that the organic carbon distribution coefficient (K(oc)) of PEs correlates well with the octanol/water partition coefficient (K(ow)). Also, a strong relationship was found between the first-order sorption rate coefficient normalized to injection pulse size and compound residence time. A similar trend of timescale dependence was found for the rate parameter in the radial diffusion model. Results also revealed that the fraction of instantaneous sorption sites is dependent on K(ow) and appears to decrease with the increase in the sorption rate parameter.

Modeling solute transport affected by heterogeneous sorption kinetics using single-rate nonequilibrium approaches

Single-rate transport models are commonly used for interpreting sorption-related mass transfer in porous media, often with the intention of approximating the kinetics of the sorption process. Among the most commonly used single-rate models are the two-site first-order (TSFO) and the two-site radial diffusion (TSRD) models. We fitted the parameters of the TSFO and TSRD models to simulated breakthrough data of hypothetical column experiments in which sorption rates were described by a γ-distributed sorption sites (GS) model. Our objective was to determine the conditions under which the assumption of a single-rate sorption parameter will be applicable to systems with heterogeneous sorption rates. We were further interested in knowing in what manner the fitted single-rate nonequilibrium model parameters depend upon the conditions under which the data were obtained. The considered hypothetical cases covered a range of experimental conditions and involved compounds with different sorption characteristics. The study revealed that the goodness of fit of the single rate models in simulating the transport of solutes exhibiting heterogeneous sorption rates is affected by solute residence time and pulse injection duration. Compared to the TSFO model, the TSRD model generally results in better prediction of solute transport affected by heterogeneous sorption kinetics. In addition, for such systems, the nonequilibrium parameters fitted using the TSFO model and their counterparts in the TSRD model are highly correlated. Moreover, an increase in the fitted mass transfer timescale of each of the single-rate models is coupled with an increase in the associated fraction of instantaneous sorption sites. A strong correlation was found between the time of the experiment and the product of the fitted characteristic time for mass transfer, pulse duration, and solute residence time. The correlation explains many of the variations in the mass transfer timescale encountered when single-rate sorption approaches were utilized to model solute transport in previous miscible displacement studies.

Characterizing Driver Behaviors Relevant to Traffic Safety: A Multistage Approach

This article used a multistage methodology to characterize driver behaviors relevant to traffic safety in the United Arab Emirates. The first stage utilized traffic violations and accident records to quantify the types and frequencies of the most common unsafe driving behaviors. Experts in traffic safety and law enforcement were interviewed in the second stage to confirm the findings of the first stage and to identify the most suitable measures to limit such unsafe behaviors. The third stage of the study entailed studying unsafe driving behaviors from a psychological perspective. Finally, drivers were surveyed to obtain their own assessment of attitudes and behaviors affecting traffic safety. A probability-based approach was adopted based on the results of statistical cross classification to identify the percentage of the frequent violators in the population, and the most common negative behavior forms (among the overall drivers population). The joint probabilities of having more than one type of frequent violations were estimated. The chi-squared values were also estimated to quantify the strength of relation among the violations and their causes. Finally, recommendations were made to limit the drivers’ negative behaviors.

Water-Transit Services in Dubai-UAE: User and Operator Survey Development and Quantitative Analysis

The Marine Agency-Roads and Transport Authority (MA-RTA) of Dubai-UAE is currently undertaking a study to develop a new transport policy for service delivery. The goal of the new policy is to increase rider share and use of MA-RTA services. To attain this goal, a five-year service policy will be adopted to establish modern, cost-effective, and efficient services to attract higher ridership. An integral part of the development of the policy is to assess baseline conditions, particularly user and operator opinions of current services. This paper focuses on developing user and operator survey questionnaires and providing quantitative statistical analyses of survey results. The majority of the assessment tools for the operations practice indicated acceptable levels of services. However, there is a need for a reduction of trip fares, better facilities at the stations, and a broader survey to identify prospective users and attraction methods to the marine services. In addition, development of a database for the marine transport system could assist in better planning, operational, and management aspects of the system.

Analysis of the effectiveness of the road-crash database in the United Arab Emirates

This study presents an approach to assess the effectiveness of road-crash databases in providing meaningful analysis. The assessment was done for the case of the database system in the United Arab Emirates. The assessment was based on feedback from two groups: one involved crash investigators and database managers, and the second involved other traffic safety experts. A quantitative geographical information system-based data analysis was also included to illustrate some deficiencies in the existing database. The survey of crash investigators and database managers focused on aspects including database utilization, technology for providing data services, methods of data collection, sufficiency of the traffic crash report, difficulties in the traffic crash report, and sufficiency of pedestrian crash data. The experts’ survey addressed aspects of difficulties of the traffic crash report and database system, required data for crash analyses, need of in-depth crash data, need for Global Positioning System data, involving a private party in handling the database system, and permissible extent of data sharing. The conducted analyses were used to subjectively quantify the importance and the perceived level of satisfaction of the surveyed aspects. This was then used to propose measures to upgrade the database system and the crash investigation process to overcome the identified deficiencies.

A Framework for the Assessment and Policy Development of Water Transit Services in Dubai, UAE

The Marine Agency-Roads and Transport Authority (MA-RTA) of Dubai-UAE recently undertook a study to develop a new transport policy for service delivery. The goal of the new policy is to increase rider share and use of MA-RTA services. The existing service policy guidelines of the water transit services were barely supported by the loading patterns, existing market coverage, adequate intermodal planning tools, economic feasibility, and capital return of services. Based on user and operators surveys of the water transit services, aiming at assessing the baseline conditions,methodologies are suggested to estimate important service attributes, including service coverage, intermodal connectivity, and market demand estimation. The estimated service attributes were used as measures to develop the service policy standards to increase water transit ridership and enhance service efficiency. It is recommended to incorporate some of the service standard/guideline measures under a framework discussed in this study for developing service plans, monitoring performance, and providing short or long term alteration to the services.

Simulation of the hydrodynamic regime of Jebel Ali Harbor, Dubai, United Arab Emirates

Jebel Ali Harbor in Dubai is one of the largest man-made ports in the world. Due to its bottle-like nature, it is anticipated that little mixing or wave action occurs within the harbor; however, no study has been conducted to investigate the harbors hydrodynamic regime. In this paper, the circulation pattern in Jebel Ali Harbor is presented. The vertical profile of temperature and salinity shows that the harbor water is fairly well-mixed, justifying the use of a depth-averaged, 2-D model. Modeling the hydrodynamic pattern of Jebel Ali Harbor was carried out using MIKE21 model. Although tide is the main driving force of circulation in the harbor, wind and Coriolis forces were taken into account in the simulation. Simulation results showed that both dead and eddy zones exis. As the harbor receives treated wastewater from several industries located in the area, flushing out of a conservative tracer by the advection-dispersion process was also simulated using MIKE21. Results revealed that the flushing time of a conservative tracer from the harbor varies significantly with the discharge location. Simulated results showed that understanding the hydrodynamic regime of Jebel Ali Harbor is useful for selecting the location and time for discharging tracers into the harbor.

A fuzzy-based decision support system for ranking the delivery methods of mega projects

Purpose The purpose of this paper is to present a framework to devise a system for ranking of traditional project delivery methods, regarding their suitability, against the varying levels of mega project attributes. Design/methodology/approach The proposed system employs input and output interfaces and a granular (fuzzy rule base) component for estimating the subjective levels of risks, opportunities, and constraints and then mapping them to a decision matrix. A questionnaire has been designed (using the SurveyGizmo® platform) to collect the perceptions of the various project stakeholders and use them. A total of 127 stakeholders completed the survey form in full. Findings The survey data were used to calibrate the fuzzy logic model of the granular component. The envisioned system computes, for each possible delivery method, an index that reflects the suitability (of the corresponding delivery method) on an ordinal scale. Originality/value The devised decision support system is likely to lessen the dependency of “accurate decision” on “the experience of the decision-makers.” It will also enable ranking the various project delivery methods based on the various project and stakeholder attributes that are likely to affect the project risks, opportunities and constraints.

Ability of Single-Rate Models to Predict Solute Distribution Coefficients in Systems with Heterogeneous Sorption Kinetics

Several sorption nonequilibrium models have been developed to gain a better understanding of solute transport in porous media, among which are those that assume a single-rate behavior. In this study, two commonly used single-rate models were fitted to computer-simulated breakthrough data from hypothetical column experiments in which multirate sorption kinetics exist at the pore scale. The objective was to determine how the sorption distribution coefficient (K) predicted using these models depends on the conditions under which the data were obtained. Simulated cases covered a range of experimental conditions and involved compounds with different sorption characteristics and different degrees of sorption rate heterogeneity. Results revealed that, for a system with a multirate sorption behavior, the true K value is under-predicted if the parameter estimation is determined by curve fitting a single-rate model. The extent of deviation between the fitted and true K increases with the decrease in residence time and increase in sorption rate heterogeneity. Functional relationships were developed between the relative reduction in K and solute residence time. Analysis using the relationships developed suggests that a major potential cause of the previously reported discrepancy between batch- and column-determined K could be attributed to the use of single-rate models for parameter prediction.